Multiple acces techniques

11

Multiple Access Techniques Multiple Access Techniques Multiple Access Techniques Multiple Access Techniques

Multiple access schemes are used to allow many Multiple access schemes are used to allow many mobile users to share simultaneously a finite mobile users to share simultaneously a finite amount of radio spectrum.amount of radio spectrum.

The sharing of spectrum is required to achieve The sharing of spectrum is required to achieve high capacityhigh capacity by simultaneously allocating the by simultaneously allocating the available bandwidth (or the available amount of available bandwidth (or the available amount of channels) to multiple users.channels) to multiple users.

For high quality communicationsFor high quality communications, this must be , this must be done without severe degradation in the done without severe degradation in the performance of the system.performance of the system.

Multiple access schemes are used to allow many Multiple access schemes are used to allow many mobile users to share simultaneously a finite mobile users to share simultaneously a finite amount of radio spectrum.amount of radio spectrum.

The sharing of spectrum is required to achieve The sharing of spectrum is required to achieve high capacityhigh capacity by simultaneously allocating the by simultaneously allocating the available bandwidth (or the available amount of available bandwidth (or the available amount of channels) to multiple users.channels) to multiple users.

For high quality communicationsFor high quality communications, this must be , this must be done without severe degradation in the done without severe degradation in the performance of the system.performance of the system.

22

Difference between multiplexing and multiple accessDifference between multiplexing and multiple accessDifference between multiplexing and multiple accessDifference between multiplexing and multiple access

33

Multiple Access Techniques Multiple Access Techniques Multiple Access Techniques Multiple Access Techniques

Multiple Access TechniquesMultiple Access Techniques

FDMAFDMA TDMATDMA CDMACDMA SDMASDMA

44

Multiple Access (MA) TechnologiesMultiple Access (MA) Technologies used in Different Wireless Systems used in Different Wireless Systems Multiple Access (MA) TechnologiesMultiple Access (MA) Technologies used in Different Wireless Systems used in Different Wireless Systems

Cellular SystemsCellular Systems MA TechniqueMA Technique AMPS ( Advanced Mobile AMPS ( Advanced Mobile Phone system ) Phone system )

FDMA / FDD FDMA / FDD

GSM ( Global System for GSM ( Global System for Mobile )Mobile )

TDMA / FDD TDMA / FDD

US DC ( U. S Digital US DC ( U. S Digital Cellular )Cellular )


JDC ( Japanese Digital JDC ( Japanese Digital Cellular ) Cellular )


55

……Multiple Access (MA) TechnologiesMultiple Access (MA) Technologies used in Different Wireless Systems used in Different Wireless Systems……Multiple Access (MA) TechnologiesMultiple Access (MA) Technologies used in Different Wireless Systems used in Different Wireless SystemsCellular SystemsCellular Systems MA TechniqueMA Technique

DECT ( Digital DECT ( Digital European Cordless European Cordless Telephone ) Telephone )

FDMA / FDD FDMA / FDD

IS – 95 ( U.S Narrowband IS – 95 ( U.S Narrowband Spread Spectrum ) Spread Spectrum )

CDMA / FDD CDMA / FDD

66

Frequency Division Frequency Division Multiple Access (FDMA)Multiple Access (FDMA)Frequency Division Frequency Division Multiple Access (FDMA)Multiple Access (FDMA)

codecode

timetime

frequencyfrequency

CC11 CC22 CCnn

CCnnCC22CC11

frequencyfrequency

77

Principles Of OperationPrinciples Of OperationPrinciples Of OperationPrinciples Of Operation

Each user is allocated a unique frequency Each user is allocated a unique frequency band or channel. These channels are band or channel. These channels are assigned on demand to users who request assigned on demand to users who request service.service.

In FDD, the channel has two frequencies – In FDD, the channel has two frequencies – forward channel & reverse channel.forward channel & reverse channel.

Each user is allocated a unique frequency Each user is allocated a unique frequency band or channel. These channels are band or channel. These channels are assigned on demand to users who request assigned on demand to users who request service.service.

In FDD, the channel has two frequencies – In FDD, the channel has two frequencies – forward channel & reverse channel.forward channel & reverse channel.

88

……Principles Of OperationPrinciples Of Operation……Principles Of OperationPrinciples Of Operation

During the period of the call, no other user During the period of the call, no other user can share the same frequency band.can share the same frequency band.

If the FDMA channel is not in use, then it If the FDMA channel is not in use, then it sits idle and cannot be used by other users sits idle and cannot be used by other users to increase or share capacity. This is a to increase or share capacity. This is a wasted resource.wasted resource.

During the period of the call, no other user During the period of the call, no other user can share the same frequency band.can share the same frequency band.

If the FDMA channel is not in use, then it If the FDMA channel is not in use, then it sits idle and cannot be used by other users sits idle and cannot be used by other users to increase or share capacity. This is a to increase or share capacity. This is a wasted resource.wasted resource.

99

Properties of FDMAProperties of FDMAProperties of FDMAProperties of FDMA

The bandwidth of FDMA channels is narrow The bandwidth of FDMA channels is narrow (30 KHz) since it supports only one call/ (30 KHz) since it supports only one call/ carrier.carrier.

