unit i limitations of conventional mobile telephone ... · fundamentals of cellular radio system...

(A70434) CELLULAR AND MOBILE COMMUNICATIONS (2018) UNIT-I

VIDYA SAGAR.P VBIT potharajuvidyasagar.wordpress.com

CMC

VIDYA SAGAR P

UNIT – I INTRODUCTION TO CELLULAR

MOBILE RADIO SYSTEMS:

Limitations of conventional mobile

telephone systems, Basic Cellular

Mobile System, First, second, third,

and fourth generation cellular

wireless systems, Uniqueness of

mobile radio environment. Fading-

Time dispersion parameters,

Coherence bandwidth, Doppler

spread and coherence time.

FUNDAMENTALS OF CELLULAR

RADIO SYSTEM DESIGN :

Concept of frequency reuse, Co-

channel interference, co-channel

interference reduction factor,

Desired C/I from a normal case in a

omnidirectional antenna system,

system capacity, trunking and grade

of service, Improving coverage and

capacity in cellular systems- Cell

splitting, Sectoring, Microcell zone

concept.



INTRODUCTION

Wireless Communications: Modern telecommunications are one of the main

application of electromagnetic theory (along with remote sensing and radars). The wireless telecommunications industry has seen incredible advances in recent years, some based on well-known principles, others on new inventions or on new practical implementations. This chapter introduces some aspects of wireless communications, including its spectrum landscape.

Past:Early smoke signals and carrier pigeons may of course be considered as a form of wireless communications, but offer little modern interest. Instead, it may be telling to look at the first appearance of several important techniques. Early coding schemes can be attributed to the British scientist Robert Hooke for inventing large mobile panels coding the letters of the alphabet (1684). More elaborate schemes appear in the late 18th century, including the noteworthy optical telegraph invented by the French physicist Claude Chappe (1791); these large signaling towers transmitted coded words (rather than letters) over long

distances, and were developed in the following years into a large network over major cities in France and surrounding countries. These precursors to radio communications already emphasize some valuable points:

1) Coding information is used for efficiency,

2) Transmission works only in line of sight, and may suffer from outages due to fog or rain. In a sense these systems are rather reminiscent of current fixed wireless systems such as microwave, millimeter-wave, or infrared radio links.

Figure 1.1: Heinrich Hertz (1857-1894) and James Clerk Maxwell (1831-1879), physicists who laid the foundation of electromagnetic theory and its wireless applications.

True radio communications were of course based on the work of Maxwell and the experiments of Hertz. The first use of radio to transmit coded information was proposed by Tesla in the 1880’s, and the first radio communication systems were described in his papers around 1891. Nearly simultaneously, Marconi patented the telegraph and demonstrated to the world the usefulness of mobile communications with ships crossing the English Channel. Interestingly the infancy of radio communications already emphasizes the importance of some important points:

1) certain radio frequencies overcome line-of-sight obstructions and weather impediments,

2) mobility is the main application,

3) patent protection is paramount.



Figure 1.2: Nicola Tesla (1856-1943) and Guglielmo Marconi (1874-1937), wireless visionaries, pioneers, inventors.

The next major advances in radio systems were developed during and after world war two, and benefitted from significant research around radar and remote sensing. Subsequently, different applications flourished: TV broadcasting in the 1940’s probably has the merit of introducing the first standardization of communications technology, leading to major television standards, (NTSC Color Standard in 1953, and recently ATSC Digital Standard in 2009). Standards have become very important in all aspects of wireless communications, and will be analyzed in more details further.

Cellular systems were devised by AT&T Bell Labs in the seventies. Continued improvements in standards and products provide increasing spectral efficiencies, lower prices, and wider consumer acceptance. Amazing growth occurred in the wireless industry during the 1980’s and 1990’s, which led to almost ubiquitous service availability and cheap service plans; some irrational exuberance in the industry also caused failures and bankruptcies around 2000 (such as excessive spectrum bidding, expensive satellite services, or some early broadband wireless initiatives).

Spectrum:

Spectrum is a very important notion for any wireless system: it refers to the range of frequencies used by the system’s electromagnetic waves; when several services use the same spectrum in the same location interferences occur that may be harmful to these services. Governments therefore step in and set rules and regulations for spectrum coordination.

Spectrum Allocation:



VLF = Very Low Frequency UHF = Ultra High Frequency

LF = Low Frequency SHF = Super High Frequency

MF = Medium Frequency EHF = Extra High Frequency

HF = High Frequency UV = Ultraviolet Light

VHF = Very High Frequency

Relationship between frequency ‘f’ and wave length ‘’ : = c/f

Where c is the speed of light 3x108m/s.

Licensed Spectrum

Different spectrum bands are made available for commercial use at different times. The

spectrum bands listed in this section are interesting for US activities, they have a very different history and different rules. Many of the detailed band plans are available at www.fcc.gov under auctions.

Cellular and PCS spectrum:

The first US cellular spectrum, at 800 MHz, was given to interested operators in 1982 and 1986 to encourage rolling out mobile wireless systems. With the booming success of these cellular systems, the FCC decided to auction more spectrum, at 1900 MHz, referred to as PCS spectrum (for Personal Communication Services).

Figure : Cellular band plan at 800 MHz: two 20-MHz blocks (A and B) allocated by the FCC in 1982, augmented in 1986 (A* and B*).

Figure : PCS band plan: the PCS band was auctioned by the FCC in 1994-1996, different block sizes combined with spectrum caps encouraged newcomers in the industry.



1.1 Evolution of Mobile Radio Communications :

The first wire line telephone system was introduced in the year 1877. Mobile com-munication systems as early as 1934 were based on Amplitude Modulation (AM) schemes and only certain public organizations maintained such systems. With the demand for newer and better mobile radio communication systems during the World War II and the development of Frequency Modulation (FM) technique by Edwin Armstrong, the mobile radio communication systems began to witness many new changes. Mobile telephone was introduced in the year 1946. However, during its initial three and a half decades it found very less market penetration owing to high costs and numerous technological drawbacks. But with the development of the cellular concept in the 1960s at the Bell Laboratories, mobile communications began to be a promising field of expanse which could serve wider populations. Initially, mobile communication was restricted to certain official users and the cellular concept was never even dreamt of being made commercially available. Moreover, even the growth in the cellular networks was very slow. However, with the development of

newer and better technologies starting from the 1970s and with the mobile users now connected to the Public Switched Telephone Network (PSTN), there has been an astronomical growth in the cellular radio and the personal communication systems. Advanced Mobile Phone System (AMPS) was the first U.S. cellular telephone system and it was deployed in 1983. Wireless services have since then been experiencing a 50% per year growth rate. The number of cellular telephone users grew from 25000 in 1984 to around 3 billion in the year 2007 and the demand rate is increasing day by day. A schematic of the subscribers is shown in Fig. 1.3.



Figure 1.3: The worldwide mobile subscriber chart.

Limitations of conventional mobile telephone systems

One of many reasons for developing a cellular mobile telephone system and deploying it in many cities is the operational limitations of conventional mobile telephone systems: limited service capability, poor service performance, and inefficient frequency spectrum utilization.

1. Limited service capability: A conventional mobile telephone system is usually designed by selecting one or more channels from a specific frequency allocation for use in autonomous geographic zones, as shown in Fig.1.4.The communications coverage area of each zone is normally planned to be as large as possible, which means that the transmitted power should be as high as the federal specification allows.

The user who starts a call in one zone has to reinitiate the call when moving into a new zone because the call will be dropped. This is an undesirable radio telephone system since there is no guarantee that a call can be completed without a handoff capability. The handoff is a process of automatically changing frequencies as the mobile unit moves into a different frequency zone so that the conversation can be continued in a new frequency zone without redialing. Another disadvantage of the conventional system is that the number of active users is limited to the number of channels assigned to a particular frequency zone.

