department of electronic engineering final year project...

I

Department of Electronic Engineering

FINAL YEAR PROJECT REPORT

BEngIE-2006/07-<QTZ-01>

Multiple Access Capability of DS-UWB

and Multi-band OFDM Schemes for IEEE

802.15.3a

Student Name: Cheuk Chun Fung

Student ID:

Supervisor: Prof. ZHANG, Kieth Q T

Assessor: Dr. Wong, W K

Bachelor of Engineering (Honours) in

Information Engineering

II

Student Final Year Project Declaration

I have read the student handbook and I understand the meaning of academic

dishonesty, in particular plagiarism and collusion. I declare that the work submitted

for the final year project does not involve academic dishonesty. I give permission for

my final year project work to be electronically scanned and if found to involve

academic dishonesty, I am aware of the consequences as stated in the Student

Handbook.

Project Title: Multiple-Access Capability of DS-UWB and Multi-band OFDM Schemes for

IEEE802.15.3a

Student Name: Cheuk Chun Fung

Student ID:

Signature

Date: 18/04/07

i

No part of this report may be reproduced, stored in a retrieval system, or transcribed in

any form or by any means – electronic, mechanical, photocopying, recording or

otherwise – without the prior written permission of City University of Hong Kong.

Contents

Abstract 1

Introduction 2

Section 1

Section 1.1 Basic concept of CDMA 3

Section 1.2 Slow fading mobile radio channel 4-5

Section 1.3 Channel Model 6-7

Section 2

Section 2.1 DS-CDMA Basic principles 8

Section 2.2 Rake Receiver Structure 9

Section 2.3 DS-CDMA Transmitter Model 10

Section 2.4 DS-CDMA Receiver Model 11

Section 3

Section 3.1 OFDM basic principles 12

Section 3.2 IFFT/FFT pairs 13-14

Section 3.3 Cyclic Prefix 15-16

ii

Section 3.4 OFDM-CDMA Basic Principles 17

Section 3.5 OFDM-CDMA Transmitter Model 18

Section 3.6 OFDM-CDMA Receiver Model 19

Section 4 Performance Analysis 20-22

Section 5 Analytical Result 23-24

Section 6 Comparisons of simulation results

Section 6.1 Performance comparison for single user 25-26

Section 6.2 Performance comparison for different

processing gain

27-28

Section 6.3 Performance comparison for multi user with

fixed Processing Gain and Diversity for type A

delay profile

29-34


fixed Processing Gain and Diversity for type B

delay profile

35-38

Section 7 Conclusion 39

References 40

Appendix

iii

List of figures

Figure 1 The multi-path environment with multiple reception 6

Figure 2 The delay profile with unequal path gain 7

Figure 3 The delay profile with equal path gain 7

Figure 4 The chipped process in time domain 8

Figure 5 Structure of Rake Receiver 9

Figure 6 OFDM system structure 12

Figure 7 Multi-carrier modulation 13

Figure 8 Effects of Channel Response when no CP introduced 15

Figure 9 Effects of Channel Response when CP introduced 15

Figure 10 The spreading on frequency domain in OFDM-CDMA system 17

Figure 11 The despreading on frequency domain in OFDM-CDMA system 17

Figure 12 Block diagram for OFDM-CDMA transmitter 18

Figure 13 The probability density function for MRC and with no diversity 20

Figure 14 The comparison between the analytical results and the simulation

results for DS-CDMA

23

Figure 15 The comparison between the analytical results and the simulation

results for OFDM-CDMA

23

Figure 16 The comparison between the performance of DS-CDMA and

OFDM-CDMA

25

Figure 17 The comparison between G=64 and G=128 for DS-CDMA 27

Figure 18 The comparison between G=64 and G=128 for OFDM-CDMA for

type A delay profile

28

Figure 19 comparison between DS-CDMA and OFDM-CDMA for type A

delay profile with G=64 and D=2

29



31



32



34

Figure 23 comparison between DS-CDMA and OFDM-CDMA for type B


35



36

iv



37



38

1

Abstract

The demand for high data rate transmission in mobile wireless communications is

growing rapidly. Apart from the traditional DS-UWB (Direct Sequence Ultra Wide

Band), Multi-band Orthogonal Frequency Division Multiplexing (OFDM) has

attracted diverse interest as a modulation technique for short range wireless data

