1

Date: 5th July 2011Date: 5th July 2011 Section –IIMultiple Access Techniques for wireless

communication

Objectives:a) To study the duplexing concept and it’s

types i.e. Frequency division duplexing (FDD) and Time division duplexing (TDD) B) To study Frequency Division Multiple

access (FDMA)

Section –IIMultiple Access Techniques for wireless

communication

Objectives:a) To study the duplexing concept and it’s

types i.e. Frequency division duplexing (FDD) and Time division duplexing (TDD) B) To study Frequency Division Multiple

access (FDMA)

2

Multiple Access Techniques For Wireless CommunicationMultiple Access Techniques For Wireless Communication

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

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

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

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

3

Multiple Access Techniques Multiple Access Techniques

4

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

Cellular Systems MA Technique AMPS ( Advanced Mobile Phone system )

FDMA / FDD

GSM ( Global System for Mobile )

TDMA / FDD

US DC ( U. S Digital Cellular )

TDMA / FDD

JDC ( Japanese Digital Cellular )

TDMA / FDD

5

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

DECT ( Digital European Cordless Telephone )

FDMA / FDD

IS – 95 ( U.S Narrowband Spread Spectrum )

CDMA / FDD

6

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

code

time

frequency

C1 C2 Cn

CnC2C1

frequency

Figure 1

7

Principles Of OperationPrinciples Of Operation

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

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

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

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

8

…Principles Of Operation…Principles Of Operation

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

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

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

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

9

Properties of FDMAProperties of FDMA

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

ISI is low since the symbol time is large compared to average delay spread

FDMA systems are simple than TDMA systems, but modern DSP is changing this factor.

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

ISI is low since the symbol time is large compared to average delay spread

FDMA systems are simple than TDMA systems, but modern DSP is changing this factor.

10

… Properties of FDMA… Properties of FDMA

FDMA systems have higher cost Cell site system due to single

call/carrier Costly band pass filters to eliminate

spurious radiation Duplexers in both T/R increase

subscriber costs FDMA requires tight RF filtering to

minimize adjacent channel interference

FDMA systems have higher cost Cell site system due to single

call/carrier Costly band pass filters to eliminate

spurious radiation Duplexers in both T/R increase

subscriber costs FDMA requires tight RF filtering to

minimize adjacent channel interference

11

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

gB gB

tB

t g

c

g

c

B 2BN

B

B GuardBand

B ChannelBandwidth

12

ExampleExample

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

t

g

c

6 3

3

B 12.5MHz

B 10KHz

B 30KHz

(12.5 10 ) 2(10 10 )N 416

30 10

13

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

code

time

frequency

C1

Cn

CnC2C1

time

Figure 2

14

Principles Of OperationPrinciples Of Operation

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

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

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

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

15

TDMA Frame StructureTDMA Frame Structure

Preamble Information message

Trail Bits

Slot 1 Slot 2 Slot N

Trail Bit Sync Bit Information Bit

Guard Bits

Figure 3

16

Components of 1 TDMA FrameComponents of 1 TDMA Frame

Preamble Address and synchronization information for base station and subscriber identification

Guard times Synchronization of receivers between a different slots and frames

Preamble Address and synchronization information for base station and subscriber identification

Guard times Synchronization of receivers between a different slots and frames

17

Principles Of Operation Principles Of Operation

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

Data Transmission for user of TDMA system is not continuous, but discrete bursts

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

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

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

Data Transmission for user of TDMA system is not continuous, but discrete bursts

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

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

18

…Principles Of Operation…Principles Of Operation

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

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

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

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

19

Efficiency of TDMA:Efficiency of TDMA:

Efficiency of a TEMA system is a measure of the % of transmitted data that contains information as opposed to providing overhead for the access scheme

Efficiency of a TEMA system is a measure of the % of transmitted data that contains information as opposed to providing overhead for the access scheme

20

Efficiency of TDMA (13th Sept. 10)Efficiency of TDMA (13th Sept. 10)

Frame Efficiency : is the % of bits per frame which contain transmitted data

Frame Efficiency : is the % of bits per frame which contain transmitted data

f

No.ofbits / frame containingtransmitted dataTotal Numberof bits / frame

OH T

T OH

T

(1 b / b ) 100

(b b )100

b

21

Frame 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

22

…Frame efficiency parameters…Frame 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

23

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

24

Example Example

GSM System uses a TDMA / FDD system.

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

GSM System uses a TDMA / FDD system.

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

25

…Example…Example

1. Time duration of a bit2. Time duration of a slot3. Time duration of a frame and 4. How long must a user occupying a single

slot must wait between two simultaneous transmissions?