2. Poor Service Performance: In the past, a total of 33 channels were all allocated to three mobile telephone systems: Mobile Telephone Service (MTS), Improved Mobile Telephone Service (IMTS) MJ systems, and Improved Mobile Telephone Service (IMTS) MK systems. MTS operates around 40 MHz and MJ operates at 150 MHs; both provide 11 channels; IMTS MK operates at 450 MHz and provides 12 channels.

These 33 channels must cover an area 50 mi in diameter. In 1976, New York City had 6 channels of( MJ serving 320 customers, with another 2400 customers on a waiting list. New York City also had 6 channels of MK serving 225 customers, with another 1300 customers on a waiting list. The large number of subscribers created a high blocking probability during busy hours. Although service performance was undesirable, the demand was still great. A high-capacity system for mobile telephones was needed.



Fig.1.4 Conventional Mobile System

3. Inefficient Frequency Spectrum Utilization: In a conventional mobile telephone system, the frequency utilization measurement Mo, is defined as the maximum number of customers that could be served by one channel at the busy hour.

Mo = Number of customers/channel: Mo = 53 for MJ, 37 for MK

The offered load can then be obtained by

A = Average calling time (minutes) x total customers / 60 min (Erlangs)

Assume average calling time = 1.76 min.

A1 = 1.76 * 53 * 6 / 60 = 9.33 Erlangs (MJ system)

A2 = 1.76 * 37 * 6 / 60 = 6.51 Erlangs (MK system)

If the number of channels is 6 and the offered loads are A1 = 9.33 and A2 = 6.51, then from the Erlang B model the blocking probabilities, B1 = 50 percent (MJ system) and B2 =30 percent (MK system), respectively. It is likely that half the initiating calls will be blocked in the MJ system, a very high blocking probability. As far as frequency spectrum utilization is concerned, the conventional system does not utilize the spectrum efficiently.Since each channel can only serve one customer at a time in a whole area. This is overcome by the new cellular system.

BASIC CELLULAR SYSTEMS:

A basic analog cellular system consists of three subsystems: a mobile unit, a cell site, and a mobile telephone switching office (MTSO), as Fig. 1.5 shows, with connections to link the three subsystems.

1. Mobile units. A mobile telephone unit contains a control unit, a transceiver, and an

antenna system.

2. Cell site. The cell site provides interface between the MTSO and the mobile units. It has a control unit, radio cabinets, antennas, a power plant, and data terminals.

3. MTSO. The switching office, the central coordinating element for all cell sites, con-tains the cellular processor and cellular switch. It interfaces with telephone company zone offices, controls call processing, provides operation and maintenance, and handles billing activities.

4. Connections. The radio and high-speed data links connect the three subsystems. Each mobile unit can only use one channel at a time for its communication link. But the channel is not fixed; it can be any one in the entire band assigned by the serving area, with each site having multichannel capabilities that can connect simultaneously to many mobile units.



The MTSO is the heart of the analog cellular mobile system. Its processor provides central coordination and cellular administration. The cellular switch, which can be either analog or digital, switches calls to connect mobile subscribers to other mobile subscribers and to the nationwide telephone network. It uses voice trunks similar to telephone company interoffice voice trunks. It also contains data links providing supervision links between the processor and the switch and between the cell sites and the processor. The radio link carries the voice and signaling between the mobile unit and the cell site. The high-speed data links cannot be transmitted over the standard telephone trunks and therefore must use either microwave links or T-carriers (wire lines). Microwave radio links or T-carriers carry both voice and data between cell site and the MTSO.

Figure 1.5 Cellular systems

First, second, third, and fourth generation cellular wireless systems (1G, 2G, 3G and 4G networks):

The "G" in wireless networks refers to the "generation" of the wireless network technology. Technically generations are defined as follows:

1G networks :( NMT, C-Nets, AMPS, and TACS) are considered to be the first analog cellular systems, which started early 1980s completed in early 1990s. There were radio telephone systems even before that. 1G networks were conceived and designed purely for voice calls with almost no consideration of data services. It provides a speed up to 2.4kbps and is based on analog system. It allows user to make call in one country, it has low capacity, unreliable handoff, poor voice links , and no security at all since voice calls were played back in radio towers, making these calls susceptible to unwanted eavesdropping by

third parties. Low capacity, unreliable handoff, poor voice links, and no security at all since voice calls were played back in radio towers, making these calls susceptible to unwanted eavesdropping by third parties. Ex: GSM Global System for Mobile Telecommunication.

2G networks : (GSM, CDMAOne, D-AMPS) are the first digital cellular systems launched early 1990s and completed in late 1990s, offering improved sound quality, better security and higher total capacity. GSM supports circuit-switched data (CSD), allowing users to place dial-up data calls digitally, so that the network's switching station receives actual ones and zeroes rather than the screech of an analog modem. 2G networks with theoretical data rates up to about 144kbps and provides a speed of up to 64 kbps.It provides services like voice and sms with more clarity.Major prominent technologies were GSM, CDMA, and IS95. Ex: GPRS Generalized Packet Radio Service, packet service for GSM (2G) networks.



2.5G technology: addition to GSM service.

EDGE Enhanced Data Rates for GSM Evolution; enhancement (data rates) of GPRS service (mainly software based, can be deployed in existing GPRS networks with software upgrades).

2.75G technology: Sometimes also seen as a 3G technology. EDGE is actually a step between GPRS and UMTS.UMTS Universal Mobile Telecommunication System.

3G networks: NTT DoCoMo launched the first commercial 3G network on 1 October 2001, using the WCDMA technology (UMTS FDD and TDD, CDMA2000 1x EVDO, CDMA2000 3x, TDSCDMA, Arib WCDMA, EDGE, IMT-2000 DECT) are newer cellular networks that have data rates of 384kbit/s and more. Bandwidth of 3G network is 128 Kbps for mobile stations, and 2 Mbps for fixed applications. The current trend in mobile systems is to support the high bit rate data services at the downlink via High Speed Downlink Packet Access (HSDPA).

3.5G technology: Enhancement of UMTS for higher speeds in Network-to-mobile direction. Mainly a software based improvement over plain UMTS. HSUPA High Speed Uplink Packet Access.

3.75G technology: Further enhancement (higher speeds in mobile-to-network direction) of UMTS and HSDPA service. LTE Long Term Evolution.

4G networks: 4G was developed in the year 2010 refers to the fourth generation of mobile phone communication standards. LTE and WiMAX are marketed as parts of this generation, even though they fall short of the actual standard. The ITI has taken ownership of 4G, bundling into a specification known as IMTAdvanced. The document calls for 4G technologies to deliver downlink speeds of 1Gbps when stationary and 100Mbps when mobile. It provides high performance like uploading and downloading speed and provides easy roaming as compared to 3G. Use of a higher Layer Protocol (IP) as transport medium affords intelligence at every stage within the network relative to a service. UMTS successor, competitor to WiMAX.

5G networks:

It is the next major phase of mobile telecommunication & wireless system. It is 10 times faster than 4G.It has an expected speed of 1gbps.Lower cost than the previous version.



Figure 1.6

UNIQUENESS OF MOBILE RADIO ENVIRONMENT :

Description of Mobile Radio Transmission Medium:

The Propagation Attenuation. In general, the propagation path loss increases not only with frequency but also with distance. If the antenna height at the cell site is 30 to 100 m and at the mobile unit about 3 m above the ground, and the distance between the cell site and the mobile unit is usually 2 km or more, then the incident angles of both the direct wave and the reflected wave are very small, as Fig. 1.7 shows. The incident angle of the direct wave is 91, and the incident angle of the reflected wave is 02. 01 is also called the elevation angle. The propagation path loss would be 40 dB/dec, 4 where "dec" is an abbreviation of decade, i.e., a period of 10. This means that a 40-dB loss at a signal receiver will be observed by the mobile unit as it moves from 1 to 10 km. Therefore C is inversely proportional to R.

