Wireless Communications Engineering

Cellular Fundamentals

Definitions – Wireless Communication

What is Wireless Communication?

Ability to communicate via wireless links.

Mobile Communication = + ?

Wireless Communication Wireless Communication are of two

types: Fixed Wireless Communication Mobile Wireless Communication.

Mobile Wireless Communication

Mobile Wireless Communication (Infrastructured Network) Single Hop Wireless Link to reach a

mobile Terminal.

Mobile Communication = + ?

Mobile Ad Hoc Networks Infrastructureless or Adhoc Network Multihop Wireless path from source to destination.

Mobile Radio Environment

Mobile Radio Environment

The transmissions over the wireless link are in general very difficult to characterize.

EM signals often encounter obstacles, causing reflection, diffraction, and scattering.

Mobility introduces further complexity. We have focused on simple models to help

gain basic insight and understanding of the wireless radio medium.

Three main components: Path Loss, Shadow fading, Multipath fading (or fast fading).

Free Space loss Transmitted signal attenuates over distance

because it is spread over larger and larger area This is known as free space loss and for isotropic

antennas

Pt = power at the transmitting antenna

Pr = power at the receiving antennaλ = carrier wavelengthd = propagation distance between the antennasc = speed of light

2

2

2

2 )4()4(

c

fdd

P

P

r

t

Free Space loss For other antennas

Gt = Gain of transmitting antenna

Gr = Gain of receiving antenna

At = effective area of transmitting antenna

Ar = effective area of receiving antenna

trtrr

t

AA

d

GG

d

P

P 2

2

2 )()4(

Thermal Noise Thermal noise is introduced due to thermal

agitation of electrons Present in all transmission media and all electronic devices a function of temperature uniformly distributed across the frequency spectrum and

hence is often referred to as white noise amount of noise found in a bandwidth of 1 Hz is N0 = k T

N0 = noise power density in watts per 1 Hz of bandwidth k = Boltzman’s constant = 1.3803 x 10-23 J/K T = temperature, in Kelvins N = thermal noise in watts present in a bandwidth of B = kTB where

Free Space loss Transmitted signal attenuates over distance

because it is spread over larger and larger area This is known as free space loss and for isotropic

antennas

Pt = power at the transmitting antenna

Pr = power at the receiving antennaλ = carrier wavelengthd = propagation distance between the antennasc = speed of light

2

2

2

2 )4()4(

c

fdd

P

P

r

t

Free Space loss For other antennas

Gt = Gain of transmitting antenna

Gr = Gain of receiving antenna

At = effective area of transmitting antenna

Ar = effective area of receiving antenna

trtrr

t

AA

d

GG

d

P

P 2

2

2 )()4(

Thermal Noise Thermal noise is introduced due to thermal

agitation of electrons Present in all transmission media and all electronic devices a function of temperature uniformly distributed across the frequency spectrum and

hence is often referred to as white noise amount of noise found in a bandwidth of 1 Hz is N0 = k T

N0 = noise power density in watts per 1 Hz of bandwidth k = Boltzman’s constant = 1.3803 x 10-23 J/K T = temperature, in Kelvins N = thermal noise in watts present in a bandwidth of B = kTB where

Data rate and error rate

Bit error rate is a decreasing function of Eb/N0.

If bit rate R is to increase, then to keep bit error rate (or Eb/N0) same, the transmitted signal power must increase, relative to noise

Eb/N0 is related to SNR as follows

B = signal bandwidth (since N = N0 B)

R

B

N

S

N

Eb 0

Doppler’s Shift When a client is mobile, the frequency of received

signal could be less or more than that of the transmitted signal due to Doppler’s effect

If the mobile is moving towards the direction of arrival of the wave, the Doppler’s shift is positive

If the mobile is moving away from the direction of arrival of the wave, the Doppler’s shift is negative

Doppler’s Shift

wherefd =change in frequency

due to Doppler’s shiftv = constant velocity of the mobile receiverλ = wavelength of the transmission

θ

S

XY

cosv

fd

Doppler’s shift

f = fc + fd

where f = the received carrier frequencyfc = carrier frequency being transmitted

fd = Doppler’s shift as per the formula in the previous slide.