FDMA systems have higher costFDMA systems have higher cost Costly band pass filters to eliminate

spurious radiation Duplexers in both T/R increase

subscriber costs

The bandwidth of FDMA channels is narrow The bandwidth of FDMA channels is narrow (30 KHz) since it supports only one call/ (30 KHz) since it supports only one call/ carrier.carrier.

FDMA systems have higher costFDMA systems have higher cost Costly band pass filters to eliminate

spurious radiation Duplexers in both T/R increase

subscriber costs

1010

Number Of Channel SupportedNumber Of Channel Supported By FDMA System By FDMA SystemNumber Of Channel SupportedNumber Of Channel Supported By FDMA System By FDMA System

gB gB

tBt g

c

g

c

B 2BN

B

B GuardBand

B ChannelBandwidth

1111

ExampleExampleExampleExample

In the US, each cellular carrier is allocated 416 In the US, each cellular carrier is allocated 416 channels,channels, In the US, each cellular carrier is allocated 416 In the US, each cellular carrier is allocated 416 channels,channels,

t

g

c

6 3

3

B 12.5MHz

B 10KHz

B 30KHz

(12.5 10 ) 2(10 10 )N 416

30 10

1212

Time Division Time Division Multiple Access (TDMA)Multiple Access (TDMA)Time Division Time Division Multiple Access (TDMA)Multiple Access (TDMA)

codecode

timetime

frequencyfrequency

CC11

CCnn

CCnnCC22CC11

timetime

1313

Principles Of OperationPrinciples Of OperationPrinciples Of OperationPrinciples Of Operation

TDMA systems divide the radio spectrum TDMA systems divide the radio spectrum into time slots and each user is allowed to into time slots and each user is allowed to either transmit or receive in each time slots.either transmit or receive in each time slots.

Each user occupies a cyclically repeating Each user occupies a cyclically repeating time slots. TDMA can allow different number time slots. TDMA can allow different number of time slots for separate user.of time slots for separate user.

TDMA systems divide the radio spectrum TDMA systems divide the radio spectrum into time slots and each user is allowed to into time slots and each user is allowed to either transmit or receive in each time slots.either transmit or receive in each time slots.

Each user occupies a cyclically repeating Each user occupies a cyclically repeating time slots. TDMA can allow different number time slots. TDMA can allow different number of time slots for separate user.of time slots for separate user.

1414

TDMA Frame StructureTDMA Frame StructureTDMA Frame StructureTDMA Frame Structure

PreamblePreamble Information Information messagemessage

Trail BitsTrail Bits

Slot 1Slot 1 Slot 2Slot 2 Slot NSlot N

Trail BitTrail Bit Sync BitSync Bit Information Information BitBit

Guard BitsGuard Bits

1515

Components of 1 TDMA FrameComponents of 1 TDMA FrameComponents of 1 TDMA FrameComponents of 1 TDMA Frame

Preamble Preamble Address and synchronization Address and synchronization information for base station and subscriber information for base station and subscriber identificationidentification

Guard times Guard times Synchronization of Synchronization of receivers between a different slots and receivers between a different slots and framesframes

Preamble Preamble Address and synchronization Address and synchronization information for base station and subscriber information for base station and subscriber identificationidentification

Guard times Guard times Synchronization of Synchronization of receivers between a different slots and receivers between a different slots and framesframes

1616

Principles Of Operation Principles Of Operation Principles Of Operation Principles Of Operation

TDMA shares the single carrier frequency TDMA shares the single carrier frequency with several users, where each user with several users, where each user makes use of non-overlapping timeslots.makes use of non-overlapping timeslots.

Data Transmission for user of TDMA Data Transmission for user of TDMA system is discrete burstssystem is discrete bursts

• The result is low battery consumption.The result is low battery consumption.• Handoff process is simpler, since it is Handoff process is simpler, since it is

able to listen for other base stations able to listen for other base stations during idle time slots.during idle time slots.

TDMA shares the single carrier frequency TDMA shares the single carrier frequency with several users, where each user with several users, where each user makes use of non-overlapping timeslots.makes use of non-overlapping timeslots.

Data Transmission for user of TDMA Data Transmission for user of TDMA system is discrete burstssystem is discrete bursts

• The result is low battery consumption.The result is low battery consumption.• Handoff process is simpler, since it is Handoff process is simpler, since it is

able to listen for other base stations able to listen for other base stations during idle time slots.during idle time slots.

1717

……Principles Of OperationPrinciples Of Operation……Principles Of OperationPrinciples Of Operation

Since different slots are used for T and R, Since different slots are used for T and R, duplexers are not required.duplexers are not required.

Equalization is required, since transmission Equalization is required, since transmission rates are generally very high as compared to rates are generally very high as compared to FDMA channels.FDMA channels.

Since different slots are used for T and R, Since different slots are used for T and R, duplexers are not required.duplexers are not required.

Equalization is required, since transmission Equalization is required, since transmission rates are generally very high as compared to rates are generally very high as compared to FDMA channels.FDMA channels.