communications described in the IEEE 802.15.3a physical layer proposal. In this

paper, two multiple access schemes for uplink system in a multi user environment are

studied with respect to their error performance. Investigation covers two schemes,

namely an uncoded orthogonal frequency division multiplexing (OFDM) based code

division multiple access scheme (OFDM-CDMA), and an uncoded direct sequence

code division multiple access (DS-CDMA) scheme. In both schemes, Maximum Ratio

Combining is applied at the receiver. The performance of the two schemes is

compared considering different channel modes which is important for the later system

design. The analytical results are confirmed by the simulation results which suggest

OFDM-CDMA outperforms DS-CDMA under slow fading multi-path environment.

2

Introduction

The popularity of mobile personal devices has created a demand for wireless

communications system which can accommodate many users simultaneously. The

promising key concept is the Code Division Multiple Access (CDMA) scheme.

Another challenging task for the transmission of data through a wireless channel is the

multipath propagation. Maximum Ratio Combining (MRC) is applied at the receiver

so as to take advantage of channel diversity to combat the multipath fading.

The objective of the study is to carry out comparison between DSCDMA and

OFDMCDMA under the uplink synchronous environment. Both schemes would be

applied MRC at the receiver and the processing gain is assumed to be the same at both

systems.

This paper is organized as follows: In Section 1 the basic concept of the CDMA, slow

fading mobile radio channel and the channel model is presented. In Section 2 the

Basic Concept of DS-CDMA is investigated. In Section 3 OFDM-CDMA system is

discussed. In Section 4 the performance analysis is shown. In Section 5 the analytical

results is investigated and the performance for single user is presented. In Section 6

the simulation results are presented to compare the performance of the two systems.

In Section 7 the conclusion is made.

3

Section 1

Section 1.1

Basic Concept of CDMA [1]

The Users are assigned unique signatures. Each transmitter sends its data streams by

modulating its own signature waveform. In order to recover the original information,

the receiver then correlates the synchronized signature sequence with the received

signal. The transmitted signal can be written as

)()()()()()()()( 332211 tctstctstctstcts NN++++ L

where si is the i-th user signal and ci is the i-th user’s signature sequence.

The Basic concept of CDMA is to choose the spreading sequence with the cross

correlation which is equal to zero, that is:

∫ =>=<T

dttctccc0

2121 0)()(,

It can be seen that if the above condition is satisfied, the desired user’s signal can be

perfectly recovered from the received signal. Since 1, 11 >=< cc and

><><>< Ncccccc ,,,,, 13121 L are all equal to zero.

The ability to combat multiple access interference (MAI) is determined by the

cross-correlation characteristics of the spreading codes. In this paper, pseudo-noise

sequences are chosen to be the signature of the users. Pseudo-Noise (PN) sequences

are binary sequences, which exhibit noise-like properties.

4

Section 1.2

Slow fading mobile radio channel [2]

For most mobile radio communication, a direct line-of-sight (LOS) does not exist

between the transmitter and the receiver. Due to multiple reflections from various

objects, the electromagnetic wave travels along different paths. Fading is caused by

interference between two or more versions of the transmitted signal which arrive at

the receiver at slightly different times.

Slow fading belongs to small scale fading which is used to describe the rapid

fluctuations of the amplitudes, phases, or multipath delays of a radio signal over a

short period of time or travel distance. In a slow fading channel, the channel impulse

response (CIR) changes at a rate much slower than the transmitted baseband signal. In

this case, the channel may be assumed to be static over one or several reciprocal

bandwidth intervals. From the point of views of frequency domain, this means that the

Doppler spread of the channel is much less than the bandwidth of the baseband signal.