1. Time duration of a bit2. Time duration of a slot3. Time duration of a frame and 4. How long must a user occupying a single

slot must wait between two simultaneous transmissions?

26

Solution Solution • Time duration of a bit

• Time duration of a slot

• Time duration of a bit

• 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

27

…Solution…Solution

• Time duration of a frame

• A user has to wait 4.615 ms before next transmission

• Time duration of a frame

• A user has to wait 4.615 ms before next transmission

slot8 T 4.615ms

28

ExampleExample

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

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

29

SolutionTime slots have 6 + 8.25 + 26 + 2(58) = 156.25 bits.A frame has 8 * 156.25 = 1250 bits / frame.

30

…Example…Example

The number of overhead bits per frame is given by

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

= 74.24 %

The number of overhead bits per frame is given by

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

= 74.24 %

31

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

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

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

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

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

32

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

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

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

33

Types Of Spread Spectrum Techniques Types Of Spread Spectrum Techniques

Frequency Hopped Multiple Access ( FHMA )

Direct Sequence Multiple Access ( CDMA )

Frequency Hopped Multiple Access ( FHMA )

Direct Sequence Multiple Access ( CDMA )

34

FHMA:FHMA: Frequency hopped multiple access (FHMA)

is a digital multiple access system in which the carrier frequencies of individual users are varied in a pseudorandom fashion within a wideband channel.

It allows multiple users to simultaneously occupy the same spectrum at the same time.

User data is broken in to uniform sized bursts which are transmitted on different channels within allocated spectrum band.

Frequency hopped multiple access (FHMA) is a digital multiple access system in which the carrier frequencies of individual users are varied in a pseudorandom fashion within a wideband channel.

It allows multiple users to simultaneously occupy the same spectrum at the same time.

User data is broken in to uniform sized bursts which are transmitted on different channels within allocated spectrum band.

35

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

code

time

frequency

C1

C2

C3

Cn

Figure 4

36

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

1m (t)

2m (t)

1S (t)

1

k

1PN (t)

KPN (t)

c 1cos(2 f t )

r(t)

c kcos(2 f t )

37

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

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

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

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

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

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

38

CDMA ReceiverCDMA Receiver

c kcos(2 f t )

r(t)km (t)

(.)dt

kiZ (t)

KPN (t)

39

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

At the receiver, the received signal is correlated with the appropriate signature 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

40

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

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

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

NTC = T

where TC is the chip period.

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

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

NTC = T

where TC is the chip period.

41

Example: Example: Assume N=4

PN Wave for N =4

1

-1

-1

1

42

Correlator output for first user Correlator output for first user

•The multiplied signal will be p2(t) = 1 for the correct signal and will yield the dispersed signal and can be demodulated to yield the message signal mi(t).

•The multiplied signal will be p2(t) = 1 for the correct signal and will yield the dispersed signal and can be demodulated to yield the message signal mi(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 )

43

Probability of bit errorProbability of bit errorProbability of bit error

Pe = Q {1/ [(K –1)/3N + (N0/2Eb)]1/2}

K = Number of users

N = Number of chips/ symbol

Now when, Eb/No

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

Probability of bit error

Pe = Q {1/ [(K –1)/3N + (N0/2Eb)]1/2}

K = Number of users

N = Number of chips/ symbol

Now when, Eb/No

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

44

Features of CDMA :Features of CDMA : Many users of a CDMA system share same

frequency. Either TDD or FDD may be used There is no absolute limit on the number of

users in CDMA. Multipath fading may be substantially

reduced because the signal is spread over a large spectrum. i.e. if the spread spectrum bandwidth is greater than the coherence B.W. of channel, the inherent frequency diversity will tone down the effects of small scale fading.

Many users of a CDMA system share same frequency. Either TDD or FDD may be used

There is no absolute limit on the number of users in CDMA.

Multipath fading may be substantially reduced because the signal is spread over a large spectrum. i.e. if the spread spectrum bandwidth is greater than the coherence B.W. of channel, the inherent frequency diversity will tone down the effects of small scale fading.

45

Features of CDMA :Features of CDMA : Since CDMA uses co-channel cells, it can

use macroscopic, spatial diversity to Provide soft handoff. Soft handoff is performed by the MSC, which can simultaneously monitor a particular user from two or more base stations.

Self-jamming is a problem in CDMA system. Self-jamming arises from the fact that the

spreading sequences of different users are not exactly orthogonal The near—far problem occurs at a CDMA receiver if

an undesired user has a high detected power as compared to the desired user

Since CDMA uses co-channel cells, it can use macroscopic, spatial diversity to Provide soft handoff. Soft handoff is performed by the MSC, which can simultaneously monitor a particular user from two or more base stations.