1818

Efficiency of TDMAEfficiency of TDMAEfficiency of TDMAEfficiency of TDMA

Frame Efficiency : Frame Efficiency : Frame Efficiency : Frame Efficiency :

f

No.ofbits / frame containingtransmitted dataTotal Numberof bits / frame

OH T

T OH

T

(1 b / b ) 100

(b b )100

b

1919

Frame efficiency parametersFrame efficiency parametersFrame efficiency parametersFrame efficiency parameters

Tb Total Number of bits per frame

f=T R

OHb =Number of overhead bits /frame

fT =Frame duration

R=Channel bit rate

r r t p t g r g=N b N b N b N b

2020

……Frame efficiency parametersFrame efficiency parameters……Frame efficiency parametersFrame efficiency parameters

r

t

r

p

g

N Number of reference bits per frame

N Number of traffic bits per frame

b Number of overhead bits per reference burst

b Number of overhead bits per preamble in each slots

b Number of equivalent bits

in each guard time interval

2121

Number of channels in TDMA System Number of channels in TDMA System Number of channels in TDMA System Number of channels in TDMA System

tot guard

c

m(B -2B )N=

B

guard

m Maximum number of TDMA users supported on each radio channelB Guard band to present user at the edge of the band

from 'bleeding over' to an adjacent radio service

2222

Example Example Example Example

GSM System uses a TDMA / FDD system.GSM System uses a TDMA / FDD system.

The GSM System uses a frame structure The GSM System uses a frame structure where each frame consist of 8 time slots, and where each frame consist of 8 time slots, and each time slot contains 156.25 bits, and data is each time slot contains 156.25 bits, and data is transmitted at 270.833 kbps in the channel. transmitted at 270.833 kbps in the channel. Find: ……Find: ……

GSM System uses a TDMA / FDD system.GSM System uses a TDMA / FDD system.

The GSM System uses a frame structure The GSM System uses a frame structure where each frame consist of 8 time slots, and where each frame consist of 8 time slots, and each time slot contains 156.25 bits, and data is each time slot contains 156.25 bits, and data is transmitted at 270.833 kbps in the channel. transmitted at 270.833 kbps in the channel. Find: ……Find: ……

2323

……ExampleExample……ExampleExample

1.1. Time duration of a bitTime duration of a bit2.2. Time duration of a slotTime duration of a slot3.3. Time duration of a frame and Time duration of a frame and 4.4. How long must a user occupying a single How long must a user occupying a single

slot must wait between two simultaneous slot must wait between two simultaneous transmissions?transmissions?

1.1. Time duration of a bitTime duration of a bit2.2. Time duration of a slotTime duration of a slot3.3. Time duration of a frame and Time duration of a frame and 4.4. How long must a user occupying a single How long must a user occupying a single

slot must wait between two simultaneous slot must wait between two simultaneous transmissions?transmissions?

2424

Solution Solution Solution Solution • Time duration of a bit Time duration of a bit

• Time duration of a slot Time duration of a slot

• Time duration of a bit Time duration of a bit

• Time duration of a slot Time duration of a slot

b 3

1 1=T = 3.692 s

bit-rate 270.833 10

slot bT 156.25 T 0.577 s ms

2525

……SolutionSolution……SolutionSolution

• Time duration of a frame Time duration of a frame

• A user has to wait 4.615 ms before next A user has to wait 4.615 ms before next transmission transmission

• Time duration of a frame Time duration of a frame

• A user has to wait 4.615 ms before next A user has to wait 4.615 ms before next transmission transmission

slot8 T 4.615ms

2626


If a normal GSM timeslot consists of 6 trailing If a normal GSM timeslot consists of 6 trailing bits, 8.25 guard bits, 26 training bits, and 2 bits, 8.25 guard bits, 26 training bits, and 2 traffic bursts of 58 bits of data, find the frame traffic bursts of 58 bits of data, find the frame efficiency efficiency

SolutionSolution Time slots have 6 + 8.25 + 26 + 2(58) = Time slots have 6 + 8.25 + 26 + 2(58) =

156.25 bits.156.25 bits. A frame has 8 * 156.25 = 1250 bits / frame.A frame has 8 * 156.25 = 1250 bits / frame.

If a normal GSM timeslot consists of 6 trailing If a normal GSM timeslot consists of 6 trailing bits, 8.25 guard bits, 26 training bits, and 2 bits, 8.25 guard bits, 26 training bits, and 2 traffic bursts of 58 bits of data, find the frame traffic bursts of 58 bits of data, find the frame efficiency efficiency

SolutionSolution Time slots have 6 + 8.25 + 26 + 2(58) = Time slots have 6 + 8.25 + 26 + 2(58) =

156.25 bits.156.25 bits. A frame has 8 * 156.25 = 1250 bits / frame.A frame has 8 * 156.25 = 1250 bits / frame.