C R—4 = aR—4 (1.3-1)

Where C = received carrier power, a = constant

R = distance measured from the transmitter to the receiver

Model of Transmission Medium:

A mobile radio signal r (t), illustrated in Fig. 1.8, can be artificially characterized5 by two components m (t) and r0 (t) based on natural physical phenomena.

r (t) = m(t )ro(t)

FIGURE 1.7 Mobile radio transmission model.

The component m(t) is called local mean, long-term fading, or lognormal fading and its variation is due to the terrain contour between the base station and the mobile unit. The factor r0 is called multipath fading, short-term fading, or Rayleigh fading and its variation is due to the waves reflected from the surrounding buildings and other structures.



FIGURE 1.8 A mobile radio signal fading representation. (a) A mobile signal fading.

(b) A short-term signal fading.

Mobile Fading Characteristics:

Rayleigh fading is also called multipath fading in the mobile radio environment. When these multipath waves bounce back and forth due to the buildings and houses, they form many standing-wave pairs in space, as shown in Fig. 1.8. Those standing-wave pairs are summed together and become an irregular wave-fading structure. When a mobile unit is standing still, its receiver only receives a signal strength at that spot, so a constant signal is observed. When the mobile unit is moving, the fading structure of the wave in the space is received. It is a multipath fading. The recorded fading becomes fast as the vehicle moves faster.

The Radius of the Active Scattered Region. The mobile radio multipath fading shown in Fig. 1.8 explains the fading mechanism. The radius of the active scattered region at 850 MHz can be obtained indirectly as shown. The radius is roughly 100 wavelengths. The active scattered region always moves with the mobile unit as its center. It means that some houses were inactive scatterers and became active as the mobile unit approached them; some houses were active scatterers and became inactive as the mobile unit drove away from them.

Signal path loss basics: The signal path loss is essentially the reduction in power density of an electromagnetic wave or signal as it propagates through the environment in which it is travelling.

There are many reasons for the radio path loss that may occur:

• Free space loss: The free space loss occurs as the signal travels through space without any other effects attenuating the signal it will still diminish as it spreads out. This can be thought of as the radio communications signal spreading out as an ever increasing sphere. As the signal has to cover a wider area, conservation of energy tells us that the energy in any given area will reduce as the area covered becomes larger.

• Absorption losses: Absorption losses occur if the radio signal passes into a medium which is not totally transparent to radio signals. This can be likened to a light signal passing through transparent glass.

• Diffraction: Diffraction losses occur when an object appears in the path. The signal can diffract around the object, but losses occur. The loss is higher the more rounded the object. Radio signals tend to diffract better around sharp edges.



• Multipath: In a real terrestrial environment, signals will be reflected and they will reach the receiver via a number of different paths. These signals may add or subtract from each other depending upon the relative phases of the signals. If the receiver is moved the scenario will change and the overall received signal will be found vary with position. Mobile receivers (e.g. cellular telecommunications phones) will be subject to this effect which is known as Rayleigh fading.

• Terrain: The terrain over which signals travel will have a significant effect on the signal. Obviously hills which obstruct the path will considerably attenuate the signal, often making reception impossible. Additionally at low frequencies the composition of the earth will have a marked effect. For example on the Long Wave band, it is found that signals travel best over more conductive terrain, e.g. sea paths or over areas that are marshy or damp. Dry sandy terrain gives higher levels of attenuation.

• Buildings and vegetation: For mobile applications, buildings and other obstructions including vegetation have a marked effect. Not only will buildings reflect radio signals, they will also absorb them. Cellular communications are often significantly impaired within buildings. Trees and foliage can attenuate radio signals, particularly when wet.

• Atmosphere: The atmosphere can affect radio signal paths. At lower frequencies, especially below 30 - 50MHz, the ionosphere has a significant effect, reflecting (or more correctly refracting) them back to Earth. At frequencies above 50 MHz and more the troposphere has a major effect, refracting the signals back to earth as a result of changing refractive index. For UHF broadcast this can extend coverage to approximately a third beyond the horizon.

FREE-SPACE PATH LOSS (FSPL):

In Mobile communication, free-space path loss (FSPL) is the loss in signal strength of an electromagnetic wave that would result from a line-of-sight path through free space (usually air), with no obstacles nearby to cause reflection or diffraction. It does not include factors such as the gain of the antennas used at the transmitter and receiver, nor any loss associated with hardware imperfections. A discussion of these losses may be found in the article on link budget.

Fig 1.9

Free-space path loss formula:

Free-space path loss is proportional to the square of the distance between the transmitter and receiver, and also proportional to the square of the frequency of the radio signal. For any type of wireless communication the signal disperses with distance. Therefore, an antenna with a fixed area will receive less signal power the farther it is from the transmitting antenna.



For satellite communication this is the primary mode of signal loss. Even if no other sources of attenuation or impairment are assumed, a transmitted signal attenuates over distance because the signal is being spread over a larger and larger area. This form of attenuation is known as free space loss, which can be express in terms of the ratio of the radiated power to the power received by the antenna or, in decibels, by taking 10 times the log of that ratio. For the ideal isotropic antenna, free space loss is

[Equation 1]

Where: λ is the signal wavelength (in metres), f is the signal frequency (in hertz),

d is the distance from the transmitter (in metres),c is the speed of light in a vacuum, 2.99792458 × 108 metres per second,

Gt and Gr are the transmit and receive antenna gains and are dimensionless quantities, Pt is the transmitted power, Pr is the received power,Pt and Pr are in same units.

Derivation of Friis Transmission Formula :

To begin the derivation of the Friis Equation, consider two antennas in free space (no obstructions nearby) separated by a distance R:

Figure 1.10. Transmit (Tx) and Receive (Rx) Antennas separated by R.

Assume that Watts of total power are delivered to the transmit antenna. For the moment, assume that the transmit antenna is omnidirectional, lossless, and that the receive antenna is in the far field of the transmit antenna. Then the power density p (in Watts per square meter) of the plane wave incident on the receive antenna a distance R from the transmit antenna is given by:

[Equation 2]

If the transmit antenna has an antenna gain in the direction of the receive antenna given

by , then the power density equation above becomes:

[Equation 3]



The gain term factors in the directionality and losses of a real antenna. Assume now that

the receive antenna has an effective aperture given by . Then the power received by this

antenna ( ) is given by:

[Equation 4]

Since the effective aperture for any antenna can also be expressed as:

[Equation 5]

The resulting received power can be written as:

[Equation 6]

This is known as the Friis Transmission Formula. It relates the free space path loss, antenna gains and wavelength to the received and transmit powers. This is one of the fundamental equations in antenna theory, and should be remembered (as well as the derivation above).

Another useful form of the Friis Transmission Equation is given in Equation [7]. Since wavelength and frequency f are related by the speed of light c (see intro to frequency page), we have the Friis Transmission Formula in terms of frequency:

[Equation 7]

Free-space path loss in decibels

A convenient way to express FSPL is in terms of dB:

[Equation 8]

LdB =



=

=

For other antennas, we must take into account the gain of the antenna, which yields the following free space loss equation:

[Equation 9]

At = effective area of transmitting antenna

Ar = effective area of receiving antenna

We can recast the loss equation as:

LdB =20 log (λ) + 20 log (d)-10 log (AtAr)

= -20 log (f) + 20 log (d)-10 log (AtAr) +169.54 dB

Time dispersion parameters:

Power delay profile P(_): the channel power spectral density as a function of delay, i.e. how “channel power” is distributed along dimension excess delay

When a signal is transmitted, this signal can suffer a distortion caused by reflections and scattered propagation paths in the radio channel, and these phenomenon’s cause that an identical signal arrives at different times at its destination. These different times are due that to the signal arrives via multiple paths and in different incident angles. The time difference between the arrival moment of the first multipath component and the last one is called delay spread.