That is cs TT < where sT represents the Doppler spread time and cT represents the

coherence time.

5

The channel model investigated in this paper is the slow fading channel which is

described by the Rayleigh distribution. The Rayleigh distribution has a probability

function (pdf) given by

<

∞≤≤

−

=

)0(0

)0(2

exp)( 2

2

2

r

rrr

rp σσ

where σ is the root-mean-square power of the received signal.

6

Section 1.3

Channel Model

Due to the reflection and scattering of obstacles, the transmitted signal is propagated

through a noisy fading multipath channel. The multipath and noise are assumed to be

statically independent and the path components are assumed to be independent

identically distributed (iid). Figure x is best to describe the channel model.

Figure 1 The multi-path environment with multiple reception

The channel impulse response is modeled as a time pulse train represented by

∑−

=

−=1

0

)()(L

l

ll tth τδα

where L is the number of multipath components, lα is the real attenuation of the l-th

multipath component which follows an independent identically distributed Rayleigh

distribution, lτ is the delay from the transmitter to the receiver for the l-th multipath

component, and δ is the delta function of the impulse.

LOS

7

We define the following delay profile model as Type A with unequal path gain:

Figure 2 The delay profile with unequal path gain

We define the following delay profile model as Type B with equal path gain:

Figure 3 The delay profile with equal path gain

In this paper, the time delay τ would be assumed to be 1 chip duration. The total

power conveyed by the channel would be normalized such that the expectation power

would be given to be 11

0

2=

∑

−

=

L

l

iE α .

τ

T 2T 3T 4T

τ

T 2T 3T 4T

Power

Power

8

Section 2

Section 2.1

DS-CDMA Basic principles

In DS-CDMA transmission, each information bit for user k would be multiplied by a

user signature code applied in time domain. The spreading can be described by

figure 4:

Figure 4 The chipped process in time domain

where τ is the bit duration ( bT ) and L is the number of chips. The information bit is

chipped by a sequence which is much faster than the information bit rate. The chip

duration thus isL

Tc

τ= . And the processing gain obtained by spreading can be

expressed as the ratio of chip to bit duration that isc

b

T

T. For simplicity, we assume that

the sequence length L is equal to the processing gain.

9

Section 2.2

Rake Receiver Structure [3]

Rake Receiver would be exploited to combat the multipath fading. It can be shown by

figure 5 :

Figure 5 Structure of Rake Receiver

As we can see, Rake Receiver consists of a bank of correlators. We call these

correlators fingers. After transmission over a Rayleigh multi-path fading channel, the

received signal is the superposition of several copies of the original signals each

arriving with different delays. We assume that there are L paths and there are M

fingers at the Rake Receivers which can obtain Mth

order of diversity. In the following

discussion, we assume that ML ≥ and no selection at the receiver would be provided.

All copies would be caught by the fingers at the receiver and combined together.

10

Section 2.3

DS-CDMA Transmitter model

Due to the simplicity, we make assumption that all carrier phases are equal to zero. As

a result, the real part of the baseband signal would be considered in the following

discussion. The data modulation is binary phase shift keying (BPSK). The transmitted

signal can be represented as:

∑∑∑−

=

−

=

−

=

−−=1

0

1

0

1

0

)(2)(U

u

M

i

N

j

cbT

u

j

u

i jTiTtpcbEtsc

where U is the total number of users

M is the total number of bits

N is the total number of chips

u

ib is the i-th information bit of u-th user

u

jc is the j-th chips of the u-th user

p(t) represents the pulse

bT is the bit duration

cT is the chip duration

11

Section 2.4

DS-CDMA Receiver Model

The receiver model for uplink synchronized DS-CDMA system can be represented

by:

)()()(11

tnlTtsAtrU

u

cu

L

l

l +−= ∑∑==

where L is the number of path

Al is the attenuation of the l-th path

U is the total number of users

Tc is 1 chip length duration

n(t) is the zero-mean additive white Gaussian noise(AWGN) with variance

2

nσ

12

Section 3

Section 3.1

OFDM basic principles [4]

Orthogonal Frequency Division Multiplexing (OFDM) provides a means of

counteracting the channel-induced linear distortions especially the inter symbol

interference (ISI) encountered when transmitting over a dispersive radio channel.