2727

……ExampleExample……ExampleExample

The number of overhead bits per frame is The number of overhead bits per frame is given bygiven by

bbOHOH = 8(6) + 8(8.25) + 8(26) = 322 bits = 8(6) + 8(8.25) + 8(26) = 322 bits Frame efficiency = (1250 – 322 ) / 1250Frame efficiency = (1250 – 322 ) / 1250

= 74.24 % = 74.24 %

The number of overhead bits per frame is The number of overhead bits per frame is given bygiven by

bbOHOH = 8(6) + 8(8.25) + 8(26) = 322 bits = 8(6) + 8(8.25) + 8(26) = 322 bits Frame efficiency = (1250 – 322 ) / 1250Frame efficiency = (1250 – 322 ) / 1250

= 74.24 % = 74.24 %

2828

Spread Spectrum Multiple Access Spread Spectrum Multiple Access Technologies (SSMA)Technologies (SSMA)Spread Spectrum Multiple Access Spread Spectrum Multiple Access Technologies (SSMA)Technologies (SSMA)

SSMA technologies uses techniques which SSMA technologies uses techniques which has a transmission bandwidth that is much has a transmission bandwidth that is much greater than maximum required RF greater than maximum required RF bandwidth.bandwidth.

This is achieved by pseudo noise (PN) This is achieved by pseudo noise (PN) sequence that contents a narrowband signal sequence that contents a narrowband signal to a wideband noise-like signal before to a wideband noise-like signal before transmission.transmission.

SSMA technologies uses techniques which SSMA technologies uses techniques which has a transmission bandwidth that is much has a transmission bandwidth that is much greater than maximum required RF greater than maximum required RF bandwidth.bandwidth.

This is achieved by pseudo noise (PN) This is achieved by pseudo noise (PN) sequence that contents a narrowband signal sequence that contents a narrowband signal to a wideband noise-like signal before to a wideband noise-like signal before transmission.transmission.

2929

……Spread Spectrum Multiple Access Spread Spectrum Multiple Access Technologies (SSMA)Technologies (SSMA)……Spread Spectrum Multiple Access Spread Spectrum Multiple Access Technologies (SSMA)Technologies (SSMA)

SSMA provides immunity to multiple SSMA provides immunity to multiple interference and has robust multiple access interference and has robust multiple access capability.capability.

SSMA provides immunity to multiple SSMA provides immunity to multiple interference and has robust multiple access interference and has robust multiple access capability.capability.

3030

Types Of Spread Types Of Spread Spectrum TechniquesSpectrum Techniques Types Of Spread Types Of Spread Spectrum TechniquesSpectrum Techniques

Frequency Hopped Multiple Access Frequency Hopped Multiple Access ( FHMA )( FHMA )

Direct Sequence Multiple Access ( CDMA )Direct Sequence Multiple Access ( CDMA )

Frequency Hopped Multiple Access Frequency Hopped Multiple Access ( FHMA )( FHMA )

Direct Sequence Multiple Access ( CDMA )Direct Sequence Multiple Access ( CDMA )

3131

Direct Sequence Spread Direct Sequence Spread Spectrum (DS-SS)Spectrum (DS-SS)Direct Sequence Spread Direct Sequence Spread Spectrum (DS-SS)Spectrum (DS-SS)

codecode

timetime

frequencyfrequency

CC11

CC22

CC33

CCnn

3232

Direct Sequence Spread Direct Sequence Spread Spectrum (DS-SS)Spectrum (DS-SS)Direct Sequence Spread Direct Sequence Spread Spectrum (DS-SS)Spectrum (DS-SS)

1m (t)

2m (t)

1S (t)

11

kk

1PN (t)

KPN (t)

c 1cos(2 f t )

r(t)

c kcos(2 f t )

3333

Principles of operation-transmitterPrinciples of operation-transmitterPrinciples of operation-transmitterPrinciples of operation-transmitter The narrowband message signal The narrowband message signal

mmii(t) is multiplied by a pseudo noise (t) is multiplied by a pseudo noise code sequence that has a chip rate >> code sequence that has a chip rate >> data rate of message.data rate of message.

All users use the same carrier All users use the same carrier frequency and may transmit frequency and may transmit simultaneously. The kth transmitted simultaneously. The kth transmitted signal is given by:signal is given by:

The narrowband message signal The narrowband message signal mmii(t) is multiplied by a pseudo noise (t) is multiplied by a pseudo noise code sequence that has a chip rate >> code sequence that has a chip rate >> data rate of message.data rate of message.

All users use the same carrier All users use the same carrier frequency and may transmit frequency and may transmit simultaneously. The kth transmitted simultaneously. The kth transmitted signal is given by:signal is given by:

k s s k k c kS (t) (2E / T )1/ 2m (t)p (t)cos(2 f t )

3434

CDMA ReceiverCDMA ReceiverCDMA ReceiverCDMA Receiver

c kcos(2 f t )

r(t)km (t)

(.)dt(.)dt

kiZ (t)

KPN (t)

3535

Principles of operation-receiverPrinciples of operation-receiverPrinciples of operation-receiverPrinciples of operation-receiverAt the receiver, the received signal is At the receiver, the received signal is correlated with the appropriate signature correlated with the appropriate signature sequence to produce desire variable.sequence to produce desire variable.

At the receiver, the received signal is At the receiver, the received signal is correlated with the appropriate signature correlated with the appropriate signature sequence to produce desire variable.sequence to produce desire variable.