In order to compare different multipath channels and to develop some general design guidelines for wireless systems, some parameters are used to quantify the multipath channel. Some of these multipath parameters are the mean excess delay, rms delay spread, and maximum excess delay, and can be determined from a power delay profile. However, the mean excess delay and the rms delay spread are frequently used to quantify the time dispersive properties of wide band multipath channels.

The Mean Excess Delay is the first moment of the power delay profile (PDP) and is defined as the first moment of power delay profile



RMS Delay Spread:

The root-mean-square (RMS) delay spread is probably the most important single measure for the delay time extent of a multipath radio channel. This parameter calculates the standard deviation value of the delay of reflections, weighted proportional to the energy in the reflected waves. This parameter can be considered like the square root of the second central moment of the power delay profile and is defined by

We must take into consideration that these delay are measured relative to the first detectable signal arriving at the receiver at = 0, and their equations do not rely on the absolute power level of P(),but only the relative amplitudes of the multipath components within P().

RMS delay spread →

Where

where Avg( τ2) is the same computation as above as used for except that

A simple way to explain this is “the range of time within which most of the delayed signals arrive”

Maximum Excess Delay (X dB) :

The maximum excess delay (X dB) of the power delay profile is defined as the time delay value after which the multipath energy falls to X dB below the maximum multipath energy (not necessarily belonging to the first arriving component). It is also called excess delay spread, but in all cases must be specified with a threshold that relates the multipath noise floor to the maximum received multipath component.

The values of these time dispersion parameters also depend on the noise threshold used to process P(), and if this noise is set too low, then the noise will be processed as multipath and thus causing the parameters to be higher.

Coherence Bandwidth

Coherence bandwidth is a statistical measure of the range of frequencies over which the channel can be considered "flat".

Two sinusoids with frequency separation greater than are affected quite differently by the channel. If the coherent bandwidth is defined as the bandwidth over which the frequency correlation function is above 0.9, then the coherent bandwidth is approximately

If we define Coherence Bandwidth as the range of frequencies over which the frequency correlation is above 0.5, then The coherence bandwidth of the channel gives a good indication about the frequency variations of the channel in relation to the bandwidth of the transmitted signal. We can have two different cases, depending on this bandwidth. If a signal with a bandwidth larger than Bc is transmitted through the channel, it will be subject to frequency selective distortion.



The channel will be, in this case, referred to as a frequency selective fading channel. However, if the signal transmitted has a bandwidth considerably less than Bc, it will experience amplitude attenuation only with no distortion since the channel characteristics will be the same all over the spectrum of the signal. In this case the channel is referred to as a frequency non-selective (flat) fading channel.

Range of freq over Which response is flat

Figure 1.11

)(tx

Time domain view

B delay spread

)( fX

Freq. domain view



Frequency Dispersion Parameters:

Doppler Spread and Coherent Time:

Doppler spread and coherent time are parameters which describe the time varying nature of the channel in a small-scale region. When a pure sinusoidal tone of fc is transmitted, the received signal spectrum, called the Doppler spectrum, will have components in the range

fc- fd and fc+ fd, where fd is the Doppler shift.

fd is a function of the relative velocity of the mobile, and the angle between the direction of motion of the mobile and direction of arrival of the scattered waves

Figure 1.12

Coherent time is the time domain dual of Doppler spread. Coherent time is used to characterize the time varying nature of the frequency depressiveness of the channel in the time domain.

fm: maximum Doppler shift given by fm =v /λ

v : speed of the mobile , λ : speed of the light

Two signals arriving with a time separation greater than Tc are affected differently by the channel. A statistic measure of the time duration over which the channel impulse response is essentially invariant. If the coherent time is defined as the time over which the time correlation function is above 0.5, then



The Cellular Concept:

The design aim of early mobile wireless communication systems was to get a huge coverage area with a single, high-power transmitter and an antenna installed on a giant tower, transmitting a data on a single frequency. Although this method accomplished a good coverage, but it also means that it was practically not possible to reuse the same frequency all over the system, because any effort to reuse the same frequency would result in interference.

The cellular concept was a major breakthrough in order to solve the problems of limited user capacity and spectral congestion. Cellular system provides high capacity with a limited frequency spectrum without making any major technological changes. It is a system-level idea in which a single high-power transmitter is replaced with multiple low-power transmitters, and small segment of the service area is being covered by each transmitter, which is referred to as a cell. Each base station (transmitter) is allocated a part of the total number of channels present in the whole system, and different groups of radio channels are allocated to the neighboring base stations so that all the channels present in the system

are allocated to a moderately small number of neighboring base stations.

Cells using the same set of Radio channels Fig. 1.13: Cellular Network

Frequency Reuse:

Conventional communication systems faced the problems of limited service area capability and ineffective radio spectrum utilization. This is because these systems are generally designed to provide service in an autonomous geographic region and by selecting radio channels from a particular frequency band. On the other hand, the present mobile communication systems are designed to offer a wide coverage area and high grade of service. These systems are also expected to provide a continuous communication through an efficient utilization of available radio spectrum. Therefore, the design of mobile radio network must satisfy the following objectives i.e., providing continuous service, and wide service area, while efficiently using the radio spectrum.

In order to achieve these objectives, the present mobile systems use cellular networks which depend more on an intelligent channel allocation and reuse of channels throughout the region. Each base station is allocated a set of radio channels, which are to be used in a geographic area called a cell. Base stations in the neighboring cells are allocated radio channel sets, which are entirely different. The antennas of base station antennas are designed to get the required coverage within the specific cell.



By restricting the coverage area of a base station to within the cell boundaries, the same set of radio channels can be used in the different cells that are separated from each other by distances which are large enough in order to maintain interference levels within limits. The procedure of radio sets selection and allocation to all the base stations present within a network is called frequency reuse; Fig. 1.13 shows the frequency reuse concept in a cell in a cellular network, in which cells utilize the same set of radio channels. The frequency reuse plan indicates where different radio channels are used. The hexagonal shape of cell is purely theoretical and is a simple model of radio coverage for each base station, although it has been globally adopted as the hexagon permits the easy analysis of a cellular system.

Channel Reuse Schemes:

The radio channel reuse model can be used in the time and space domain. Channel reuse in the time domain turns out to be occupation of same frequency in different time slots and is also called Time Division Multiplexing. Channel reuse in the space domain is categorized

into:

a) Same channel is allocated in two different areas, e.g. AM and FM radio stations using same channels in two different cities.

b) Same channel is frequently used in same area and in one system the scheme used is cellular systems. The entire spectrum is then divided into K reuse sets.

Locating Co-channel Cells in a Cellular Network:

Cells, which use the same set of channels, are called co-channels cells. For determining the location of co-channel cell present in the neighborhood, two shift parameters i and j are used where i and j are separated by 600, as shown in Fig. 2.3 below. The shift parameters can have any value 0, 1, 2...., n.

To find the location of nearest co-channel cell, mark the center of the cell as (0, 0) for which co-channel cells are to be located. Define the unit distance as the distance of center of two adjacent cells, and follow the two steps given below:

Step 1: Move i number of cells along i axis

Step 2: Turn 600 anti-clockwise and move j number of cells The technique of locating co-channel cells using the preceding procedure is shown in Fig. 2.4 for i=3 and j=2. The shift parameters i and j measures the number of neighboring cells between co-channel cells.