The basic principle of OFDM is that the serial data stream of a traffic channel is

passed through a serial-to-parallel (S/P) converter which splits the data into a number

of parallel channels. The data in each sub channel is then modulated by Inverse Fast

Fourier Transform (IFFT). The transmitted signal is then passed through the

parallel-to-serial (P/S) converter. It is then followed by the Cyclic Prefix (CP)

padding. At the receiver, the inverse operations would be applied. The CP in the

received signal would first be removed and then passed through the (S/P) converter.

The Fast Fourier Transform (FFT) would then be exploited to demodulate the

received signal and finally passed through the (P/S) converter.

Figure 6 OFDM system structure

13

Section 3.2

IFFT/FFT pairs [5]

The Basic concept of OFDM is based on the multi-carrier transmission. Each

transmitted signal is modulated by a specific carrier frequency denoted by nf . The

transmission scenario is described by figure 7

Figure 7 Multi-carrier modulation

In OFDM, the orthogonal properties is maintained by the sub-carrier spacing T

1

which can be expressed as

( ) ( )∫T

tfjtfjdtee

T 0

2*2 211 ππ

= dteT

Ttffj

∫−

0

)(2 121 π

≠

=

21

21

0

,1

ff

ff

Discrete Fourier Transform (DFT) and Inverse Discrete Fourier Transform are two

critical components in the implementation of OFDM system especially reduce the

complexity of sub-channel modem. It can be shown mathematically that taking the

14

DFT of the original signal and then transmitting the DFT coefficients serially is

exactly equivalent to apply the data directly to the modulator transmitting at a

carrier frequency positioned at the centre of the transmission band 0f ,……, Nf .[]

That is:

1 0

2

Nf f− +

The modulated signal m(t) is given by

{ }02( ) Re ( )

j f tm t b t e

π= where b(t) is the equivalent baseband signal and f0 is the

carrier frequency.

15

Section 3.3

Cyclic Prefix [6]

After the IFFT modulation, the OFDM time domain samples would then be guard

padded by enough samples to eliminate the ISI between the OFDM symbols due to

the delay spread of the multi-path channel. The method is best explained with

reference to figure 8 and figure 9.

Without CP,

Figure 8 Effects of Channel Response when no CP introduced

With CP,

Figure 9 Effects of Channel Response when CP introduced

Each block of length T modulated signal segment is extended by a length of Tg the

guard time interval that repeating Ng samples of the original information symbol. The

total sequence length would be N+ Ng which corresponds to duration T+ Tg. The

extended parts would be corrupted by the channel impulse response. The first Ng

samples of the received block would be ignored by the receiver. Only the central

OFDM1 OFDM2 channel OFDM1 OFDM2

OFDM1 channel OFDM2 CP CP

OFDM1 OFDM2

16

useful information block would be demodulated by the FFT.

The number of Ng samples depends on the channel’s impulse response. It is required

that the guard time interval must be greater than the delay time spread of the channel.

Tg >> Tdelay. It is assumed that the GI Note that the method of adding cyclic prefix

would actually waste the channel capacity as well as the transmitted power. In order

to minimize the effects, the block size is usually set greater or equal to 128. As a

result, the extended length would only be 10% of the useful information block length.