Multipath Propagation Wireless signal can arrive at the receiver

through different paths LOS Reflections from objects Diffraction

Occurs at the edge of an impenetrable body that is large compared to the wavelength of the signal

Multipath Propagation (source: Stallings)

Mobile Radio Channel: Fading

Limitations of Wireless Channel is unreliable Spectrum is scarce, and not all

ranges are suitable for mobile communication

Transmission power is often limited Battery Interference to others

Advent of Cellular Systems Noting from the channel model, we know

signal will attenuated with distance and have no interference to far users.

In the late 1960s and early 1970s, work began on the first cellular telephone systems.

The term cellular refers to dividing the service area into many small regions (cells) each served by a low-power transmitter with moderate antenna height.

Cell Concept

Cell A cell is a small geographical area served by a

singlebase station or a cluster of base stations

Areas divided into cells Each served by its own antenna Served by base station consisting of

transmitter, receiver, and control unit Band of frequencies allocated Cells set up such that antennas of all

neighbors are equidistant

Cellular Networks

Cellular Network Organization

Use multiple low-power transmitters Areas divided into cells

Each served by its own antenna Served by base station consisting of

transmitter, receiver, and control unit Band of frequencies allocated Cells set up such that antennas of all

neighbors are equidistant

Consequences Transmit frequencies are re-used across

these cells and the system becomes interference rather than noise limited the need for careful radio frequency planning

– colouring in hexagons! a mechanism for handling the call as the

user crosses the cell boundary - call handoff (or handover)

increased network complexity to route the call and track the users as they move around

But one significant benefit: very much increased traffic capacity, the ability to service many users

Cellular System Architecture

Cellular Systems Terms Mobile Station

users transceiver terminal (handset, mobile) Base Station (BS)

fixed transmitter usually at centre of cell includes an antenna, a controller, and a

number of receivers Mobile Telecommunications Switching

Office (MTSO) /Mobile Switch Center (MSC) handles routing of calls in a service area tracks user connects to base stations and PSTN

Cellular Systems Terms (Cont’d) Two types of channels available between

mobile unit and BS Control channels – used to exchange

information for setting up and maintaining calls

Traffic channels – carry voice or data connection between users

Handoff or handover process of transferring mobile station from

one base station to another, may also apply to change of radio channel within a cell

Cellular Systems Terms (Cont’d) Downlink or Forward Channel

radio channel for transmission of information (e.g.speech) from base station to mobile station

Uplink or Reverse Channel radio channel for transmission of information

(e.g.speech) from mobile station to base station Paging

a message broadcast over an entire service area, includes use for mobile station alert (ringing)

Roaming a mobile station operating in a service area other

than the one to which it subscribes

Steps in an MTSO Controlled Call between Mobile Users

Mobile unit initialization Mobile-originated call Paging Call accepted Ongoing call Handoff

Frequency Reuse Cellular relies on the intelligent

allocation and re–use of radio channels throughout a coverage area.

Each base station is allocated a group of radio channels to be used within the small geographic area of its cell

Neighbouring base stations are given different channel allocation from each other

Frequency Reuse (Cont’d) If we limit the coverage area within the

cell by design of the antennas we can re-use that same group of

frequencies to cover another cell separated by a large enough distance

transmission power controlled to limit power at that frequency to keep interference levels within tolerable limits

the issue is to determine how many cells must intervene between two cells using the same frequency

Radio Planning Design process of selecting and

allocating channel frequencies for all cellular base stations within a system is known as frequency re-use or frequency planning.

Cell planning is carried out to find a geometric shape to

tessellate a 2D space represent contours of equal transmit power

Real cells are never regular in shape

Two-Dimensional Cell Clusters Regular geometric shapes tessellating a

2D space: Square, triangle, and hexagon. ‘Tessellating Hexagon’ is often used to

model cells in wireless systems: Good approximation to a circle (useful when

antennas radiate uniformly in the x-y directions).

Also offer a wide variety of reuse pattern Simple geometric properties help gain basic

understanding and develop useful models.