Fig. 1.15: Shift Parameters i and j in Hexagonal Network



Fig. 1.14: K-Cell Reuse Pattern

Fig. 1.16: Locating Co-channel Cells when i=3 & j=2



The relationship between cluster size K and shift parameters i & j, is given below:

Let 'R' be the distance between the center of a regular hexagon to any of its vertex. A regular hexagon is one whose all sides are also of equal length i.e. 'R'. Let 'd' be the distance between the centre of two neighboring hexagons, and following steps are followed while calculating the size of a cluster ‘K’.

Step 1: To show that d = 3R

P Q

O A B Fig. 1.17: Distance Between two adjacent cells

From the geometry of the Fig. 2.5, OA = R and AB = R/2 (1.1)

Then, OB = OA + AB = R + R/2 = 3R/2 (1.2)

Then, in right-angled OAP

OP = OA sin 600 = (1.3)

Let the distance between the centers of two neighboring hexagonal cells, OQ, be

denoted by ‘d’, then,

OQ = OP + PQ (where OP = PQ)

Therefore,

Hence, d =

(1.4)

Step 2: Area of a small hexagon, Asmall hexagon W

The area of a hexagonal cell with radius R is given as

Asmall hexagon =

(1.5)

Step 3: To find the relation between D, d and shift parameters

Let ‘D’ be the distance between the center of a particular cell under consideration to the center of the nearest co-channel cell.



Fig. 1.18: Relationship between K and Shift Parameters (i & j)

Using cosine formula XYZ in Fig. 1.18, we have

(1.6)

Step 4: To find the area of a large hexagon, Alarge hexagon

By joining the centers of the six nearest neighboring co-channel cells, a large hexagon is

formed with radius equal to D, which is also the co-channel cell separation. Refer Fig. 1.18

The area of the large hexagon having a radius D can be given as

Alarge hexagon = (1.7)

Using equation 1.6 Alarge hexagon = (1.8)



Fig. 1.19

Step 7: To find the number of cells in the large hexagon (L)

Number of cells in large hexagon L = Alarge hexagon / Asmall hexagon (1.9)

Using equations 1.7&1.8we get

L = 3 x (i2 + j2 + i x j) (1.10)

Step 8: Find the correlation between L and cluster size K

It can be seen from Fig. 1.19, that the larger hexagon is created by joining the centers of co-channel cells present in the first tier contains 7 cells of the central cluster plus 1/3rd of the number of 7 cells of all the neighboring six clusters. Therefore, it can be calculated that the larger hexagon consisting of the central cluster of K cells plus 1/3rd the number of the cells connected with six neighboring clusters present in the first tier.

Fig. 1.20: Number of Clusters in the First Tier for N=7

Hence, the total number of cells enclosed by the larger hexagon is L = K + 6 x [(1 / 3) x K]

L = 3 x K (1.11)



Step 9: To establish relation between K and shift parameters from equation 1.11 and 1.10, we get

3 x K = 3 x (i2 + j2 + i x j)

K = (i2 + j2 + i x j) (1.12)

The Table 1.1 shows the frequency reuse patterns along with the cluster sizes

Table 1.1: Frequency Reuse Pattern and Cluster Size

Frequency Reuse Pattern Cluster Size (I, j) K = (i2 + j2 + i j)

(1, 1) 3

(2, 0) 4

(2, 1) 7

(3, 0) 9

(2, 2) 12

(3, 1) 13

(4, 0) 16

(2, 3) 19

(4, 1) 21

(5, 0) 25

Frequency Reuse Distance

To reuse the same set of radio channels in another cell, it must be separated by a distance called frequency reuse distance, which is generally represented by D. Reusing the same frequency channel in different cells is restricted by co-channel interference between cells. So, it is necessary to find the minimum frequency reuse distance D in order to minimize the

co-channel interference. Fig. 1.21 illustrates the separation of cells by frequency reuse distance in a cluster of 7 cells. In order to derive a formula to compute D, necessary properties of regular hexagon cell geometry are first discussed.



Fig. 1.21

The frequency reuse distance (D), which allows the same radio channel to be reused in co-channel cells, depends on many factors:

the number of co-channel cells in the neighborhood of the central cell

the type of geographical terrain

the antenna height

the transmitted signal strength by each cell-site

Suppose the size of all the cells in a cellular is approximately same, and it is usually calculated by the coverage area of the proper signal strength in every cell. The co-channel interference does not depend on transmitted power of each, if the cell size is fixed, i.e., the threshold level of received signal at the mobile unit is tuned to the size of the cell.

The co-channel interference depends upon the frequency reuse ratio, q, and is defined as

q = D / R (1.13)

Where D is the distance between the two neighboring co-channel cells, and R is the radius of the cells. The parameter q is also referred to as the frequency reuse ratio or co- channel reuse ratio. The following steps are used to find the relationship between frequency reuse ratio q and cluster size K

Fig. 1.22 shows an array of regular hexagonal cells, where R is the cell radius. Due to the hexagonal geometry each hexagon has exactly six equidistant neighbors.

7 2

6 1 3

5 4 d

Fig. 1.22: Distance between Two Adjacent Cells (d)



Let d be the distance between two cell centers of neighboring cells. Therefore, d = 3R

The relationship between D, d, and shift parameters is

D2 = 3 * R2 *(i2 + j2 + i X j)

As K = i2 + j2 + i * j

D2 = 3 * R2 * K

D2 = 3 * K

R2

(1.14)

As q = D/R q = 3K

Thus, the frequency reuse ratio q can be computed from the cluster size K. Table 1.2 shows the frequency reuse ratios for different cluster sizes, K

As the D/R measurement is a ratio, if the cell radius is decreased, then the distance between co-channels cells must also be decreased by the same amount, for keeping co-channel interference reduction factor same. On the other hand, if a cell has a large radius, then the distance between frequency reusing cells must be increased proportionally in order to have the same D/R ratio.

As frequency reuse ratio (q) increases with the increase in cluster size (K), the smaller value of K largely increase the capacity of the cellular system. But it will also increase the co-channel interference. Therefore, the particular value of q (or K) is selected in order to keep the signal-to-cochannel interference ratio at an acceptable level. If all the antennas transmit the same power, then with the increase in K, the frequency reuse distance (D) increases, and reduce the likelihood that co-channel interference may occur. Therefore, the challenge is to get the optimal value of K so that the desired system performance can be achieved in terms of increased system capacity, efficient radio spectrum utilization and signal quality.

Cluster Size Frequency Reuse Ratio

K q =√3K

3 3.00

4 3.46

7 4.58

9 5.20

12 6.00

13 6.24

19 7.55

21 7.94

27 9.00

Table 1.2: Frequency Reuse Ratio and Cluster Size



COCHANNEL INTERFERENCE:

The frequency-reuse method is useful for increasing the efficiency of spectrum usage but results in cochannel interference because the same frequency channel is used repeatedly in different cochannel cells.

Higher values of “q”

Reduces co-channel interference,

Leads to higher value of “N” more cells/cluster,

Less number of channels/cells,

Less traffic handling capacity.

Lower values of “q”

Increases co-channel interference,

Leads to lower value of “N” fewer cells / cluster,

More number of channels / cells,

More traffic handling capacity.

COCHANNEL INTERFERENCE REDUCTION FACTOR :

Reusing an identical frequency channel in different cells is limited by cochannel interfer-ence between cells, and the cochannel interference can become a major problem. Here we would like to find the minimum frequency reuse distance in order to reduce this cochannel interference.

Assume that the size of all cells is roughly the same. The cell size is determined by the coverage area of the signal strength in each cell. As long as the cell size is fixed, cochannel interference is independent of the transmitted power of each cell. It means that the received threshold level at the mobile unit is adjusted to the size of the cell. Actually, cochannel interference is a function of a parameter q defined as

q = D (1.15)

R

The parameter q is the cochannel interference reduction factor. When the ratio q increases, cochannel interference decreases. Furthermore, the separation D in Eq. (1.15) is a function of K I and C/I ,

D = f (K I, C/I) (1.16)

where K I is the number of cochannel interfering cells in the first tier and C/I is the received carrier-to-interference ratio at the desired mobile receiver.3

(1.17)

In a fully equipped hexagonal-shaped cellular system, there are always six cochannel inter-fering cells in the first tier, as shown in Fig. 1.23; that is, K I = 6. The maximum number of KI in the first tier can be shown as six (i.e., 2π D/ D ≈ 6).