17

Section 3.4

OFDM-CDMA Basic Principles [7]

In OFDMCDMA, instead of applying spreading sequences in the time domain, the

spreading sequences would be applied in the frequency domain, mapping a different

chip of spreading sequence to an individual OFDM subcarrier. Hence each OFDM

subcarrier has a data rate identical to the original input data. The processing gain is

due to the spreading in a wider frequency band.

The additional component would be shown in figure 10

At transmitter,

Figure 10 The spreading on frequency domain in OFDM-CDMA system

At receiver,

Figure 11 The despreading on frequency domain in OFDM-CDMA system

18

Section 3.5

OFDM-CDMA Transmitter model

After the investigation of the OFDMCDMA system, in the following section the

OFDMCDMA uplink transmission applying MRC would be shown. In this paper,

BPSK would be chose to be the modulation technique. The transmitted signal can be

expressed as:

11 1 12 ( )

0 0 0

( )U M G

j gM m tu u T

c m g

u m g

s t E b c eπ− − − +

= = =

=∑∑∑

where U is the number of users, which has a maximum of G.

M is the number of bits transmitted per user.

The processing gain G is given as N/M, where N is the number of carriers.

Ec is the energy per subcarrier, and Ec = Eb/N, where Eb is the energy per bit

before spreading.

T is the signaling interval in which M numbers of bits per user are

transmitted and 1/T is equal to the spacing between adjacent subcarriers.

For BPSK signal,

{ }1±∈u

mb is the m-th bit of user u

{ }1±∈u

mc is the g-th chip of the u-th user’s spreading sequence.

Figure 12 Block diagram for OFDM-CDMA transmitter

19

Section 3.6

OFDM-CDMA Receiver model

Assume there is no inter-subcarrier interference. At the receiver the received signal

can be represented by

∑∑−

=

−

=

+=1

0

1

0

G

g

U

u

gg

u

g

u

c nHcbEr

where Hg is the frequency domain channel transfer factor

G is the processing gain

U is the total number of users

Ec is the energy per sub-carrier

bu is the u-th user information

cu is the u-th user spreading sequence

ng is a discrete AWGN process having zero mean and a one-sided

power spectral density N0

20

Section 4

Performance Analysis

In this paper, a diversity combining technique which is called Maximum Ratio

Combining (MRC) would be applied at the receiver. Diversity combining includes

range of techniques that exploits the channel redundancy of the channel. The idea can

be interpreted as if one path is in deep fade, the other paths may still allow data

transmission. Diversity can improve the performance by providing multiple branches

for the transmission.

In mathematical mean, the performance improvement due to maximum ratio

combining is not only due to a shift in the mean of the pdf of the SNR, but also to a

change in the shape of the distribution.

Figure 13 The pdf for MRC and with no diversity

21

It can be seen that MRC is succeed to combat deep fades as it provides more

realization of the SNR. MRC combines each diversity branch to provide the largest

possible performance improvement.

We consider the single user scenario. Assume that the demodulator provides prefect

estimates of the channel. The decision variable D is given by

∑∑ ∑∑= = = =

+=L

l

G

m

L

l

G

m

lmlc

p p

nmbED1 1 1 1

2)( αα

where mn is Gaussian noise. The mean value of D is then given by

∑=

=L

l

lcp EGDE1

2][ α

and the variance

∑=

=L

l

lp

NGD

1

20

2]var[ α

The probability of error is given by

=

)var(

])[()(

2

D

DEQP bb γ

∴

= ∑

=

L

l

l

b

bbN

EQP

1

2

0

2)( αγ

We denote iγ the instantaneous bit energy to noise spectral density ratio in one branch.

