1

Multiple Access Techniques

2

Brief Overview of Mobile Communications

• Earliest Systems used by Military and Police– Simplex

– Bulky

• Late 1940’s: Push to Talk FM Systems– Half Duplex Mode

– 120 kHz RF Bandwidth (3 kHz used)

• Improved Mobile Telephone Service (ITMS)– Fully Duplex, Auto-dial, Auto-trunking Phone Systems

– Quickly Became Saturated in Major Markets

3

Advent of the Cellular Concept• Techniques and Theory Developed in 1950’ & 60’s

• AT&T Proposes Cellular System to FCC in 1968

• 1983 FCC Authorizes US Advanced Mobile Phone System (AMPS)– Deployed in Chicago– 666 Duplex Channels (40 MHz in the 800MHz band)

• Late 1991- U.S. Digital Cellular (USDC) Implemented

• Code Division Multiple Access Developed by Qualcomm.

• New Personal Communication Service– Licenses in the 1800/1900 MHz Band Auctioned

4

Techno-politics of Spectrum

• Fundamental Driver of New Technology

• Progressive Governmental Policies Vital

• Explosive Growth of Wireless Market

5

Future Wireless Industry Growth Tied to:

• Governmental Regulatory Decisions

• Radio Spectrum Allocations

• Consumer Needs

• Technological Advances in the Areas of :– Signal Processing– Access– Network Areas

6

Multiple Access

• Term has its Origin in Satellite Communications

• System of Earth Stations and a Satellite

• Used to Mean Sharing a Communications Channel (of M Hz) among a Group of Users

• Signal Space of Time Bandwidth TW– Where T = k/R– Signal Space D = 2TW

7

Partition of Signal Space

8

Duplex Transmission

• Frequency Division Duplexing

9

Frequency/Time Division Duplexing

10

Trade-offs between FDD and TDD

• Frequency Division Duplexing– Geared toward Individual Channels for each User

– Frequency Separation must use Inexpensive Technology

• Time Division Duplexing– Eliminates Need for Forward and Reverse Channels

– Time Latency

11

Multiple Access Techniques

• Frequency Division Multiple Access (FDMA)

• Time Division Multiple Access (TDMA)

• Code Division Multiple Access (CDMA)

• Grouped as Narrowband or Wideband Systems

12

Narrowband Systems

• “Narrow” relates single channel BW to coherent BW• Normally operated using Frequency Division Duplexing• Narrowband FDMA

– Channel not Shared with other users

– Called FDMA/FDD Access Systems

• Narrowband TDMA– Users Separated by Time

– Called TDMA/FDD or TDMA/TDD Access Systems

13

Wideband Systems

• Transmission BW is much larger than Coherence BW

• Multipath Fading is not a Problem

• Users Transmit in a Large Part of the Spectrum

• Many Transmitters use the Same Channel using TDMA

• TDMA Allocates Time Slots

14

Frequency Division Multiple Access

• Users Receive Unique Channel

• Channels Assigned as Users Request Service

15

FDMA vs. TDMA• If an FDMA channel not in use, it is idle, and can not be used by other users.• After the assignment the reverse and forward channel may transmit simultaneously and

continuously• FDMA implemented as Single channel per carrier (SCPC), and is narrow band 30 KHz• The symbol time is large as compared to average delay spread. ISI is low, and no

equalization required for NB FDMA• FDMA mobile system is less complex as compared to TDMA. Due to advancement in DSP

this is changing• FDMA is continuous tx scheme, so fewer overhead bits are required as compared to

TDMA• FDMA systems have higher cell site costs as compared to TDMA, because of SCPC in

FDMA, and use of expensive pass band filters to eleminate the spurious radiations at base stations.

• The FDMA mobile unit uses duplexers, since both TX and RCVR operate at the same time, resulting in an incresae in the cost of mobie units and base stations

• FDMA requires tight RF filtering to minimize adjacent channel interference

16

Multiple Access Techniques used in wireless systems

17

Nonlinear Effects in FDMA

• Antenna at Base Station Shared by Channels

• Nonlinearities of Power Amps and Combiners

• Results in Signal Spreading – Generates Intermodulation– Causes Adjacent Channel Interference– And Adjacent Service Interference

18

Example 9.1

• Find the IM products, if base station antenna transmits 2 carrier frequencies, at 1930, and 1932 MHz, that are amplified by a saturated clipping amplifier. If mobile radi band is allocated from 1920 to 1940 Mhz, find the in-band and out of band IM products.

• Solution• IM products are mf1+nf2 for all integer values of m and n.• Possible IM products are:• (2n+1)f1-2nf2, (2n+2)f1-(2n+1)f2, (2n+1)f2-2nf, (2n+2)f2-

(2n+1)f1 for n=0,1,2-----

N=0 N=1 N=2 N=3

1930 1926 1922 1918

1928 1924 1920 1916

1932 1936 1940 1944

1934 1938 1942 1946

19

Advanced Mobile Phone System (AMPS)

• First U.S. Analog Cellular System• Based on FDMA/FDD• NBFM Modulates the Carrier

• Total Number of Channels is Given by:

Example: Given allocated BW =12.5 Mhz for each simplex band.Guard band = 10 Khz, and Channel BW =30 Khz. Find the number of channels

N =(12.5*10^3-20)/30 = 416

In US each cellular carrier is allocated 416 channels

20

Time Division Multiple Access• Enables Users to Use the Whole Bandwidth on a time Basis

21

TDMA Time Frame

• Data Transmitted in a Buffer-and-Burst Method

22

Time Division Multiple Access Features

• In TDMA a single carrier frequency with a wide bandwidth is shared among multiple users. Ecah user is assigned non-overlapping time slot.