Cochannel interference can be experienced both at the cell site and at mobile units in the center cell. If the interference is much greater, then the carrier-to-interference ratio C/I at the mobile units caused by the six interfering sites is (on the average) the same as the C/I received at the center cell site caused by interfering mobile units in the six cells. According to both the reciprocity theorem and the statistical summation of radio propagation, the two C/I values can be very close. Assume that the local noise is much less than the interference

level and can be neglected. C/I then can be expressed, from Eq. ), as

(1.18)

Where γ is a propagation path-loss slope5 determined by the actual terrain environment. In a mobile radio medium, γ usually is assumed to be 4. K I is the number of cochannel interfering cells and is equal to 6 in a fully developed system, as shown in Fig.1.23. The six cochannel interfering cells in the second tier cause weaker interference than those in the first tier.

FIGURE 1.23 Six effective interfering cells of cell 1

Therefore, the cochannel interference from the second tier of interfering cells is negli-gible. Substituting Eq. (1.15) into Eq. (1.16) yields

(1.19)

where qk is the cochannel interference reduction factor with kth cochannel interfering cell

(1.20)



DESIRED C/I FROM A NORMAL CASE IN A OMNI DIRECTION ANTENNA SYSTEM:

ANALYTIC SOLUTION: There are two cases to be considered:

(1) The signal and cochannel interference received by the mobile unit and

(2) The signal and cochannel interference received by the cell site.

Both the cases are shown in Figure 1.24. Nm, and Nb are the local noises at the mobile unit and the cell site, respectively. Usually, Nm and Nb are small and can be neglected as compared with the interference level. The system is called a balanced system as long as the received CIRs at both the mobile unit and the cell site are the same. In order to analyze the system requirement, either of the cases can be chosen in a balanced system.

FIGURE 1.24 Cochannel interference from six interferers, (a) Receiving at the cell site; (b) receiving at the mobile unit

In a balanced system, we can choose either one of the two cases to analyze the system requirement; the results from one case are the same for the others.

Assume that all Dk are the same for simplicity, as shown in Fig. 1.24; then D = Dk , and q = qk , and

(1.21)

(1.22)

(1.23)

In Eq. (1.23), the value of C/I is based on the required system performance and the specified value of γ is based on the terrain environment. With given values of C/I and γ , the cochannel interference reduction factor q can be determined. Normal cellular practice is to specify C/I to be 18 dB or higher based on subjective tests and the criterion described in previous Sec. Because a C/I of 18 dB is measured by the acceptance of voice quality from present cellular mobile receivers, this acceptance implies that both mobile radio multipath fading and cochannel interference become ineffective at that level. The path-loss slope γ is equal to about 4 in a mobile radio environment.

q = D/ R = (6 × 63.1)1/4 = 4.41 (1.24)



The 90th percentile of the total covered area would be achieved by increasing the transmitted power at each cell; increasing the same amount of transmitted power in each cell does not affect the result of Eq. (1.24). This is because q is not a function of transmitted power. The computer simulation described in the next section finds the value of q = 4.6, which is very close to Eq. (1.24). The factor q can be related to the finite set of cells K in a hexagonal-shaped cellular system by

(1.25)

Substituting q from Eq. (1.18) into Eq. (1.19) yields

K = 7 (1.26)

Equation (1.26) indicates that a seven-cell reuse pattern* is needed for a C/I of 18 dB. The seven-cell reuse pattern is shown in Fig. 1.14.

Based on q = D/ R, the determination of D can be reached by choosing a radius R in Eq. (1.24). Usually, a value of q greater than that shown in Eq. (1.24) would be desirable. The greater the value of q, the lower the cochannel interference. In a real environment, Eq. (1.19) is always true, but Eq. (1.21) is not. Because Eq. (1.24) is derived from Eq. (1.21), the value q may not be large enough to maintain a carrier-to-interference ratio of 18 dB.

Co-channel Interference and System Capacity:

The channel reuse approach is very useful for increasing the efficiency of radio spectrum

utilization but it results in co-channel interference because the same radio channel is

repeatedly used in different co-channel cells in a network. In this case, the quality of a

received signal is very much affected both by the amount of radio coverage area and the co-

channel interference.

Co-channel interference takes place when two or more transmitters located within a

wireless system, or even a neighboring wireless system, which are transmitting on the same

radio channel. Co-channel interference happens when the same carrier frequency (base

station) reaches the same receiver (mobile phone) from two different transmitters.

This type of interference is generally generated because channel sets have been allocated to

two different cells that are nor far enough geographically, and their signals are strong

enough to cause interference to each other. Thus, co-channel interference can either modify

the receiver or mask the particular signal. It may also merge with the particular signal to

cause severe distortions in the output signal.

The co-channel interference can be evaluated by picking any particular channel and

transmitting data on that channel at all co-channel sites. In a cellular system with

hexagonal shaped cells, there are six co-channel interfering cells in the first tier. Fig. 1.25

shows a Test 1 which is set-up to calculate the co-channel interference at the mobile unit,

in this test mobile unit is not stationary but is continuously moving in its serving cell.



First tier

Interfering

Mobile Cells

Serving

Cell

Fig. 1.25: Co-channel Interference Measurement at the Mobile Unit

In a small cell system, interference will be the major dominating factor and thermal noise can be neglected. Thus the S/I can also be written as:

Where S/I = Signal to interference ratio at the desired mobile receiver, S = desired signal

power, I = Interference power, 2 5 is the propagation path-loss slope and depends on

the terrain environment. If we assume, for simplification, that Dk is the same for the six interfering cells, i.e., D = Dk, then the formula above becomes:

(1.27)

(1.28) For analog systems using frequency modulation, normal cellular practice is to specify an S/I ratio to be 18 dB or higher based on subjective tests. An S/I of 18 dB is the measured value for the accepted voice quality from the present-day cellular mobile receivers.

Using an S/I ratio equal to 18dB (101810 63.1 ) and =4 in the Eq. (1.28), then

q 6 63.10.25 4.41 (1.28)



Substituting q from Eq. (1.28) into Eq. (1.11) yields

N (4.41)2

6.49 7 . (1.29) 3

Eq. (1.29) indicates that a 7-cell reuse pattern is needed for an S/I ratio of 18 dB. Therefore, the performance of interference-limited cellular mobile system can be calculated

from the following results.

a) If the signal-to-interference ratio (S/I) is greater than 18 dB, then the system is said to

be correctly designed.

b) If S/I is less than 18 dB and signal-to-noise ratio (S/N) is greater than 18 dB, then the

system is said to be experiencing with a co-channel interference problem.

c) If both S/I and S/N are less than 18 dB and S/I is approximately same as S/N in a cell,

then the system has a radio coverage problem.

d) If both S/I and S/N are less than 18 dB and S/I is less than S/N, the system has both

co-channel interference and radio coverage problem.

Therefore, the reciprocity theorem can be used to study the radio coverage problem, but it

does not give accurate results when used for the study of co-channel interference problem.

Therefore, it is suggested to perform Test 2 in order to measure co-channel interference at

the cell-site. In Test 2 shown in Fig. 1.26, both the mobile unit present in the serving cell

and six other mobile units present in the neighboring cells are transmitting simultaneously

at the same channel.

First tier

Interfering Cells

Fig. 1.26: Co-channel Interference Measurement at the Cell-site



Co-channel Interference Reduction Methods:

Interference is major factor affecting the performance of cellular communication systems. Sources of interference may consist of a different mobile working in the same or in the neighboring cells, which are operating in the same frequency band that may leak energy into the cellular band.