It can be represented by

2

0

l

b

iN

Eαγ =

22

The total instantaneous bit energy to noise spectral density ratio, denoted by bγ is

given by

∑ ∑= =

==L

l

L

l

ll

b

bN

E

1 1

2

0

γαγ

If lα is Rayleigh distributed then lγ has a chi square distribution with two degrees

of freedom. The pdf can be represented as

01

)( ≥=−

l

l

ll

l

ep γγ

γ γ

γ

where lγ is the average bit to noise spectral density ratio defined as

][2

0

l

b

l EN

Eαγ =

where ][2

lE α is the average value of 2

lα

The probability density function of bγ can be represented by

l

b

eL

p L

bL

l

b

γ

γ

γγ

γ−

−

−= 1

)!1(

1)(

The average bit error probability is obtained by averaging the conditional error

probability over all bγ that is

∫∞

⋅=0

)()( bbb dpPbPb γγγ

which get the closed form solution

∑−

=

+

−

−=

1

0 2

11

2

1 L

l

lL

l

LPb

µµ where

l

l

γ

γµ

+=

1[8]

23

Section 5

Analytical results

Figure 14 The comparison between the analytical results and the simulation results for

DS-CDMA

Figure 15 The comparison between the analytical results and the simulation results for

OFDM-CDMA

24

Figure 14 shows the BER for the analytical results for single user scenario discussed

above. The simulation results for DS-CDMA are close to the analytical results for

diversity order equal to 1, 2 and 4. The BER is decreased when the diversity order

increased. DS-CDMA exploits the diversity combining technique to obtain the

diversity gain. The comparison between the simulation results for OFDM-CDMA and

the theoretical results would be shown in Fig 15. The trend is similar to DS-CDMA

and the simulation results are also agreed to the analytical results.

Figure 14 and Figure 15 can be divided into three regions in order to perform the

comparison:

1st region:

0N

Eb ranging from 0 to 4 dB

2nd

region: 0N


3rd

region: 0N


In the 1st region, when the diversity order is doubled there would be 2 to 3 db

performance improvement.

In the 2nd

region, when the diversity order is doubled there would be 4 to 5 db


In the 3rd

region, when the diversity order is doubled there would be 5 to 6 db


25

Section 6 Comparisons of simulation results

Section 6.1 Performance comparison for single user

Figure 16 The comparison between the performance of DS-CDMA and

OFDM-CDMA

Figure 16 shows the simulation results for DS-CDMA and OFDM-CDMA in which

the processing gain pG of both system is equal to 64 and the utilization of the system is

1 that means only one user is active.

When diversity order is equal to four, OFDM-CDMA perform about 1 db better than

DS-CDMA for 0N

Eb ranging from 4 dB to 10 dB. The performance improvement by

OFDM-CDMA is mainly due to absence of ISI and no cross correlation between the

information signal and the delay copies of the original information i.e.

26

∫ −T

dttctc0

11 )()( τ ( >−< )(),( 11 τtctc )

where )(1 tc is the spreading sequence and τ is the delay with 1 chip length.

When diversity order is equal to two, the three curves are slightly moved upward due

to the less diversity gain. In addition, OFDM-CDMA outperforms DS-CDMA only by

about 0.5 dB. It is because the cross correlation >−< )(),( 11 τtctc is small when

compared to that of diversity order 4 and the ISI effect is also reduced.

At last, it can be seen that the performance for both system is nearly the same when

the diversity order is equal to one. Since in this situation the ISI effects is minimized

and there is no cross correlation between the information signal and its delay. That is

no other paths would provide the copies of the original signal. The trade off is no

diversity gain would be obtained. As a result, even 0N

Eb archives 16 dB. The BER

would only stand at between 10-2

and 10-3

.

27

Section 6.2 Performance comparison for different processing

gain

Figure 17 The comparison between G=64 and G=128 for DS-CDMA

From figure 17, we can see that in DS-CDMA when then processing gain is doubled

the BER performance is improved.

In two users scenario, when processing gain of 128 is employed corresponding to 64,

the improvement would maintain at about 1 dB, 2 dB and 4 dB for 0N

Eb <10 dB, 10

dB<0N

Eb <14 dB and 0N

Eb >16 dB respectively.