1

1

iT1i 1 1 c 1 1

(i 1)T

Z (t) r(t)p (t )cos[2 f (t ) ]dt

3636

Message Signal Message Signal Message Signal Message Signal m(t) is a time sequence of non-overlapping m(t) is a time sequence of non-overlapping

pulses of duration T, each of which has an pulses of duration T, each of which has an amplitude (+/-) 1.amplitude (+/-) 1.

The PN waveform consists of N pulses or The PN waveform consists of N pulses or chips for message symbol period T.chips for message symbol period T.

NTNTC C = T= T

where Twhere TCC is the chip period. is the chip period.

m(t) is a time sequence of non-overlapping m(t) is a time sequence of non-overlapping pulses of duration T, each of which has an pulses of duration T, each of which has an amplitude (+/-) 1.amplitude (+/-) 1.

The PN waveform consists of N pulses or The PN waveform consists of N pulses or chips for message symbol period T.chips for message symbol period T.

NTNTC C = T= T

where Twhere TCC is the chip period. is the chip period.

3737

Example:Example: Example:Example: Assume N=4Assume N=4

PN Wave for N =4

11

-1-1

-1-1

11

3838

Correlator output for first user Correlator output for first user Correlator output for first user Correlator output for first user

•The multiplied signal will be pThe multiplied signal will be p22(t) = 1 for the (t) = 1 for the correct signal and will yield the dispersed correct signal and will yield the dispersed signal and can be demodulated to yield the signal and can be demodulated to yield the message signal mmessage signal mii(t).(t).

•The multiplied signal will be pThe multiplied signal will be p22(t) = 1 for the (t) = 1 for the correct signal and will yield the dispersed correct signal and will yield the dispersed signal and can be demodulated to yield the signal and can be demodulated to yield the message signal mmessage signal mii(t).(t).

1

1

iT1i 1 1 c 1 1

(i 1)T

Z (t) r(t)p (t )cos[2 f (t ) ]dt

1/ 21 s s 1 1 c 1S (t) (2E / T ) m (t)p (t)cos(2 f t )

3939

Probability of bit errorProbability of bit errorProbability of bit errorProbability of bit errorProbability of bit error Probability of bit error

PPee = Q {1/ [(K –1)/3N + (N = Q {1/ [(K –1)/3N + (N00/2E/2Ebb)])]1/21/2}}

KK = Number of users= Number of users

NN = Number of chips/ symbol= Number of chips/ symbol

Now when, ENow when, Ebb/N/Noo

PPee = Q{[3N/(K-1)] = Q{[3N/(K-1)]1/21/2 } }

Probability of bit error Probability of bit error

PPee = Q {1/ [(K –1)/3N + (N = Q {1/ [(K –1)/3N + (N00/2E/2Ebb)])]1/21/2}}

KK = Number of users= Number of users

NN = Number of chips/ symbol= Number of chips/ symbol

Now when, ENow when, Ebb/N/Noo

PPee = Q{[3N/(K-1)] = Q{[3N/(K-1)]1/21/2 } }

4040

Important Advantages of CDMAImportant Advantages of CDMAImportant Advantages of CDMAImportant Advantages of CDMA

Many users of CDMA use the same Many users of CDMA use the same frequency. Either TDD or FDD may be used.frequency. Either TDD or FDD may be used.

Multipath fading may be substantially Multipath fading may be substantially reduced because of large signal bandwidth.reduced because of large signal bandwidth.

There is no absolute limit on the number of There is no absolute limit on the number of users in CDMA. The system performance users in CDMA. The system performance gradually degrades for all users as the gradually degrades for all users as the number of users is increased.number of users is increased.

Many users of CDMA use the same Many users of CDMA use the same frequency. Either TDD or FDD may be used.frequency. Either TDD or FDD may be used.

Multipath fading may be substantially Multipath fading may be substantially reduced because of large signal bandwidth.reduced because of large signal bandwidth.

There is no absolute limit on the number of There is no absolute limit on the number of users in CDMA. The system performance users in CDMA. The system performance gradually degrades for all users as the gradually degrades for all users as the number of users is increased.number of users is increased.

4141

Drawbacks of CDMADrawbacks of CDMADrawbacks of CDMADrawbacks of CDMA

Self-jamming is a problem in a CDMA Self-jamming is a problem in a CDMA system. Self-jamming arise because the PN system. Self-jamming arise because the PN sequence are not exactly orthogonal, non-sequence are not exactly orthogonal, non-zero contributions from other users in the zero contributions from other users in the system arisesystem arise

The near- far problem occurs at a CDMA The near- far problem occurs at a CDMA receiver if an undesired user has high receiver if an undesired user has high detected power as compared to the desired detected power as compared to the desired user.user.

Self-jamming is a problem in a CDMA Self-jamming is a problem in a CDMA system. Self-jamming arise because the PN system. Self-jamming arise because the PN sequence are not exactly orthogonal, non-sequence are not exactly orthogonal, non-zero contributions from other users in the zero contributions from other users in the system arisesystem arise

The near- far problem occurs at a CDMA The near- far problem occurs at a CDMA receiver if an undesired user has high receiver if an undesired user has high detected power as compared to the desired detected power as compared to the desired user.user.