Coverage Patterns

Cellular Coverage Representation

Geometry of Hexagons

Hexagonal cell geometry and axes

Geometry of Hexagons (Cont’d) D = minimum distance between centers

of cells that use the same band of frequencies (called co-channels)

R = radius of a cell d = distance between centers of

adjacent cells (d = R√3) N = number of cells in repetitious

pattern (Cluster) Reuse factor Each cell in pattern uses unique band of

frequencies

Geometry of Hexagons (Cont’d)

The distance between the nearest cochannel cells in a hexagonal area can be calculated from the previous figure

The distance between the two adjacent co-channel cells is D=√3R.

(D/d)2 = j2 cos2(30) + (i+ jsin30)2 = i2 + j2 +ij = N D=Dnorm x √3 R =(√3N)R In general a candidate cell is surrounded by 6k

cells in tier k.

Geometry of Hexagons (Cont’d)

Using this equation to locate co-channel cells, we start from a reference cell and move i hexagons along the u-axis then j hexagons along the v-axis. Hence the distance between co–channel cells in adjacent clusters is given by:

D = (i2 + ij + j2)1/2

where D is the distance between co–channel cells in adjacent clusters (called frequency reuse distance).

and the number of cells in a cluster, N is given by D2

N = i2 + ij + j2

Hexagon Reuse Clusters

3-cell reuse pattern (i=1,j=1)

4-cell reuse pattern (i=2,j=0)

7-cell reuse pattern (i=2,j=1)

12-cell reuse pattern (i=2,j=2)

19-cell reuse pattern (i=3,j=2)

Relationship between Q and N

Proof

Cell ClustersReuse coordinates Number of

cells in re-use pattern

Normalised reuse

distance i j N SQRT(N) 1 0 1 1 1 1 3 1.732 1 2 7 2.646 2 2 12 3.464 1 3 13 3.606 2 3 19 4.359 1 4 21 4.583

since D = SQRT(N)

Co–channel Cell Location

Method of locating co–channel cells Example for N=19, i=3, j=2

Cell Planning Example Suppose you have 33 MHz bandwidth

available, an FM system using 25 kHz channels, how many channels per cell for 4,7,12 cell re-use? total channels = 33,000/25 = 1320 N=4 channels per cell = 1320/4 = 330 N=7 channels per cell = 1320/7 = 188 N=12 channels per cell = 1320/12 = 110

Smaller clusters can carry more traffic However, smaller clusters result in larger

co-channel interference

Remarks on Reuse Ratio

Co-channel Interference with Omnidirectional Cell Site

Propagation model

Cochannel interference ratio

Worst-case scenario for co-channel interference

Worst-case scenario for co-channel interference

Reuse Factor and SIR

Remarks SIGNAL TO INTERFERENCE LEVEL IS

INDEPENDENT OF CELL RADIUS! System performance (voice quality) only

depends on cluster size What cell radius do we choose?

Depends on traffic we wish to carry (smaller cell means more compact reuse or higher capacity)

Limited by handoff

Adjacent channel interference So far, we assume adjacent channels to

be orthogonal (i.e., they do not interfere with each other).

Unfortunately, this is not true in practice, so users may also experience adjacent channel interference besides co-channel interference.

This is especially serious when the near-far effect (in uplinks) is significant Desired mobile user is far from BS Many mobile users exist in the cell

Near-Far Effect

Near-Far Effect (Cont’d)

Reduce Adjacent channel interference Use modulation schemes which have

small out-of-band radiation (e.g., MSK is better than QPSK)

Carefully design the receiver BPF Use proper channel interleaving by

assigning adjacent channels to different cells, e.g., for N = 7

Reduce Adjacent channel interference (Cont’d) Furthermore, do not use adjacent

channels in adjacent cells, which is possible only when N is very large. For example, if N =7, adjacent channels must be used in adjacent cells

Use FDD or TDD to separate the forward link and reverse link.

Improving Capacity in Cellular Systems

Adding new channels – often expensive or impossible

Frequency borrowing (or DCA)– frequencies are taken from adjacent cells by congested cells

Cell splitting – cells in areas of high usage can be split into smaller cells (microcells with antennas moved to buildings, hills, and lamp posts)

Cell sectoring – cells are divided into a number of wedge-shaped sectors, each with their own set of channels

Sectoring Co-channel interference reduction

with the use of directional antennas (sectorization) Each cell is divided into sectors

and uses directional antennas at the base station.