The co-channel interference can be reduced by the following methods:

a. Increasing the distance(D) between two co-channel cells, D As D increases, the strength of interfering signal from co-channel interfering cells decreases significantly. But it is not wise to increase D because as D is increased, K must also be increased. High value of K means fewer number of radio channels are available per cell for a given spectrum. This results into decrease of the system capacity in terms of channels that are available per cell.

b. Reducing the antenna heights

Reducing antenna height is a good method to minimize the co-channel interference in some environment, e.g., on a high hill. In the cellular system design effective antenna height is considered rather than the actual antenna height. Therefore, the effective antenna height changes according to the present location of the mobile unit in such a difficult terrain.

When the antenna is put up on top of the hill, the effective antenna height gets more than the actual antenna height. So, in order to minimize the co-channel interference, antenna with lower height should be used without decreasing the received signal strength either at the cell-site or at the mobile device. Similarly, lower antenna height in a valley is very useful in minimizing the radiated power in a far-off high-elevation area where the mobile user is believed to be present.

However, reducing the antenna height does not always minimize the co-channel interference, e.g., in forests, the larger antenna height clears the tops of the longest trees in the surrounding area, particularly when they are located very close to the antenna. But reducing the antenna height would not be appropriate for minimizing co-channel interference because unnecessary attenuation of the signal would occur in the vicinity of the antenna as well as in the cell boundary if the height of the antenna is below the treetop level.

c. Using directional antennas.

The use of directional antennas in every cell can minimize the co-channel interference if the co-channel interference cannot be avoided by a fixed division of co-channel cells. This will also improve the system capacity even if the traffic increases. The co-channel interference can be further minimized by smartly setting up the directional antenna.

d. Use of diversity schemes at the receiver.

The diversity scheme used at the receiving end of the antenna is an efficient technique for

minimizing the co-channel interference because any unwanted action performed at the

receiving end to increase the signal interference would not cause further interference. For

example, the division of two receiving antennas installed at the cell-site meeting the

condition of h/s=11, (where h is the antenna height and s is the division between two

antennas), would produce the correlation coefficient of 0.7 for a two-branch diversity

system. The two correlated signals can be combined with the use of selective combiner. The

mobile transmitter could suffer up to 7 dB minimization in power and the same

performance at the cell-site can be achieved as a non-diversity receiver. Therefore,

interference from the mobile transmitters to the receivers can be significantly reduced.



Trunking and Grade of Service (GoS) :

Trunking is the concept that allows large number of users to use a smaller number of channels (or phone lines, customer service representatives, parking spots, public bathrooms …) as efficiently as possible. It is clear that Trunking is based on statistics.

• The number of available channels in a trunked system is directly related to the probability of call blocking during peak time. In some systems, because of high system demand, calls that cannot be initiated are

o Blocked (caller will have to make the call later with not priority at all). Such systems are

sometimes called Blocked Calls Cleared systems.

o Queued (call is placed in a queue for several seconds until a free channel becomes

available). Such systems are sometimes called Blocked Calls Queued systems.

• Trunking and Queuing theories were first studied by a mathematician called Erlang

What is an Erlang?

One Erlang is defined as the amount of traffic intensity carrier by a channel that is completely occupied

Therefore,1 Erlang = 1 call with a duration of 1 hour over a channel every hour

= 2 calls with a duration of 0.5 hours over the channel every hour

= 30 calls with a duration of 4 minutes over the channel every 2 hours (120 minutes)

A channel that carries 2 calls of duration 5 minutes each per hour carries (2*5 min/60 min = 1/6 Erlangs)

Grade of Service (GOS)

The grade of service (GOS) is related to the ability of a mobile phone to access the trunked mobile phone system during the busiest hour. To meet a specific GOS, the maximum required capacity of the system must be estimated and the proper number of channels must be allocated for the system

• GOS is a measure of the congestion of the system which is specified as the probability of a call being blocked (Erlang B system) or the probability of a call being delayed beyond a certain amount of time (Erlang C system).

Traffic Intensity

Each user in a trunked system generates a Traffic Intensity per User of U A Erlangs given by

AU = λ ⋅H

where λ = average number of call request per unit time (Request Rate), and H = average duration of a call (Holding Time).

For a system with U users, total offered traffic intensity A is (Offered Traffic Intensity)

U A =U ⋅ A =U ⋅λ ⋅H



In a trunked system with C channels with traffic that is equally distributed among them, Traffic Intensity per Channel C A is given by

When offered traffic intensity ( A) > Maximum capacity of system � carrier traffic becomes limited due to limited capacity of the system.

To study the traffic capacity of a trunked system, we will assume the following three assumptions:

A) There are memoryless arrivals of call requests: all users including users who had blocked called may request a channel at any time. Also, because a user has just had a call blocked, does not affect his decision in making another call or the time to make that other call.

B) The probability of a user occupying a channel is exponentially distributed. So, longer calls have lower probability.

C) There are a finite number of channels available in for trunking.

Based on these assumptions, it is found that the probability of a call getting blocked in an Erlang B system is

and the probability of a call getting delayed for any period of time greater than zero is

The probability of a call getting delayed for a period of time greater than some T is



= GOS (Erlang C for a delay of length or longer)

Pr Delay > Pr Delay > 0 GOS (Erlang C for a delay of length or longer)

The average delay in this case is

Pr[Delay > 0] Avg

The following plots are for Pr[Blocking] in an Erlang B system and the Pr[Delay > 0] in an Erlang C system for different number of trunked channels (C). These figures can be used to simplify the computations in many problems related to system capacity and GOS.

The capacity of a trunked radio system in which blocked calls are lost is shown in Table.



Improving Capacity in Cellular Systems:

With the rise in the demand for wireless services, the number of radio channels allocated to each cell could become inadequate in order to satisfy this increase in the demand. Therefore, to increase the capacity (i.e. a cellular system can take up more calls) of a cellular system, it is very important to allocate more number of radio channels to each cell in order to meet the requirements of mobile traffic. Various techniques that are proposed for increasing the capacity of a cellular system is as follows:

i. Cell splitting

ii. Cell sectoring

iii. Repeaters for extending range

iv. Micro zone method

Cell Splitting:

Cell splitting is a method in which congested (heavy traffic) cell is subdivided into smaller cells, and each smaller cell is having its own base station with reduction in antenna height and transmitter power. The original congested bigger cell is called macrocell and the smaller cells are called microcells. Capacity of cellular network can be increased by creating micro-cells within the original cells which are having smaller radius than macro-cells, therefore, the capacity of a system increases because more channels per unit area are now available in a network.

Fig. 1.27: Cell Splitting

Fig. 1.27 shows a cell splitting in which a congested cell, divided into smaller micro-cells, and the base stations are put up at corners of the cells. The micro-cells are to be added in such a way in order to the frequency reuse plan of the system should be preserved. For

micro-cells, the transmit power of transmitter should be reduced, and each micro-cell is having half the radius to that of macro-cell. Therefore, transmit power of the new cells can be calculated by analyzing the received power at the cell boundaries. This is required in order to make sure that frequency reuse plan for the micro-cells is also working the same way as it was working for the macro-cells.

Where Ptp is the transmit power of macro-cell PtN is the transmit power of macro-cell

n is the path loss exponent is the radius of macro and micro-cells.



In cell splitting, following factors should be carefully monitored;

1. In cell splitting, allocation of channels to the new cells (micro-cells) must be done very cautiously. So, in order to avoid co-channel interference, cells must follow the minimum reuse distance principle.

2. Power levels of the transmitters for new and old cells must be redesigned. If the transmitter of the old cell has the same power as that of new cells, then the channels in old cell interfere with the channels of new cell. But, if the power level of transmitter is too low then it may result into in sufficient area coverage.