4242

Capacity of Cellular SystemsCapacity of Cellular SystemsCapacity of Cellular SystemsCapacity of Cellular Systems

Channel capacity for a radio system is Channel capacity for a radio system is defined as the maximum number of defined as the maximum number of channels or users that can be provided in a channels or users that can be provided in a fixed frequency band fixed frequency band spectrum efficiency spectrum efficiency of wireless system.of wireless system.

Channel capacity for a radio system is Channel capacity for a radio system is defined as the maximum number of defined as the maximum number of channels or users that can be provided in a channels or users that can be provided in a fixed frequency band fixed frequency band spectrum efficiency spectrum efficiency of wireless system.of wireless system.

4343

……Capacity of Cellular SystemsCapacity of Cellular Systems……Capacity of Cellular SystemsCapacity of Cellular Systems

For a Cellular SystemFor a Cellular System m = Radio Capacity Matrix = Bm = Radio Capacity Matrix = Btt / (B / (BCC * N) * N) BBtt = Total allocated spectrum for the = Total allocated spectrum for the

systemsystem BBCC = Channel bandwidth = Channel bandwidth NN = Number of cells in frequency reuse = Number of cells in frequency reuse

patternpattern

For a Cellular SystemFor a Cellular System m = Radio Capacity Matrix = Bm = Radio Capacity Matrix = Btt / (B / (BCC * N) * N) BBtt = Total allocated spectrum for the = Total allocated spectrum for the

systemsystem BBCC = Channel bandwidth = Channel bandwidth NN = Number of cells in frequency reuse = Number of cells in frequency reuse

patternpattern

4444

Channel capacity design Channel capacity design Channel capacity design Channel capacity design

CELL A CELL A

CELL A

CELL A

CELL ACELL A

4545

……Channel capacity design Channel capacity design for given C/I ratiofor given C/I ratio……Channel capacity design Channel capacity design for given C/I ratiofor given C/I ratio

Carrier to Interference ratio Carrier to Interference ratio

For maximum interference DFor maximum interference D00 = R = R

Carrier to Interference ratio Carrier to Interference ratio

For maximum interference DFor maximum interference D00 = R = R

no

n

o

DCI 6 D

D =Distance from desired base station to mobile

n

min

n

min

C 1 RI 6 D

C 1 QI 6

4646


Q Co- Channel reuse ratio

0.5Also ,Q (3 N)

min

C= 6 I

2(Q)Therefore , N 3 2 / n

min

C6 I eqn23

n

4747


t

2 / n

cmin

Bsubstituting, mCB 6 1/ 3I

t

(1/ 2)

cmin

BWhen n 4, m radio channels / cellsCB 2/ 3 I

min

CTypical Values of 18 dBfor Analog FM and12 dB for DigitalI

4848

Equation of C/I for digital Equation of C/I for digital cellular systemcellular systemEquation of C/I for digital Equation of C/I for digital cellular systemcellular system

b b(E R )CI I

c c(E R )I

b

b

c

c

R Channel bit rate

E Energy per bit

R Rate of channel code

E Energy per code symbol

4949

Comparison of FDMA and Comparison of FDMA and TDMA systems - FDMATDMA systems - FDMAComparison of FDMA and Comparison of FDMA and TDMA systems - FDMATDMA systems - FDMA The total bandwidth BThe total bandwidth Btt is divided into M is divided into M

channels, each with Bandwidth Bchannels, each with Bandwidth Bcc. The . The radio capacity for FDMA is given byradio capacity for FDMA is given by

The total bandwidth BThe total bandwidth Btt is divided into M is divided into M channels, each with Bandwidth Bchannels, each with Bandwidth Bcc. The . The radio capacity for FDMA is given byradio capacity for FDMA is given by

0.5

Mm

C23 I

b b

o c

o

C E R

I I B

where I Interference power /Hz

5050

TDMATDMATDMATDMA

Assume FDMA occupies the same Assume FDMA occupies the same spectrum as a single channel TDMA.spectrum as a single channel TDMA.

Assume FDMA occupies the same Assume FDMA occupies the same spectrum as a single channel TDMA.spectrum as a single channel TDMA.

b b

o c

b

b

b

C E R

I I B

where R Transmission rate of TDMA system

R Transmission rate of FDMA system

E Energy per bit

5151


Consider a FDMA system with 3 channels, Consider a FDMA system with 3 channels, each having a bandwidth of 10 KHz and each having a bandwidth of 10 KHz and transmission rate of 10 kbps. A TDMA transmission rate of 10 kbps. A TDMA system has 3 time slots, channel bandwidth system has 3 time slots, channel bandwidth of 30 KHz, and a transmission rate of 30 of 30 KHz, and a transmission rate of 30 kbps.kbps.

Consider a FDMA system with 3 channels, Consider a FDMA system with 3 channels, each having a bandwidth of 10 KHz and each having a bandwidth of 10 KHz and transmission rate of 10 kbps. A TDMA transmission rate of 10 kbps. A TDMA system has 3 time slots, channel bandwidth system has 3 time slots, channel bandwidth of 30 KHz, and a transmission rate of 30 of 30 KHz, and a transmission rate of 30 kbps.kbps.