• Number of time slots per frame depends on (1) available bandwidth, (2) modulation techniques etc

• Transmission for TDMA users is not continuous, but occurs in bursts, resulting in low battery consumption. The subscriber transmitter may be turned off during non-transmission periods

• Hand off process is simpler for a subscriber, sinc eit can listen to other base stations during non-transmit times. – AN ENHANCED LINK CONTROL SUCH AS THAT PROVIDED BY

MAHO CAN BE implemented by a subscribet by listening in an idle time slot in the TDMA frame

– TDMA uses different time slots for TX and reception, thus duplexers are not required.

• Even in TDMA/FDD a switch rather than a duplexer is required in the mobile unit to switch between the TX and RCRVR

23

Time Division Multiple Access Features (Cont’d)

• Adaptive equalization is usually required since Tx speed for a TDMA system is higher as compared to FDMA

• In TDMA the guard band should be minimized. IF the TX signal at the edges of a time slot are suppressed sharply in order to reduce the guard band, the TX spectrum will expand, and cause interference to the adjacent channels

• A high synchronization overhead is required, because of burst transmission.

• TDMA transmissions are slotted, this requires the receievr to be synchronized for each data burst. In addition guard slots are necessary, to separate users. TDMA systems have larger overhead as compared to FDMA

• TDMA is more flexible, as a number of time slots may be combined to give a higher capacity to a users. Further more number of slots combined can be varied for serving different user requirements. Bandwidth on demand

24

Efficiency of TDMA

25

Frame Efficiency for TDMA

• bOH = Overhead bits

• Nr = Number of ref bursts per frame

• br = number of overhead bits per ref burst

• Nt = Traffic bursts per farme

• bp = Number of preamble bits traffic burst

26

Number of Channels in TDMA System

• Number of Channels is Total Number of Slots Multiplied by the Channels Available

Example 9.3Consider GSM system: TDMA/FDDForward link BW = 25 MHz, which is segmented into 200 KHZ channels. IF 8 speech channels are supported on a single radio channelFind the number of simultaneous users Assume no guard bandSolutionNumber of channels = 8* 25 *10^3/200*10^3 =1000

27

Example 9.4/9.5

• Each frame in GSM supports 8 time slots• Each time slot transmits 156.25 bits, and data is transmitted at

270.833 Kbits in a channel, determine• (1) Time duration of a bit (2) time duration of a slot, (3) time

duration of a frame, (4) how long a user must wait between two transmissions, assuming the user is using one time slot

• Example 9.5• TDMA efficiency

– If a normal time slot 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

28

Solution 9.4/9.5

1. Bit duration : Tb = 1/270.833 Kbps = 3.692 microsecs

2. Slot duration = 156.25* 3.692 =0.577 milliseconds

3. Frame duration = 8 * 0.577 = 4.615 milli secs

4. A user has to wait for a duration of a frame before it gets a chance for its next transmission

Solution for 9.5

1. A time slot = 6+8.25+26+2*58 = 156.25 bits

2. A frame has 8* 156.25 = 1250 bits

3. Number of overhead bits per farme = 8 *(6+8.25+26) =322 bits

4. Frame efficiency = (1250 -322)/1250 = 74.24 %

29

Overview of Wireless Systems and Standards

30

Characteristics of AMPS

31

Characteristics of USDC IS 54

32

Characteristics of GSM

33

Capacity Gains of Digital over AMPS

34

Comparison of Standards

35

Data Rates and Efficiency

36

References

• On-Ching Yue, “Spread Spectrum Mobile Radio, 1977-1982,” IEEE Trans. On Vehicular Technology, Febuary 1983

• Rappaport, T. S., “Wireless Communication,” Prentice Hall, 1996

• Shankar, P. M., “Introduction to Wireless Systems,” J. W. Wiley & Sons, 2002

37

Multiple Access Techniques for Wireless Communication

38

Overview

• Spread Spectrum Multiple Access

• Space Division Multiple Access

• Packet Radio

39

Spread Spectrum Multiple Access

• Frequency Hopped Multiple Access(FHMA)• Code Division Multiple Access• Hybrid Spread Spectrum Techniques

40

Spread Spectrum multiple access (SSMA) uses signals which have a transmission bandwidth that is several orders of magnitude greater than the minimum required RF bandwidth. A PN sequence converts a narrowband signal to a wideband noise-like signal before transmission.

Two main types of SSMA:

1. Frequency hopped multiple access (FH)

2. Direct sequence multiple access (DS)

Code division multiple access(CDMA)

Advantage:

1. Immune to multipath interference and robust multiple access capability.

2. Efficient in a multiple user environment

Spread Spectrum multiple access (SSMA)

41

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

• In a FH transmitter:

1. The digital data is broken into uniform sized bursts which are transmitted on different carrier frequencies.

2. The instantaneous bandwidth of any one transmission burst is much smaller than the total spread bandwidth.

3. The pseudorandom change of the carrier frequencies of the user randomizes the occupancy of a specific channel at any given time.

Frequency Hopped Multiple Access (FHMA)

42

In FH receiver:

1. A locally generated PN code is used to synchronize the receivers instantaneous frequency with that of the transmitter.

2. At any given point in time, a frequency hopped signal only occupies a single, relatively narrow channel since narrowband FM or FSK is used.