In four users and eight users scenario, the performance would further be improved in

which maximum 6 dB improvement can be archived for 0N

Eb >10 dB.

The performance improvement mainly due to the low cross correlation between

28

different users spreading sequences when the spreading factor is increased. And the

increment in processing gain can also spread the interferer power.

The similar results can be found in OFDM-CDMA which is shown by figure 18.

Figure 18 The comparison between G=64 and G=128 for OFDM-CDMA for type A

delay profile

As we can see from figure 18, the spreading factor is critical in OFDM-CDMA

system. Even in two users scenario, spreading factor of 128 would perform 6 dB

better than the spreading factor of 64 for 0N

Eb >10 dB. The improvement would be

more obvious in four users and eight users scenario. We can see that the cross

correlation between different users sequences would be a critical factor.

29


fixed Processing Gain and Diversity for type A delay profile

Comparison for G=64, D=2

Figure 19 comparison between DS-CDMA and OFDM-CDMA for type A delay

profile with G=64 and D=2


the processing gain pG of both systems is equal to 64. The diversity order is equal to 2.

The system with utilization for 2, 4 and 8 users would be investigated.

In two user scenario, OFDM-CDMA outperforms DS-CDMA by about 2 dB. There

was no ISI in OFDM-CDMA. In addition, there was no cross correlation between the

information signal and the delay copies of itself. The only factor affects the

30

performance is the cross correlation between different user which exists in both

DS-CDMA and OFDM-CDMA. The cross correlation between two uses can be

represented by ∫T

dttctc0

21 )()( ( >< )(),( 21 tctc ) where )(1 tc is the spreading

sequence for the first user and )(2 tc is the spreading sequence for the second user.

Such cross correlation would results in multi access interference (MAI).

In four users’ scenario, the bit error rate increased as the MAI is increased. Therefore,

the performance of both systems is degraded. The result is that OFDM-CDMA system

perform about 1 dB better than DS-CDMA when 0N

Eb is equal to 10 dB. For high

0N

Eb ranging from 14 to 20 dB, there was 2 dB improvements. The noise floor would

be appeared after 18 dB.

In eight users scenario, the bit error rate increased significantly mainly due to the

increased amount of MAI. The performance of both systems would nearly be the

same from 0N

Eb ranging from 2 dB to 20 dB. The MAI outweighs the ISI effects and

the inter path interference due to the cross correlation >−< )(),( 11 τtctc . Therefore

the performance for both systems would almost be the same. The noise floor would be

appeared after 16 dB. For 0N

Eb higher than 16 dB, there would not be further

improvement.

31





the processing gain pG of both systems is equal to 64. The diversity order is equal to 4.

The system with utilization for 2, 4 and 8 users would be investigated.

In two users’ scenario, both OFDM-CDMA and DS-CDMA exploit the diversity

combining to obtain performance improvement. For 0N

Eb ranging from 2 dB to 20

dB OFDM-CDMA perform about 1 dB better than DS-CDMA.

In four users’ scenario, OFDM-CDMA outperforms DS-CDMA by 1 dB for 0N

Eb

ranging from 2 dB to 14 dB. The noise floor would be appeared after 0N

Eb =16 dB.

In eight users’ scenario, the performance for both system is similar due to the

increased amount of MAI. And the noise floor would be started at 14 dB.

32





the processing gain pG of both systems is equal to 128. The diversity order is equal

to 2. The system with utilization for 2, 4 and 8 users would be investigated.

In two users scenario, as discussed before the performance for both system would be

improved when compared to 64=pG with same diversity order. For 0N

Eb <10 dB,

OFDM-CDMA outperforms DS-CDMA by about 1 dB. For 0N

Eb >10 dB,

OFDM-CDMA outperforms DS-CDMA by about 2 dB. There is no ISI effects and

inter path interference in OFDM-CDMA, therefore the performance is improved.