5252

Example …Example …Example …Example …

For the TDMA scheme, the received carrier For the TDMA scheme, the received carrier to interference ratio for a single user is to interference ratio for a single user is measured for 1/3 of the time the channel is measured for 1/3 of the time the channel is in use.in use.

Compare the radio capacity of the 2 Compare the radio capacity of the 2 systems.systems.

For the TDMA scheme, the received carrier For the TDMA scheme, the received carrier to interference ratio for a single user is to interference ratio for a single user is measured for 1/3 of the time the channel is measured for 1/3 of the time the channel is in use.in use.

Compare the radio capacity of the 2 Compare the radio capacity of the 2 systems.systems.

5353

Solution Solution Solution Solution For FDMA systemFor FDMA system For FDMA systemFor FDMA system

4b b b

o c o

4b b

4o o

C E R E 10

I I B I 10 kHz

10E ECI I I10

5454

Solution …Solution …Solution …Solution … For TDMA systemFor TDMA system For TDMA systemFor TDMA system

4b

b b

3o c o

4b b

3oo

E 10C E R 0.333sI I B I 30 10

E 10 1 ECI 0.333s II 30 10

5555

Capacity of Digital Cellular CDMACapacity of Digital Cellular CDMACapacity of Digital Cellular CDMACapacity of Digital Cellular CDMA

Capacity of FDMA and TDMA system is Capacity of FDMA and TDMA system is bandwidth limited.bandwidth limited.

Capacity of CDMA system is interference Capacity of CDMA system is interference limited.limited.

The link performance of CDMA increases as The link performance of CDMA increases as the number of users decreases.the number of users decreases.

Capacity of FDMA and TDMA system is Capacity of FDMA and TDMA system is bandwidth limited.bandwidth limited.

Capacity of CDMA system is interference Capacity of CDMA system is interference limited.limited.

The link performance of CDMA increases as The link performance of CDMA increases as the number of users decreases.the number of users decreases.

5656

Single Cell SystemSingle Cell SystemSingle Cell SystemSingle Cell System

The cellular network consists of a large The cellular network consists of a large number of mobile users communicating number of mobile users communicating with a base station.with a base station.

The cell site transmitter consist of linear The cell site transmitter consist of linear combiner which adds spread signal of combiner which adds spread signal of individual users for the forward channel.individual users for the forward channel.

The cellular network consists of a large The cellular network consists of a large number of mobile users communicating number of mobile users communicating with a base station.with a base station.

The cell site transmitter consist of linear The cell site transmitter consist of linear combiner which adds spread signal of combiner which adds spread signal of individual users for the forward channel.individual users for the forward channel.

5757

Single Cell System …Single Cell System …Single Cell System …Single Cell System …

A pilot signal is also included in the cell-site A pilot signal is also included in the cell-site transmitter and is used by each mobile to transmitter and is used by each mobile to set its own power control for the reverse set its own power control for the reverse link.link.

A pilot signal is also included in the cell-site A pilot signal is also included in the cell-site transmitter and is used by each mobile to transmitter and is used by each mobile to set its own power control for the reverse set its own power control for the reverse link.link.

5858

Capacity of single cell systemCapacity of single cell systemCapacity of single cell systemCapacity of single cell system

Let the number of users be N and the Let the number of users be N and the signal power from each of N users be Ssignal power from each of N users be S

Let the number of users be N and the Let the number of users be N and the signal power from each of N users be Ssignal power from each of N users be S

SSignal to noise ratio SNR= [ N-1 S]

1N 1

5959

Bit energy-to-noise ratio Bit energy-to-noise ratio of single cell systemof single cell system Bit energy-to-noise ratio Bit energy-to-noise ratio of single cell systemof single cell system The bit energy to noise ratio is an important The bit energy to noise ratio is an important

factor in communication systemsfactor in communication systems The bit energy to noise ratio is an important The bit energy to noise ratio is an important

factor in communication systemsfactor in communication systems

b

o

SE R

N S(N 1)

W WR= Baseband information bit rateW= Total RF bandwidth, W

= Background thermal Noise

6060

Number of possible of Number of possible of single cell systemsingle cell systemNumber of possible of Number of possible of single cell systemsingle cell system

b

o

WE R

N(N 1)

S

6161

Number of users that Number of users that can access the systemcan access the system Number of users that Number of users that can access the systemcan access the system

b

o

WRN 1 SEN

Wwhere Pr ocessing GainR

6262

Number of users that Number of users that can access the system …..can access the system …..Number of users that Number of users that can access the system …..can access the system …..

In order to increase the capacity, the In order to increase the capacity, the interference due to other users should be interference due to other users should be reduced. There are mainly two techniques.reduced. There are mainly two techniques.

In order to increase the capacity, the In order to increase the capacity, the interference due to other users should be interference due to other users should be reduced. There are mainly two techniques.reduced. There are mainly two techniques.