3. FHMA systems often employ energy efficient constant envelope modulation.

4. Linearity is not an issue, and the power of multiple users at the receiver does not degrade FHMA performance.

Frequency Hopped Multiple Access (FHMA) (Cont’d)

43

The difference between FHMA and FDMA

The difference between FHMA and FDMA is that the frequency hopped signal changes channels at rapid interval.

If the rate of change of the carrier frequency is greater than the symbol rate, it is referred to as a fast frequency hopping. (FDMA)

If the channel changes at a rate less than or equal to the symbol rate, it is called slow frequency hopping system.

44

Advantage of FH system

1. A frequency hopped system provides a level of security, since an unintended receiver that does not know the pseudorandom sequence of frequency slots must retune rapidly to search for the signal it wishes to intercept.

2. FH signal is somewhat immune to fading, since error control coding and interleaving can be used to protect the frequency hopped signal against deep fades which may occasionally occur during the hopping sequence.

FH system

45

Code Division Multiple Access CDMA

• The spreading signal is a pseudo-noise code sequence that has a chip rate which is orders of magnitudes greater than the data rate of the message.

• All users in CDMA system, use the same carrier frequency and may transmit simultaneously.

• In CDMA, the narrowband message signal is multiplied by a very large bandwidth signal called the spreading signal.

• Each user has its own pseudorandom codeword which is approximately orthogonal to all other codewords.

• The receiver performs a time correlation operation to detect only the specific desired codeword.

46

SSMA

47

The near-far problem

• when many mobile users share the same channel, the strongest received mobile signal will capture the demodulator at a base station.

• In CDMA, stronger received signal levels raise the noise floors at the base station demodulators for the weaker signals, thereby decreasing the probability that weaker signals will be received.

CDMA (Cont’d)

48

Solution: power control

• Power control is provided by each base station in a cellular system and assures that each mobile within the base station coverage area provides the same signal level to the base station receiver. This solves the problem of a near by subscriber over powering the base station

• Power control is implemented at the base station by rapidly sampling the radio signal strength indicator levels of each mobile and then sending a power change command over the forward radio link.

CDMA (Cont’d)

49

The features of CDMA1. Many users of a CDMA system share the same frequency.

2. CDMA has a soft capacity. Increasing the number of users in a CDMA system raises the noise floor in a linear manner.

3. Multipath fading may be substantially reduced, because the signal is spread over a large bandwidth. If the spread spectrum BW greater than the coherence BW of the channel, the inherent freq diversty will mitigate the efefcts of small scale fading.

4. Channel data rates are very high in CDMA system. A Chip (symbol duration) is usually much smaller than the delay spread. The PN sequences have low autocorrelation, multipath which is delayed by more than a chip will appear as noise. A rake receiver can be used to improve reception by collecting time delayed versions of the signal

50

The features of CDMA (cont’d)

• Macroscopic Spatial Diversity

• CDMA uses co-channel cells it can use macroscopic spatial diversity to provide soft handoff. Soft handoff is performed by the MSC, which can monitor the user from 2 or more base stations. MSC then chooses the best version of the signal at any time.

• Self Jamming

• It arises because the PN codes of the users are not exact orthogonal. Hence in despreading of a specific code, there may be non-zero contributions from other users, which influences the receiver decision process

• The near-far problem occurs in CDMA system.

51

Comparison of DS and FH system

DS FH

Bandwidth PN sequence clock rate or chip rate

The tuning range of frequencies

Synchronization Very crucial Less critical

Spectrum Very wide narrow

Near-far problem More likely to occur Less likely to occur

52

Feature FDMA TDMA CDMA

High carrier frequency stability

Required Not necessary Not necessary

Timing/synchronization Not required Required Required

Near-far problem No No Yes,power control tech.

Variable transmission rate

Difficult Easy Easy

Fading mitigation Equalizer not needed

Equalizer may be needed

RAKE receiver possible

Power monitoring Difficult Easy Easy

Zone size Any size Any size Large size difficult

Comparison of FDMA, TDMA, CDMA

53

1. Hybrid FDMA/CDMA(FCDMA)

2. Hybrid Direct Sequence/Frequency Hopped Multiple Access(DS/FHMA)

3. Time Division CDMA(TCDMA)

4. Time Division Frequency Hopping(TDFH)

Hybrid Spread Spectrum Techniques

54

The available wideband spectrum is divided into a number of subspectras with smaller bandwidths. Each of these smaller subchannels becomes a narrowband CDMA system having processing gain lower than the original CDMA system.

Hybrid FDMA/CDMA(FCDMA)

Advantage: the required bandwidth need not be contiguous and different users and be allotted different subspectrum bandwidths depending on their requirement.

55

Advantage: they avoid the near-far effect.

Disadvantage: they are not adaptable to the soft handoff process.

Hybrid Direct Sequence/Frequency Hopped Multiple Access(DS/FHMA)

This technique consists of a direct sequence modulated signal whose center frequency is made to hop periodically in a pseudorandom fashion.

56

Different spreading codes are assigned to different cells. Within each cell, only one user per cell is allotted a particular time slot.Thus at any time, only one CDMA user is transmitting in each cell. When a handoff takes place, the spreading code of the user is changed to that of the new cell.

Advantage: it avoids the near-far effect .

Time Division CDMA(TCDMA)

57

Time Division Frequency Hopping (TDFH)

Time Division Frequency Hopping(TDFH)• The subscriber can hop to a new frequency at the

start of a new TDMA frame. In GSM standard, hopping sequence is predefined and the subscriber is allowed to hop only on certain frequencies which are assigned to a cell.