33

In four users’ scenario, the similar results would be found. The 1 dB performance

improvement can be found for 0N

Eb <10 dB and 2 dB performance improvement for

0N

Eb >10 dB. Notice that no noise floor would be found for 0N

Eb <20 dB.

In eight users’ scenario, it can be seen that the results are agreed to the two users and

four users scenario. The effects of MAI can not totally outweigh the effects of ISI and

the inter path interference.

34





the processing gain pG of both systems is equal to 128. The diversity order is equal to

4. The system with utilization for 2, 4 and 8 users would be investigated.

In two users’ scenario, the results are similar to that when the diversity order is equal

to two with the same number of active users.

In four users scenario, OFDM-CDMA perform 1 dB better than DS-CDMA system

for 0N

Eb <12 dB and 2 dB improvement for 0N

Eb >12 dB.

In eight users scenario, the performance for both system is very similar to each other

for 0N

Eb <10 dB. The noise floor would be appeared at about 16 dB at which the BER

= 10-3

.

35


fixed Processing Gain and Diversity for type B delay profile


Figure 23 comparison between DS-CDMA and OFDM-CDMA for type B delay


From figure 23, we can see that OFDM-CDMA perform better than DS-CDMA after

v>6 dB with diversity order equal to 2 and the processing gain is fixed to 64.

When c is small, the performance is affected by the Gaussian Noise and the

performance of both systems would be close to each other.

After 6 dB, the performance is affected by the inter path interference and the ISI. As a

result, for two users and four users scenario, OFDM-CDMA outperforms DS-CDMA

36

by about 1 dB for 0N

Eb >10 dB.

The MAI effects would become dominant when the number of active users increased,

therefore for eight users scenario the performance of both systems would be similar.

The noise floor for both systems would be appeared at 16 dB.




The general trend of the performance for both systems would follow to those results

when the delay profile A is applied except that the performance would be even better.

It can be verified by the two users’ scenario. The BER can archive 10-4

when 0N

Eb =11

dB for OFDM-CDMA system. The case is also valid for DS-CDMA system in which

37

the BER can archive 10-4

when 0N

Eb is almost 12 dB.




From figure 25, OFDM-CDMA performs 2 dB better than DS-CDMA for high SNR

for using type B delay profile. The results hold for when the diversity order is

increased to 4. It can be shown by figure 26

38



From figure 26, we can see OFDM-CDMA perform about 2 dB better than

DS-CDMA when the SNR is high. At the eight users scenario, even the MAI effects

cannot totally outweighs the effect of ISI and the inter path interference. The

performance for both systems would be improved for using type B delay profile rather

than using type A.

39

Section 7

Conclusions

Two multiple access schemes DS-CDMA and OFDM-CDMA have been compared

for the uplink synchronized communication over slow Rayleigh fading channel. Both

systems will apply MRC. The analytical results are confirmed by simulation results. A

performance comparison of the DS-CDMA system and the OFDM-CDMA system

shows that for a low number of active users OFDM-CDMA outperforms DS-CDMA

by about 2 dB. However, with an increasing number of active users both systems

perform similarly due to the increased amount of MAI.

40

References

[1] Thomas Wharton., “Multiuser detection,” Cambridge University

Press., pp. 4-10, 1998.

[2] Theodore S. Rappaport., “Wireless Communications Principles and

Practice,” Prentice Hall., p. 209.

[3] Savo Glisic, Branka Vucetic., “Spread Spectrum CDMA Systems for

Wireless Communications,” Artech House Publishers., p. 237.

[4] L Hanzo, M. Munster, B. J. Choi, T. Keller, “OFDM and MC-CDMA

for Broadband Multi-User Communications, Wlans and Broadcasting,”

Wiley., p. 33.



Wiley., pp. 27-29.



Wiley., p. 41.



Wiley., pp. 250-251.

[8] Savo Glisic, Branka Vucetic., “Spread Spectrum CDMA Systems for

Wireless Communications,” Artech House Publishers., pp. 237-239.

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