Self-jamming is a problem in CDMA system. Self-jamming arises from the fact that the

spreading sequences of different users are not exactly orthogonal The near—far problem occurs at a CDMA receiver if

an undesired user has a high detected power as compared to the desired user

46

Important Advantages of CDMAImportant Advantages of CDMA

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

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

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

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

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

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

47

Drawbacks of CDMADrawbacks of CDMA

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

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

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

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

48

Space Division Multiple Access (SDMA)Space Division Multiple Access (SDMA) Space division multiple access (SDMA)

controls the radiated energy for each user in space

SDMA serves different users by using spot beam antennas.

Space division multiple access (SDMA) controls the radiated energy for each user in space

SDMA serves different users by using spot beam antennas.

49

50

51

Capacity of Cellular SystemsCapacity of Cellular Systems

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

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

52

…Capacity of Cellular Systems…Capacity of Cellular Systems

For a Cellular System m = Radio Capacity Matrix = Bt / (BC * N) Bt = Total allocated spectrum for the

system BC = Channel bandwidth N = Number of cells in frequency reuse

pattern

For a Cellular System m = Radio Capacity Matrix = Bt / (BC * N) Bt = Total allocated spectrum for the

system BC = Channel bandwidth N = Number of cells in frequency reuse

pattern

53

Channel capacity design Channel capacity design

CELL A CELL A

CELL A

CELL A

CELL ACELL A

54

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

Carrier to Interference ratio

For maximum interference D0 = R

Carrier to Interference ratio

For maximum interference D0 = 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

55

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

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

56

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

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

57

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

58

Comparison of FDMA and TDMA systems - FDMAComparison of FDMA and TDMA systems - FDMA The total bandwidth Bt is divided into M

channels, each with Bandwidth Bc. The radio capacity for FDMA is given by

The total bandwidth Bt is divided into M channels, each with Bandwidth Bc. The radio 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

59

TDMATDMA

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

Assume FDMA occupies the same 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

60

ExampleExample

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

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

61

Example …Example …

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

Compare the radio capacity of the 2 systems.

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

Compare the radio capacity of the 2 systems.

62

Solution Solution For FDMA system For 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

63

Solution …Solution … For TDMA system For 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

64

Capacity of Digital Cellular CDMACapacity of Digital Cellular CDMA

Capacity of FDMA and TDMA system is bandwidth limited.

Capacity of CDMA system is interference limited.

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

Capacity of FDMA and TDMA system is bandwidth limited.

Capacity of CDMA system is interference limited.

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

65

Single Cell SystemSingle Cell System

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

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

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

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

66

Single Cell System …Single Cell System …

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

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

67

Capacity of single cell systemCapacity of single cell system

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

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

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

1N 1

68

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

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

factor in communication systems

b

o

SE R

N S(N 1)

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

= Background thermal Noise

69

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

b

o

WE R

N(N 1)

S

70

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

b

o

WRN 1 SEN

Wwhere Pr ocessing GainR

71

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

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

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

72

Techniques to improve capacityTechniques to improve capacity

Antenna Sectorization:

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

Antenna Sectorization:

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

73

Techniques to improve capacity …Techniques to improve capacity …

Monitoring or Voice activity:

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

Monitoring or Voice activity:

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

74

SNR Improvement SNR Improvement

b

os

s

WE R

NN 1 S

where N Number of users per sector

75

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

)

76

CDMA Power ControlCDMA Power Control

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

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

77

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

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

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

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

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

78

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

minimum acceptable Eb/ No is 10 dB, determine the maximum number of users that can be supported in a single cell CDMA system using

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

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

If W = 1.25 MHz, R= 9600 bps, and a minimum acceptable Eb/ No is 10 dB, determine the maximum number of users that can be supported in a single cell CDMA system using

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

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

79

SolutionSolution(a)(a)

b

o

WRN 1 SEN

1.25 10

96001 010

1 13.02 14

80

Solution …Solution …(b)(b)

b

o

WRN 1 SEN

1.25 101 96001 03 10

835.7

81

Solution …Solution …

Total amount of users N

Total amount of users N

s3N

3 35.7107 users / cell

82

Space Division Multiple Access (SDMA)Space Division Multiple Access (SDMA)

It controls the radiated energy for each user in space

SDMA serves different users by using spot beam antennas

It controls the radiated energy for each user in space

SDMA serves different users by using spot beam antennas

83

84

85

86

87

88

89

90

91

92

93