3. In order to overcome the problem of point (2); the channels of macro-cell is divided into two parts. The channels in the first part are for the new cell and other part consists of channel for the old cell. Splitting of cells is done according to the number of subscribers present in the areas, and the power levels of the transmitters must be redesigned according to the allocated channels to old and new cells.

4. Antennas of different heights and power levels are used for smooth and easy handoff, and this technique is called Umbrella cell approach. Using this approach large coverage area is provided for high speed users and small coverage area to low speed users. Therefore, the number of call handoffs is maximized for high speed users and provides more channels for slow speed users.

5. The main idea behind cell splitting is the rescaling of entire system. In cell splitting, reuse factor (D/R) is kept constant because by decreasing the radius of cell (R) and, at the same time, the separation between co-channels (D) is also decreased. So, high capacity can be achieved without changing the (D/R) ratio of system.

Sectoring:

Another way of improving the channel capacity of a cellular system is to decrease the D/R ratio while keeping the same cell radius. Improvement in the capacity can be accomplished by reducing the number of cells in a cluster, hence increasing the frequency reuse. To achieve this, the relative interference must be minimized without decreasing the transmit power.

For minimizing co-channel interference in a cellular network, a single omni-directional antenna is replaced with multiple directional antennas, with each transmitting within a smaller region. These smaller regions are called sectors and minimizing co-channel interference while improving the capacity of a system by using multiple directional antennas is called sectoring. The amount up to which co-channel interference is minimized depends on the amount of sectoring used. A cell is generally divided either into three 120 degree or six 60 degree sectors. In the three-sector arrangement, three antennas are generally located in each sector with one transmit and two receive antennas. The placement of two receive antennas provide antenna diversity, which is also known as space diversity.

Space diversity greatly improves the reception of a signal by efficiently providing a big target for signals transmitted from mobile units. The division between the two receive antenna depends on the height of the antennas above ground.

When sectoring technique is used in cellular systems, the channels used in a particular sector are actually broken down into sectored groups, which are only used inside a particular sector. With 7-cell reuse pattern and 120 degree sectors, the number of interfering cells in the neighboring tier is brought down from six to two. Cell sectoring also improves the signal-to-interference ratio, thereby increasing the capacity of a cellular system. This method of cell sectoring is very efficient, because it utilized the existing system structures. Cell sectoring also minimized the co-channel interference, with the use of directional antennas, a particular cell will get interference and transmit only a fraction of the available co-channel cells.



It is seen that the reuse ratio q = (NI*S/I) 1/n, where NI depends on the type of antenna

used. For an omni-directional antenna with only first-tier of co-channel interferer, the number of co-channel interfering cells NI = 6, but for a 120 degree directional antenna, it is 2.

n = path loss exponent, NI = Number of co-channel interfering cells q = frequency reuse ratio = D/R

Thus, S/I ratio increases with the increase in number of sectors, but at the cost of additional handoff that might be required for the movement of a user from one sector to another.

Microcell Zone Concept:

The micro-cell zone concept is associated with sharing the same radio equipment by different micro-cells. It results in decreasing of cluster size and, therefore, increase in system capacity. The micro-cell zone concept is used in practice to improve the capacity of cellular systems To improve both capacity and signal quality of a cellular system, cell sectoring depends upon correct setting up of directional antennas at the cell-site. But it also gives rise to increase in the number of handoffs and trunking inefficiencies. In a 3-sector or 6-sector cellular system, each sector acts like a new cell with a different shape and cell. Channels allocated to the un-sectored cell are divided between the different sectors present in a cell, thereby decreasing number of channels available in each sector. Furthermore, handoff takes place every time a mobile user moves from one sector to another sector of the same cell. This results in significant increase of network load on BSC and MSC of the cellular system. The problem of channel partitioning and increase in network load become very hard if all the 3 or 6-sectored directional antennas are placed at the centre of the cell.

As shown in the Fig. 1.28, three directional antennas are put at a point, Z1, also called zone-site, where three adjacent cells C1, C2, and C3 meet with each other. Z1, Z2 and Z3 are three zone-sites of the cell C1, and each zone-site is using three 135 degree directional antennas. All the three zone-sites also behave as receivers, which also receive signals transmitted by a mobile user present anywhere in the cell. All the three zone-sites are linked to one common base station, as shown in Fig. 1.29. This arrangement is known as

Lee's micro-cell zone concept.

Fig. 1.28: Location of Zone-sites in Sectored Cells



Fig. 1.29: Lee’s Microcell Zone Concept

Advantages of micro zone concept:

1. When the mobile user moves from one zone to another within the same cell, the mobile user can keep the same channel for the call progress.

2. The effect of interference is very low due to the installation of low power transmitters.

3. Better signal quality is possible.

4. Fewer handoffs when a call is in progress.

Repeaters for Range Extensions:

Wireless operators want to provide dedicated coverage for users located within buildings, or in valleys or tunnels as these areas are sometimes very hard to reach. Radio re-transmitters, also known as repeaters, are frequently used to provide coverage in such areas where range extension capabilities are required. Repeaters are bidirectional devices, as the signals can be concurrently transmitted to and received from a base station.

120° Cell Sectoring 60° Cell Sectoring



APPENDIX:

Full Duplex Systems: are communication systems in which transmission between the mobile and base stations occurs in both directions at the same time (transmit and receive at the same time) such as cellular phone systems. The regular phone at your house is a type of full duplex systems because you can talk and listen to other side talking at the same time.

Half Duplex Systems: are communication systems in which transmission between the mobile and base stations occurs at different times (transmit and receive at different times) such as push-to-talk systems.

Simplex Systems: are communication system in which transmission of information occurs in one direction only such as a garage door opening system.

Forward Channel: is the communication channel used to transmit information from the base station to the mobile station.

Forward Control Channel (FCC): is the channel used by the base station to inform mobile stations of a call directed to them, and to instruct mobile stations of the voice channels they should use to send and receive information.

Forward Voice Channel (FVC): is the channel used by the base station to transmit the voice signal to the mobile station.

Reverse Channel: is the communication channel used to transmit information from the mobile station to the base station.

Reverse Control Channel (RCC): is the channel used by the mobile station to Request from a cellular tower to initiate a phone call.

Reverse Voice Channel (RVC): is the channel used by the mobile station to transmit the voice signal to the base station.

Multiple Access Techniques: are methods by which multiple mobile stations in a communication system request that part of the limited spectrum of the system be reserved for its communication and then release the reserved spectrum once the communication is completed.

Time Division Multiple Access (TDMA): the system assigns different time slots to transmit/receive information for each mobile station that would like to use the resources of the system. Multiple mobile stations using the system transmit/receive information at the same frequency.

Frequency Division Multiple Access (FDMA): the system assigns different frequency slots to transmit/receive information for each mobile station that would like to use the resources of the system. All mobile stations using the system transmit/receive information at the same time.

Code Division Multiple Access (CDMA): the system assigns different SPREADING codes to transmit/receive information for each mobile station that would like to use the resources of the system. Multiple mobile stations using the system transmit/receive information at the same frequency and the same time. The different spreading codes assigned to different mobile stations are orthogonal to allow the use of the same codes to extract the desired information from the spread signal without the interference of transmissions of other mobile stations.

Call Blocking: occurs when more calls are initiated or received in a region or zone beyond the number of channels (or phone lines) dedicated to that region or zone. In this situation, some calls will be blocked.

Coverage Footprint: is the region around a base station in which a mobile stations will receive service from that base station as long as it is in its coverage footprint. Once the mobile station leaves the coverage footprint of a base station, its service will either be transferred to another base station or it will loose coverage completely.

unit i limitations of conventional mobile telephone ... · fundamentals of cellular radio system...

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