• Advantage: – Avoiding a severe fade or erasure event on a

particular channel.– Avoiding the co-channel interference

problems between neighboring cells if two interfering base station transmitters are made to transmit on different frequencies at different times.

58

SDMA controls the radiated energy for each user in space. From Fig 9.8., we see that different areas covered by the antenna beam may be served by the same frequency or different frequencies.

Space Division Multiple Access(SDMA)

59

Space Division Multiple Access(Cont’d))

• Sectorized antennas may be thought of as a primitive application of SDMA. In the future, adaptive antennas will likely be used to simultaneously steer energy in the direction of many users at once and appear to be best suited for TDMA and CDMA base station architectures.

60

Packet Radio

• Packet Radio Protocols

• Carrier Sense Multiple Access(CSMA) Protocol

• Reservation Protocols

61

Packet Radio

• In packet radio (PR) access techniques, many subscribers attempt to access a single channel in an uncoordinated( or minimally coordinated) manner. Collisions from the simultaneous transmissions of multiple transmitters are detected at the base station receiver, in which case an ACK or NACK signal is broadcast by the base station to alert the desired user of received transmission.

• The subscribers use a contention technique to transmit on a common channel. ALOHA protocols are the best examples of contention techniques.

• The performance of contention techniques can be evaluated by the throughput(T), and the average delay(D).

62

pV

Packet Radio Protocol

• Vulnerable period : the time interval during which the packets are susceptible to collisions with transmissions form other users.

63

Assumption:• All packets sent by all users have a constant packet length

and fixed, channel data rate.• All other users may generate new packets at random time

intervals.• Packet transmissions occur with a Poisson distribution

having a mean arrival rate of packets per second.

The traffic occupancy or throughput R:

R

Is theis the packet duration in seconds

Is the mean arrival rate in packets per second

Packet Radio Protocol (Cont’d)

64

]Pr[]Pr[ collisionnocollisionnoRT

T: the normalized throughput.

Pr[no collision]: the probability of a user making a successful packet transmission

!)Pr(

n

eRn

Rn

Pr(n): the probability that n packets are generated by the user population during a given packet duration interval is assumed to be Poisson

Packet Radio Protocol (Cont’d)

A packet is assumed to be successfully transmitted if no other packets are transmitted during the given packet interval. The prob that no packets are generated during this interval is given by (1) with n =0 , ie Pr (0) = e -R

(1)

65

Packet contention protocols

• Based on the type of access, contention protocols are categorized as :– Random access

• No contention among the users, and packets are transmitted, as they arrive from the user

– Scheduled access• Based on coordinated access by the users. Messages

are transmitted within assigned time slots

– Hybrid access

66

The pure ALOHA protocol is a random access protocol used for data transfer. A user accesses the channel as soon as the packet is ready to be transmitted. Then it waits for an ack or nack, if it gets a nack (a collision) then it waits for a random amount of time and then retransmits the packet. The delay increases as the number of users increases, as the prob of collision increases

!

)2()Pr(

2

n

eRn

Rn

The vulnerable period is , the probability of no collision during the vulnerable period Pr(n):

The throughput:RT 2Re

2

At n=0

Pure ALOHA

(9.11)

67

In slotted ALOHA, time is divided into equal time slots of length greater than the packet duration . The subscribers have a synchronized time clock and transmit only at the beginning of the time slot.

The vulnerable period for slotted ALOHA is only one packet duration.

The probability that no other packets will be generated during the vulnerable period is .Re

The throughput:

RT Re

Slotted ALOHA

(9.12)

68

Fig(9.10) shows how ALOHA and slotted ALOHA systems trade-off throughput for delay

Fig(8.10)

ALOHA Vs. Slotted ALOHA

69

Example 9.6

Determine the maximum throughput that can be achieved using ALOHA and slotted ALOHA protocols.

Solution:The rate of arrival which maximizes the throughput for ALOHA is found by taking the derivative of Eq(9.11) and equating it to zero.

Maximum throughput achieved by using the ALOHA protocol is found by substituting in Eq(9.11), and this value can be seen as the maximum throughput in Fig(9.10)

2

1

0Re2

max

22

R

edR

dT RR

RT 2Re For ALOHA

Throughput = 0.5 exp (-1) = 18. 4%

70

Thus the best traffic utilization one can hope for using ALOHA is 0.184 Erlangs.

Maximum throughput is found by substituting in Eq(9.12), and this value can be seen as the maximum throughput in Fig 9.10.

Notice that slotted ALOHA provides a maximum channel utilization of 0.368 Erlangs, double that of ALOHA.

The maximum throughput for slotted ALOHA is found by taking the derivative of Eq(9.12) and equating it to zero.

3679.01 eT

1

0Re

max

R

edR

dT RR

1839.02

1 1 eT

Example 9.6 (Cont’d)

71

CSMA protocols are based on the fact that each terminal on the network is able to monitor the status of the channel before transmitting information. If the channel is idle, then the user is allowed to transmit a packet based on a particular algorithm which is common to all transmitters on the network.

