CELLULAR SYSTEM

Multiple Access Techniques are used to allow a large number of mobile users to share the allocated spectrum in

the most efficient manner. As the spectrum is limited, so the sharing is required to increase the capacity

of cell or over a geographical area by allowing the available bandwidth to be used at the same time by different users.

Multiple Access Techniques for Wireless Communication• A cellular system divides any given area into cells where a mobile unit in each cell

communicates with a base station. The main aim in the cellular system design is to be able to increase the capacity of the channel i.e. to handle as many calls as possible in a given bandwidth with a sufficient level of quality of service.

There are several different ways to allow access to the channel:

1) Frequency division multiple-access (FDMA) 2) Time division multiple-access (TDMA) 3) Code division multiple-access (CDMA)

FDMA,TDMA and CDMA are the three major multiple access techniques thatare used to share the available bandwidth in a wireless communication system.Depending on how the available bandwidth is allocated to the users these techniques can be classified as narrowband and wideband systems.

Narrowband Systems

It is used to relate the bandwidth of the single channel to the expected coherence bandwidth of the channel.

Transmission experiences nonselective fading. This means that when fades occur, all of the information is affected.

Channel System: generally total spectrum is divide into a number of relatively narrow radio channels.

Unused bandwidth in each channel cannot be used by the other users.

In narrow band FDMA, a user is assigned a particular channel which is not shared by other users in the vicinity and if FDD is used then the system is called FDMA/FDD.

Narrow band TDMA allows users to use the same channel but allocated a unique time slot to each user on the channel, thus separating a small number of users in time on a single channel.

Wideband Systems

The transmission bandwidth of a single channel is much larger than the coherence bandwidth of the channel.

The main feature of wideband systems is that either all the spectrum available or a considerable portion of it is used by each user.

The advantage of wideband systems is that the transmission bandwidth always exceeds the coherence bandwidth for which the signal experiences only selective fading. That is, only a small fraction of the frequencies composing the signal is affected by fading.

Signal can be distorted and therefore equalization is needed but unlikely that a total signal fade will occur.

Frequency Division Duplexing (FDD)

A method for establishing a full-duplex communication link that uses two different radio frequencies for transmitter and receiver operation.

The transmit direction and receive direction frequencies are separated by a define frequency offset.

Time Division Duplexing (TDD)

Is a method for emulating full-duplex communication over a half-duplex communication link.

The transmitter and receiver both use the same frequency but the transmit and receive traffic is switched in time.

MA techniques in different wireless communication systems:

Frequency Division Multiple Access (FDMA)

Each individual user is assigned a pair of frequencies while making or receiving a call.

One frequency is used for downlink and one pair for uplink. This is called frequency division duplexing (FDD).

The allocated frequency pair is not used in the same cell or adjacent cells during the call so as to reduce the cochannel interference.

Different users can use the same frequency in the same cell except that they must transmit at different times.

The features of FDMA are as follows: The FDMA channel carries only one phone circuit at a time. If an FDMA channel is not in use, then it sits idle and it cannot be used by other users to increase share capacity. After the assignment of the voice channel the BS and the MS transmit simultaneously and continuously.

Properties of FDMA:

• Bandwidth of FDMA channel is narrow (30KHz)• No equalization is required, since the symbol time is large compared to

average delay spread.• FDMA systems have higher cost

• Costly bandpass filters to eliminate spurious radiation• Duplexer in both T/R increase subscriber costs

Number Of channels in FDMA System:

N=Bt−2Bg

BC

where:N - number of channelsBt - total spectrum allocationBg - Guard BandBc - channel bandwidth

Advantages of FDMA There is no need for network timing The channel operation in FDMA are simple Simplicity in FDMA algorithm There is almost no need for bits that are responsible for synchronization

Disadvantage of FDMA It is impossible for the station to receive data from more than one

transmission source Because of the guard bands the capacity of the FDMA will be decreased The maximum data rate which is small and fixed for every channel in FDMA

Time Division Multiple-Access (TDMA) TDMA is a system divided the channel time into frames, each frame is further

partitioned into time slots. In each slot only one user is allowed to either transmit or received.

Unlike FDMA, only digital data and digital modulation must be used.

Properties of TDMA: Data Transmission for user of TDMA system occurs in discrete bursts The result is low battery consumption. Handoff process is simpler

Since different slots are used for T and R, duplexers are not required. Equalization is required, since transmission rates are higher than FDMA

channels

Number of Channels in a TDMA system:

N=m∗(B¿¿ t−2Bg)

BC

¿

where:N - number of channelsm - number of TDMA users per radio channelBt - total spectrum allocationBg - Guard BandBc - channel bandwidth

Advantages of TDMA Low battery consumption Single Carrier Frequency for user Transmission in Burst Very high transmission rate

Disadvantage of TDMA TDMA technology, the users has a predefined time slot. When moving from

one cell site to another, if all the times slots in this cell are full the user might be disconnected.

It is subjected to multipath distortion. To overcome the distortion, a time limit can be used on the system. Once the time limit expired the signal is ignored.

Code Division Multiple Access (CDMA) Refers to any of several protocols used in second-generation (2G) and third-

generation (3G) wireless communications. Is a form of multiplexing, which allows numerous signals to occupy a single

transmission channel, optimizing the use of available bandwidth.  used in ultra-high-frequency (UHF) cellular telephone systems in the

800-MHz  and 1.9-GHz bands. Unlike FDMA, CDMA separates calls by code. Every bit of a conversation is

been tagged with a specific and unique code. The system gets a call, it allocates a unique code to that particular conversation,

and now the data is split into small parts and is tagged with the unique code given to the conversation of which they are part of. Now, this data in small pieces is sent over a number of the discrete frequencies available for use at any time in the specified range. The system then at the end reassembles the conversation from the coded bits and deliver it.

CDMA Advantages Random access possible

Users can start their transmission at any time Higher capacity than TDMA and FDMA No frequency management No equalizer needed No guard time needed

CDMA Disadvantages As the number of users increases, the overall quality of service decreases Near- Far- problem arises

The near-far problem occurs when a transmitter sends a strong signal to a receiver which then makes it difficult for any weaker signals to be received