Each sector is assigned a set of channels (frequencies).

Site Configurations

Sectorized Cell Sites120

Worst case scenario

60Sectorizd Cell Sites

Worst case scenario

Illustration of cell splitting 1

Illustration of cell splitting 2

Illustration of cell splitting 3

Cell Splitting

Design Tradeoff Smaller cell means higher capacity (frequency

reused more).

However, smaller cell also results in higher handoff probability, which also means higher

overhead

Moreover, cell splitting should not introduce too much interference to users in other cells

Handoff (Handover) Process Handoff: Changing physical radio channels

of network connections involved in a call, while maintaining the call

Basic reasons for a handoff MS moves out of the range of a BTS (signal level

becomes too low or error rate becomes too high) Load balancing (traffic in one cell is too high, and

shift some MSs to other cells with a lower load) GSM standard identifies about 40 reasons for a

handoff!

Phases of Handoff MONITORING PHASE - measurement of the quality of the current and

possible candidate radio links

- initiation of a handover when necessary HANDOVER HANDLING PHASE - determination of a new point of attachment - setting up of new links, release of old links

- initiation of a possible re-routing procedure

Handoff Types Intra-cell handoff – narrow-band interference => change carrier frequency – controlled by BSC Inter-cell, intra-BSC handoff – typical handover scenario – BSC performs the handover, assigns new radio channel in

the new cell, releases the old one Inter-BSC, intra-MSC handoff – handoff between cells controlled by different BSCs – controlled by the MSC Inter-MSC handoff – handoff between cells belonging to different MSCs – controlled by both MSCs

Handoff Types (cont’d)

Handoff Strategies Relative signal strength Relative signal strength with threshold Relative signal strength with hysteresis Relative signal strength with hysteresis

and threshold Prediction techniques

Intra-MSC Handoff (Mobile Assisted)

Handover Scenario at Cell Boundary

Handoff Based on Receive Level

How to avoid ping-pong problem?

Handoff – 1G (Analog) systems

Signal strength measurements made by the BSs and supervised by the MSC

BS constantly monitors the signal strengths of all the voice channels

Locator receiver measures signal strength of MSs in neighboring cells

MSC decides if a handover is necessary

Handoff – 2G (Digital) TDMA Handoff decisions are mobile assisted Every MS measures the received power

from surrounding BSs and sends reportsto its own BS

Handoff is initiated when the power received from a neighbor BS begins to exceed the power from the current BS (by a certain level and/or for a certain period)

Handoff – 2G (Digital) CDMA

CDMA uses code to differentiate users.

Soft handoff: a user keeps records of several neighboring BSs.

Soft handoff may decrease the handoff blocking probability and handoff delay

Avoiding handoff: Umbrella cells

Mixed Cell Architecture

Handoff Prioritization The idea of reserving channels for

handoff calls was introduced in the mid 1980s as a way of reducing the handoff call blocking probability

Motivation: users find calls blocked in mid-progress a far greater irritant than unsuccessful call attempts.

The basic idea is to reserve a certain portion of the total channel pool in a cell for handoff users only.

Performance Metrics

Call blocking probability – probability of a new call being blocked

Call dropping probability – probability that a call is terminated due to a handoff

Call completion probability – probability that an admitted call is not dropped before it terminates

Handoff blocking probability – probability that a handoff cannot be successfully completed

Performance Metrics (Cont’d) Handoff probability – probability that

a handoff occurs before call termination Rate of handoff – number of handoffs

per unit time Interruption duration – duration of

time during a handoff in which a mobile is not connected to either base station

Handoff delay – distance the mobile moves from the point at which the handoff should occur to the point at which it does occur

Summary cellular mobile uses many small cells hexagonal planning, clusters of cells cell repeat patterns 3,7,12 etc... re-uses frequencies to obtain capacity is interference not noise (kTB) limited S/I is independent of cell radius choose cell radius to meet traffic

demand N=7 is a good compromise between S/I

and capacity. handoff