There are two important parameters for CSMA protocol:

1. Detection delay

2. Propagation delay

Carrier Sense Multiple Access(CSMA)

72

Detection delay: a function of the receiver hardware and is the time required for a terminal to sense whether or not the channel is idle.Propagation delay: a relative measure of how fast it takes for a packet to travel from a base station to a mobile terminal.

m

Rtt bp

d

: the propagation time in seconds

: the channel bit rate

: the expected number of bits in a data packet

: propagation delay in packet transmission unitsdt

pt

bR

m

Carrier Sense Multiple Access(CSMA)

73

There are several variations of the CSMA strategy:

•1-persistent CSMA: the terminal listens to the channel, and as soon as it finds channel idle it transmits its message

•Non-persistent CSMA: When a terminal receives a nack, it waits for a random period of time before retransmission. Popular in wireless applications, where packet transmission interval is much greater than the propagation delay to the farthest user

•p-persistent CSMA: used in slotted ALOHA. IF channel is detected to be idle, the packet is transmitted in first available slot with prob p, or in the next slot with prob (1-p)

•CSMA/CD: The user monitors its transmission for collisions. If 2 or more terminals start the transmission at the same time, a collision is detected and transmission aborted. This can be done if receiver has the capability of “listen while you transmit”

•DSMA—Data Sense Multiple Access

Carrier Sense Multiple Access (CSMA)

74

1. Reservation ALOHA: a packet access scheme based on time division multiplexing. In this protocol, certain packet slots are assigned with priority, and it is possible for users to reserve slots for the transmission of packets.

2. Packet Reservation Multiple Access (PRMA): it uses a discrete packet time techniques similar to reservation ALOHA and combines the cyclical frame structure of TDMA in a manner that allows each TDMA time slot to carry either voice or data, where voice is given priority.

Reservation Protocols

75

Capture Effect in Packet Radio

Capture Effect in Packet Radio -------Near-far effect

•Advantage: a particular transmitter may capture an intended receiver, many packets may survive despite collision on the channel

•Disadvantage: a stronger transmitter which is attempting to communicate to the same receiver.

A useful parameter in analyzing the capture effects in packet radio protocols is the minimum power ratio of an arriving packet, relative to the other colliding packets. This radio is called the capture ratio, and is dependent upon the receiver and the modulation used.

76

Reference:

1. Wireless Communications—Theodore S.Rappaport

2. Introduction to Wireless system—P.Mohana Shankar

77

Channel data rates are very high in CDMA systems. Consequently, the symbol(chip) duration is very short and usually much less than the channel delay spread. Since PN sequences have low autocorrelation, multipath which is delayed by more than a chip will appear as noise. A RAKE receiver can be used to improve reception by collecting time delayed versions of the required signal

back

78

Soft handoff is performed by the MSC, which can simultaneously monitor a particular user from two or more base stations. The MSC may chose the best version of the signal at any time without switching frequencies.

back

79

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, hence in the despreading of a particular PN code, non-zero contributions to the receiver decision statistic for a desired user arise from the transmissions of other users in the system.

back

80

Capacity of Cellular Systems

81

Introduction

• Co-channel Interference

• Capacity of cellular system

• Comparison of the capacity of analog cellular system and digital cellular system

• Capacity of Cellular CDMA

• Power Control in CDMA

82

Definition of Channel capacity

Channel capacity: Radio capacity, the maximum number of channels or users that can be provided in a fixed frequency band.This parameter is determined by the required carrier-to-interference ratio (C/I) and the channel bandwidth (Bc)The radio capacity of a cellular system is defined as:

NB

Bm

c

t radio channels/cell (9.19)

Where m is the radio capacity metric, Bt is the total allocated spectrum for the system, Bc is the channel bandwidth, and N is the number of cells in a frequency reuse pattern. N is relatedto the co-channel reuse factor (Q).

83

Co-channel Interference

Reverse channel interference: interference at a base station receiver comes from the subscriber units in the surrounding cells.Forward channel interference: interference from the surrounding co-channel base stations to a particular subscriber unit.

Co-channel reuse ratio: R

DQ (9.14)

The minimum ratio of D/R is required to provide a tolerable level of co-channel interference.

NQ 3For a hexagonal geometry:

N is the number of cells in a frequency reuse pattern

(9.20)

84

Carrier-to-Interference Ratio

85min6

1

I

C

D

Rn

n0: path loss exponent in the desired cellD0: distance from the desired base station to the mobileDk: distance of the kth cell from the mobilenk: path loss exponent to the kth interfering base station

M

k

nk

n

kD

D

I

C

1

00

(9.15)

n

n

D

D

I

C

6

0 (9.16)

When D0=R, and for acceptable performance:n

I

CQ

/1

min

6

Co-channel Interference

86

n

nc

t

c

t

IC

B

B

QB

Bm /2

min2/

2

36

3

n

I

CQ

/1

min

6

Radio capacity

When n=4:

min32

IC

B

Bm

c

t(9.22)

87

Comparison of different system

Bc: bandwidth of a particular system(C/I)min: tolerable value for the same systemBc’: channel bandwidth for a different system(C/I)eq: minimum C/I value for the different system

2

'min

c

c

eq B

B

I

C

I

C (9.23)

'min

log20)()(c

c

eq B

BdB

I

CdB

I

C

Let Bc= 6.25, C/I=9 dB for Bc’=12.5 , C/I eq=3dB so more capacity

88

Example 9.7

Evaluate four different cellular radio standards, and choose the

one with the maximum radio capacity.

dBICkHzBASystem c 18)/(,30:_ min dBICkHzBBSystem c 14)/(,25:_ min

dBICkHzBCSystem c 12)/(,5.12:_ min dBICkHzBDSystem c 9)/(,25.6:_ min

Solution to Example 9.7: Consider each system for 6.25kHz bandwidth, and use Eq.(9.23)

Based on comparison, the smallest value of (C/I)eq should be selected for maximum capacity in Eq.(9.22). System D offers the best capacity.