6363

Techniques to improve capacityTechniques to improve capacityTechniques to improve capacityTechniques to improve capacity

Antenna Sectorization:Antenna Sectorization:

A cell site with 3 antennas, each having a A cell site with 3 antennas, each having a beamwidth of 1200 , has interference No’, beamwidth of 1200 , has interference No’, which is 1/3 of the interference received by which is 1/3 of the interference received by omni-directional antenna. This increases the omni-directional antenna. This increases the capacity by a factor of 3.capacity by a factor of 3.

Antenna Sectorization:Antenna Sectorization:

A cell site with 3 antennas, each having a A cell site with 3 antennas, each having a beamwidth of 1200 , has interference No’, beamwidth of 1200 , has interference No’, which is 1/3 of the interference received by which is 1/3 of the interference received by omni-directional antenna. This increases the omni-directional antenna. This increases the capacity by a factor of 3.capacity by a factor of 3.

6464

Techniques to improve capacity …Techniques to improve capacity …Techniques to improve capacity …Techniques to improve capacity …

Monitoring or Voice activity:Monitoring or Voice activity:

Each transmitter is switched off during Each transmitter is switched off during period of no voice activity. Voice activity is period of no voice activity. Voice activity is denoted by a factor denoted by a factor

Monitoring or Voice activity:Monitoring or Voice activity:

Each transmitter is switched off during Each transmitter is switched off during period of no voice activity. Voice activity is period of no voice activity. Voice activity is denoted by a factor denoted by a factor

6565

SNR Improvement SNR Improvement SNR Improvement SNR Improvement

b

os

s

WE R

NN 1 S

where N Number of users per sector

6666

SNR Improvement …SNR Improvement …SNR Improvement …SNR Improvement …

sb

o

WR1N 1 ,)0 1(SEN

If = 3/8 and number of sector is equal to 3 , SNR increases by a factor o .f 8

)

6767

CDMA Power ControlCDMA Power ControlCDMA Power ControlCDMA Power Control

In CDMA, the system capacity is controlled In CDMA, the system capacity is controlled if each mobile transmitter power level is if each mobile transmitter power level is controlled so that its signal arrives at the controlled so that its signal arrives at the cell site with minimum required S/I.cell site with minimum required S/I.

In CDMA, the system capacity is controlled In CDMA, the system capacity is controlled if each mobile transmitter power level is if each mobile transmitter power level is controlled so that its signal arrives at the controlled so that its signal arrives at the cell site with minimum required S/I.cell site with minimum required S/I.

6868

CDMA Power Control ...CDMA Power Control ...CDMA Power Control ...CDMA Power Control ...

If the signal power of all mobile transmitters If the signal power of all mobile transmitters within the area of cell site are controlled, within the area of cell site are controlled, then total signal power received at all site then total signal power received at all site from all mobile will be equal to average from all mobile will be equal to average received power times the number of mobiles received power times the number of mobiles operating in region of coverage.operating in region of coverage.

Optimal power is desired, never too weak or Optimal power is desired, never too weak or too strong.too strong.

If the signal power of all mobile transmitters If the signal power of all mobile transmitters within the area of cell site are controlled, within the area of cell site are controlled, then total signal power received at all site then total signal power received at all site from all mobile will be equal to average from all mobile will be equal to average received power times the number of mobiles received power times the number of mobiles operating in region of coverage.operating in region of coverage.

Optimal power is desired, never too weak or Optimal power is desired, never too weak or too strong.too strong.

6969

ExampleExampleExampleExample If W = 1.25 MHz, R= 9600 bps, and a If W = 1.25 MHz, R= 9600 bps, and a

minimum acceptable Eminimum acceptable Ebb/ N/ Noo is 10 dB, is 10 dB, determine the maximum number of users determine the maximum number of users that can be supported in a single cell CDMA that can be supported in a single cell CDMA system using system using

(a) omni directional base station antennas (a) omni directional base station antennas and no voice activity detection and no voice activity detection

(b) 3 sectors at base station and (b) 3 sectors at base station and = 3/8. = 3/8. Assume the system is interference limited. Assume the system is interference limited. = 0. = 0.

If W = 1.25 MHz, R= 9600 bps, and a If W = 1.25 MHz, R= 9600 bps, and a minimum acceptable Eminimum acceptable Ebb/ N/ Noo is 10 dB, is 10 dB, determine the maximum number of users determine the maximum number of users that can be supported in a single cell CDMA that can be supported in a single cell CDMA system using system using

(a) omni directional base station antennas (a) omni directional base station antennas and no voice activity detection and no voice activity detection

(b) 3 sectors at base station and (b) 3 sectors at base station and = 3/8. = 3/8. Assume the system is interference limited. Assume the system is interference limited. = 0. = 0.

7070

SolutionSolutionSolutionSolution(a)(a)(a)(a)

b

o

WRN 1 SEN

1.25 10

96001 010

1 13.02 14

7171

Solution …Solution …Solution …Solution …(b)(b)(b)(b)

b

o

WRN 1 SEN

1.25 101 96001 03 10

835.7

7272

Solution …Solution …Solution …Solution …

Total amount of Total amount of users Nusers N

Total amount of Total amount of users Nusers N

s3N

3 35.7107 users / cell

Multiple acces techniques

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