dBICkHzBASystem eqc 62.31)25.6/30log(2018)/(,25.6:_ dBICkHzBBSystem eqc 25)25.6/25log(2014)/(,25.6:_

dBICkHzBDSystem eqc 9)25.6/25.6log(209)/(,25.6:_ dBICkHzBCSystem eqc 15)25.6/5.12log(2012)/(,25.6:_

89

Digital cellular system

Rb: channel bit rate Eb: energy per bitRc: rate of the channel code Ec: energy per code symbol

I

RE

I

RE

I

C ccbb (9.24)

2'

''

c

c

cc

cc

eq

B

B

IRE

IRE

IC

IC

(9.25)

Rc and Bc is always linear, if the I is the same in the mobile environment for two different digital systems, then

3'

'

c

c

c

c

B

B

E

E(9.26)

90

Comparison between FDMA and TDMA

bbREC cBII 0FDMA:

'' bbREC '' 0 cBII TDMA:

IC

M

B

Bm

t

t

32

Radio capacity for FDMA:

Eb: Energy per bit, I0: interference power per Herz

Rb: radio transmission rates Bc: Channel bandwidth

91

Example 9.8

Consider an FDMA system with three channels, each having a bandwidth of 10kHzand a transmission rate of 10kbps. A TDMA system has three time slots, channelbandwidth of 30kHz, and a transmission rate of 30 kbps. For the TDMA scheme, thereceived carrier-to-interference ration for a single user is measured for 1/3 of the timethe channel is in use. For example, C’/I’ can be measured in 333.3 ms in one second.Thus C’/I’ is given by:

IkHzIBII

CERs

bitsEREC

c

bbb

bb

330''

33333.0

10'

00

4

It can be seen that the received carrier-to-interference ratio for a user in this TDMA system C’/I’ is the same as C/I for a user in the FDMA system. Therefore, for this example, FDMA and TDMA have the same radio capacity and consequently the same spectrum efficiency. However, the required peak power for TDMA is 10logk higher than FDMA, where k is the number of time slots in a TDMA system of equal bandwidth.

92

Capacity of Digital Cellular TDMA

In practice, TDMA systems improve capacity by a factor of 3 to6 times as compared to analog cellular radio system.

1. Powerful error control2. Speech coding3. Mobile assisted handoff (MAHO)4. Adaptive channel allocation (ACA)

How to improve capacity in digital cellular TDMA

93

Table 9. 3 Comparison of AMPS with Digital TDMA based Cellular Systems[Rai91]

Parameter AMPS GSM USDC PDC

Bandwidth(MHz) 25 25 25 25

Voice Channels 833 1000 2500 3000

Frequency Reuse(Cluster sizes)

7 4 or 3 7 or 4 7 or4

Channels/Site 119 250 or 333 357 or 625 429 or 750

Traffic(Erlangs/sq.km) 11.9 27.7 or 40 41 or 74.8 50 or 90.8

Capacity Gain 1.0 2.3 or 3.4 3.5 or 6.3 4.2 or 7.6

94

Capacity of Cellular CDMA

How to reduce interference in CDMA:- Use multi-sectorized antennas- Operate in a discontinuous transmission mode (DTX) In DTX, the transmitter is turned off during the periods of silence in speech.

1. The capacity of CDMA system is interference limited, while it is bandwidth limited in TDMA/FDMA.

2. The capacity of CDMA system is soft while the capacity of TDMA/FDMA system is hard.

3. While TDMA/FDMA reuse frequencies depending on the isolation between cells provided by the path loss in terrestrial radio propagation, CDMA can reuse the entire spectrum for all cells, and this results in a increase of capacity by a large percentage over the normal frequency reuse factor.

95

Start by considering a single cell with N users who share the cell:

)1(

1

)1(

NSN

SSNRSignal-to-Noise Ratio:

Bit energy-to-noise ratio:1

/

)/)(1(

/

0

N

RW

WSN

RS

N

Eb

R: base-band information bit rate; W: total RF bandwidth

Considering the background thermal noise η in the spread bandwidth

)/()1(

/

0 SN

RW

N

Eb

)/(/

/1

0

SNE

RWN

b

The number of users:

W/R: processing gain

Capacity of Cellular CDMA (Cont’d)

96

How to increase the capacity in CDMA

♠ Antenna sectorization♠ Incorporate a voice activity monitor Monitoring of voice activity such that each transmitter is switched off during periods of no voice activity.

Using these two technique, the new Eb/N0’ within a sector is:

)/()1(

/

'0 SN

RW

N

E

s

b

The number of users:

'

/11

0NE

RWN

bs

97

Antenna sectorization

N0’=1/3N0

N’=3NN0 is the interferenceN is the number of user

98

Incorporate a voice activity monitor

Monitor voice activity and switch off transmitter during periods of no voice activity.When we speak, we normally use pauses between words, and by turning off the transmitter when no voice activity is detected, we can reduce the effective number of interfering users. It is known that speakers are active only 35-40% of the time.

Designate α as the voice activity factor, the interference termbecomes (Ns-1) α, where Ns is the number of users per sector.

The typical value of voice activity factor α is assumed to be 3/8.

99

Example 9.9

If W=1.25MHz, R=9600bps, and a minimum acceptable is found to be 10dB, 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, and (b) 3-sectors at the base station and activity detection with =3/8. Assume the system is interference limited.

0/ NEb

Solution to Example 9.9 (a) using Eq. (9.31)

1402.13110

9600/1025.11

6

N

(b) Using Eq. (9.33) for each sector we can find Ns

7.3510

9600/1025.1

375.0

11

6

sN

The total number of users is given by 3Ns, since three sectors existWithin a cell; therefore N=3*35.7=107users/cell.

100

CDMA Power Control

Why use power control?♦ Since all the mobile users transmit over the same frequency band, power control is essential to overcome near-far problems.♦ In CDMA, the system capacity is maximized if each mobile transmitter power level is controlled so that its signal arrives at the cell site with the minimum required signal-to-interference ratio.♦ Power control also plays a role in conserving transmitted signal power, thereby increasing the battery recharge cycle.

How to implement power control?☻forward power control-sampling the RSSI levels ☻open-loop method☻closed-loop method

Power control is a key technique in CDMA system

101

Reference

1. Wireless communications principles & practice, Theodore S. Rappaport

2. Introduction to wireless communication systems, P. Mohana Shankar

3. Mobile Communications Enginnering Theory and Applications, Willian C. Y. Lee

4. http://www.anlian.com/trainingonline.htm

102

Capacity of CDMA with Multiple Cells

103

1. CDMA 2. Capacity of Cellular Systems3. Capacity of Cellular CDMA4. Compare single cell CDMA with Multiple cell CDMA5. Capacity of CDMA with Multiple Cells

(i) Frequency reuse factor, f(ii) Impact of Propagation Pathloss on Frequency reuse of CDMA systems(iii) Weighing Factor(iv) CDMA Capacity

6. Conclusions7. References

Contents

104

CDMA

CDMA : is a “spread spectrum” technology, which spreadsthe information contained in a particular signal of interestover a much greater bandwidth than the original signal.

This is achieved by multiplying the signal by a very large bandwidth signal called the spreading signal.

105

1. All The users in CDMA share the same carrier frequency and may transmit simultaneously

2. Each user has its own pseudorandom codewordwhich is approximately orthogonal to all other codewords.

3. The receiver needs to know this codeword used bythe transmitter and it performs time correlation to

detect only the specific desired signal.

Features

106

4. The near-far problem occurs at a CDMA receiver if an undesired user has a high detected power as compared to the desired user. To combat the near-far problem a power control is used in most CDMA implementations.

5. Capacity: There is no absolute limit on the number of users in CDMA. But as the number of users in the CDMA system increase the system performance decreases. The system performance increases as the number of users dec- rease.

Features

107

Capacity of Cellular SystemsChannel Capacity: of 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 a wireless system is determined by its “radio capacity”, which depends on the requiredCarrier-to-interference ratio (C/I) and the channel band-width (Bc).

108

InterferenceInterference in a cellular system can be due to the surroun-ding base stations or due tothe surrounding subscriber units. That is there are two kinds of interferences:

1. Forward channel interference: This is the interference due to the surrounding co-channel base stations.

2. Reverse channel interference: This is the interference due to the surrounding subscriber units.

Interference damages the performance of the system, it can be tolerated to a certain extent that is given by the “co-channel reuse ratio”.

109

Capacity of Cellular CDMAThere is no absolute limit on the capacity of CDMA, but there is a tradeoff between the capacity and performance.

The capacity of CDMA is interference limited, which is unlike that TDMA and FDMA, which are bandwidthlimited. So to increase the capacity of CDMA we shouldreduce the interference.

Interference reducing techniques:1. Using Multisectorized antenna2. Operating in discontinues transmission mode

110

1. Using Multisectorized antenna: In this technique we reduce the interference by using a directional antenna

which spatially isolates the users by receiving signals from only a fraction of the current users.

2. Discontinues Transmission Mode(DTX): This techn-ique takes advantage of the intermittent nature of speech.

Here the transmitter is turned off during the period of silence. If the voice activity factor is “ ”, then the

average bit-energy to noise density ratio, without apply- DTX is

Interference Reduction Technique

111

Where

is the background noise, B is the bandwidth, R

R is the information bit rate.

With the application of the sectorization, DTX techniques the bit energy to noise density ratio

)/()1(

// 0 SN

RWNEb

That is reduces the interference term from (N-1) to This increases the bit energy ratio by almost a factor of 8

)1( N

)/()1(

// 0 SN

RWNEb

112

Single Cell CDMA vs Multicell CDMA Multicell Single cell

1. All the base stations The base stations are inde-are interconnected by pendent.the mobile switchingcenter.

2. The weighting factors of The weighting factors of allall the users are not equal. the users are equal.

3. We need power control In the forward link no power in both the forward and control is required. Since for reverse links. a subscriber any interference

caused by the other subscriber remains at the same level as

desired signal.

113

Capacity of CDMA With Multiple Cells

In CDMA cellular system each base station can only cont-rol the transmit power of each of its own in-cell users, but itcan not control the power of users in the neighboring cells.

These neighboring users add to the noise floor and decreasethe capacity of the particular cell of interest.

114

Frequency reuse factorFrequency reuse factor (f): is determined by the amount of out-of-cell interference. It is defined as

iaii NUN

Nf

0

0

WhereN0: is the total interference power received from

the N-1 usersUi : is the number of users in the i th adjacent cell

Nai: is the average received power from users in the adjacent

cells

115

Frequency reuse efficiency

Frequency reuse efficiency (F): is the percentage of frequ-ency reuse factor

F= f x 100%

Average received power (Nai ): is defined as follows

j i

ijai U

NN

Where NiJ is the power received at the base station of int-erest from the j th user in the i th cell.

Each adjacent cell may have different number of users and hence may receive different levels of interference. ThereforeNai is different for each cell user

116

Impact of propagation path loss on the frequency reuse

Recursive geometric technique: This technique is also called “concentric circle cellular geometry”. It considers that

1. All cells have equal geographic area.2. Cell of interest is circular and located at the center of

all the surrounding cells.3. Interference cells are wedged shaped and are arranged

in layers around the center cell of interest.

117

Concentric circle cellular geometry

Fig

118

1. R: is the radius of the cell of interest2. d0: is the minimum distance, such that users in the

center are located no closer than this distance3. d: is the distance from the base station at which all

the users in the center cell of interest are located, suchthat

Rdd 0

Capacity of CDMA

119

First layer of adjacent interfering cells is found onRdR 3

Second layer is located on

RdR 53 The i th interfering layer is located on

RidRi )12()12( The area A of the cell of interest is

220

2 RdRA

120

If A1 denotes the entire area of the region. If each cell have the same area A, then there should be M1

cells that each span a particular angle ( ), such that:1

11

2

M

Solving the above equations gives M1=8 and 01 45

In general: Ai=Mi A=i. MiA= i.8A 1i

ii 41

1

=

AMRRA 122

1 )3(

121

SectorsInterfering layers can be broken into two sublayers, an inner sublayer which is on

outer sublayer which is on

02)12( diRdRi

RiddiR )12(2 0

Partitioning of layers provide two sectors within each Wedge shaped cell in a given layerInner sector: Which contain a small fraction of area of The area of the cell and hence fewer usersOuter sector: Which contain much greater fraction of the area of the cell and hence more users

122

Weighing FactorsWeighing Factors: are used to redistribute the users in theinner and outer sectors of an adjacent cell, since there is a wide range of user distributions in the interfering layers.

If K is the user density then the number of users (U) withinthe center cell is given be

U=KA.

In the first surrounding layer, the inner and outer sectors ofeach cell have areas given byInner sector:

8/38/))2((/ 2211 ARMA in

123

Outer sector:

8/58/)2()3((/ 2211 ARMA out

For each first layer cell to possess U=KA users, weighingfactors for the user densities within the inner (W1in) and outer (W1out) sectors may be applied such that

111111 /)(/)( MAKWMAKWKAU outoutinin

)8/58/3( 11 outin WWKAKAU

124

ConditionsOptimistic conditions: for frequency reuse (or upperbound) is seen when W1in=1 and W1out=1, then 3/8 of theusers will be in the inner sectors and 5/8 of the users will be in the outer sector and will offer smaller levels of interf-erence to the center cell.

Worst case: In this case all the users in each of the first layer cells would be located in the inner sector. The weighfactors in this case are W1in=8/3 and W1out=0

125

Concentric circle geometry to find CDMA Capacity

To find the capacity of a multicell CDMA system, the con-centric circle geometry is used along with a propagation path loss model to determine the interference from the adjacent cell users.

We can find the frequency reuse factor from

iaii NUN

Nf

0

0

Where N0 is given by

KAPUPUPN 0000 )1( Where P0 is the power received from any of the users in thecenter cell

126

In the adjacent cells, each subscriber is power controlledwithin its own cell and at a distance d’ from its own base station .

Since propagation path loss are based on all distances greaterThan d0, a small forbidden zone having a width 2d0 is assu-med to exist in all surrounding rings.

This forbidden zone occupies negligible area and provides virtually the same interference results as of the case when there is no forbidden region

127

Interference PowerInterference Power ( ): At the center of the cell from thethe j th user in the i th interfering cell is given by

nnjiji ddddPNdrP )/()/(),,( 00

'0,0,,0

jiP ,,0

To evaluate this expression we need to calculate d’

128

Fig

129

Using the law of cosines it can be shown that, within anycell in the i th layer

Evaluating d’

20

22' )cos2(sin ddRidd 02)12( diRdRi For

20

22' )cos2(sin ddRidd For RiddiR )12(2 0

Inner sublayer:

Outer sublayer:

130

Using d’, we can determine the interface power( ) as

jiP ,,0

nnjiji ddddPNdrP )/()/(),,( 00

'0,0,,0

Where n is the propagation path loss exponent. Using this power we can evaluate the average received power (Nai)

from

j i

ijai U

NN

Using which we can evaluate the frequency reuse factor (f)

iaii NUN

Nf

0

0

131

Conclusions1. The two factors, voice activity and spatial isolation thro-

ugh the use of multibeam or multisectorized antennas, are sufficient to render CDMA capacity atleast double that of FDMA and TDMA under similar assumptions.

2. CDMA can reuse the same (entire) spectrum for all thecells, thereby increasing the capacity by a large percent-

age of the normal frequency reuse factor. The net increasein capacity due to the above factors in CDMA over

digital TDMA or FDMA is of the order of 4 to 6 and cu-rrent analog FM/FDMA it is nearly a factor of 20.

132

Space Division Multiple Access

133

Directional Programmable Antenna

134

135

References1. Theodore S.Rappaport, “ Wireless Communications”,

Principles and Practice.

2. Jerry D.Gibson, “Mobile Communications”, Handbook,second edition.

3. T.S. Rappoport, “Effect of Radio Propagation Path Losson DS-CDMA Cellular Frequency Reuse Efficiency forthe Reverse Channel, IEEE Vehicular Technology,

Vol.41, Aug 19924. Klien S. Gilhousen, “On the capacity of a cellular CDMA

system”, IEEE Vehicular Technology,Vol.40, May 1991.