On the Performance of Multiuser Multiband DS­

UWB System for IEEE 802.15.3a Channel with

Hybrid PIC Rake Receiver
Aamish Hasanl, Asim Anwar, Hasan Mahmood3
Department ofElectronics, Quaid-i-Azam University, Islamabad, Pakistan
laamishsn®gmail.com, 2quaidian_asim@yahoo.com, 3hasan@qau.edu.pk

Abstract- In this paper, we study the performance of receiver complexity, the allocated UWB frequency range
multiband Direct Sequence Ultra Wideband (DS-UWB) is divided into several sub-bands [6-7]. The transmit
system in multiuser environment for IEEE 802.15.3a power of the user can be spread in different bands by
channel. The receiver employs hybrid Parallel Interference using the interleaving techniques, which essentially
Cancellation (PIC) technique with Selective Rake (S Rake) spreads the symbols among sub-bands [8].
and Partial Rake (P Rake) receivers. The system In [9], the performance analysis of multiband (MB)
performance is also evaluated for multistage PIC. The
UWB systems for the case of imperfect synchronization
system is simulated for three different scenarios of IEEE
and inter symbol interference is considered. Although the
802.15.3a channeL The simulation results show that hybrid
performance enhancement is achieved, the system does
multistage PIC with S Rake receiver performs better than
not consider the multiuser environment and different
single stage Rake receiver (S Rake and P Rake) and
detection schemes. In another approach [10], the authors
multistage P Rake receiver, for all three cases of the
show improvement in the performan ce of DS-UWB and
channel.
MB-UWB systems at MAC layer, but any enhancement
Keywords- IEEE 802.15.3a; multiuser multiband DS-UWB; at the physical layer design is not considered. The use of
parallel interference cancellation; PIC; Rake receiver. moments in interference analysis of UWB is presented in
[11]. In this approach, the interference analysis presents
I. INTR ODUCTI ON some interesting results; the performance of this approach
In recent years, Ultra Wideband (UWB ) systems are is not discussed with various receivers. The performance
emerging as a new research area in the field of short of space time block coded multiband orthogonal
distance communications. This technology has several frequency division multiplexing in ultra wide band
attractive features and advantages such as high tolerance systems is investigated for log normal shadowing and
to multipath fading effects, simple design of transceiver, IEEE 802.15.3a channel in [11]. The analysis presents
good performance under low signal to noise ratios useful results, but lacks the discussion on multiuser
channels, more available bandwidth, higher security environments. In [13], different multiuser detection
level with low probability of intercept, and ability to schemes are considered for single band UWB systems,
operate with low transmit power. While these systems but this paper lacks MB-UWB scenario, while in [14],
have numerous benefits, their realization and operation multiuser MB-UWB systems are considered for only
face many challenges [1-2]. Gaussian channels.
In the presences of multipath propagation, the signal at The performance of multiuser single band UWB
the receiver suffers Inter Symbol Interference (lSI) and system for Gaussian channel has been studied extensively
the lack of efficient receiver design degrades the system in literature, the multiuser multiband UWB systems are
performance. In order to mitigate lSI, sophisticated not commonly evaluated under IEEE 802.15.3a channel
receiver with precise synchronization is required, which model. In this work, we present a multiuser multiband
in turn increases the cost, complexity, and power DS-UWB system and evaluate its performance for IEEE
consumption of the system. The frequency selective 802.15.3a channel with hybrid PIC Rake receiver.
fading also reduces the performance in UWB systems. The proposed system enhances a DS-UWB system by
Furthermore, in the case of multiuser communication, the introducing multiple bands in UWB spectrum and
signals from different users interfere with each other, implementing Orthogonal Frequency Division
creating additional performance bottlenecks [3-4]. Multiplexing (OFDM) in each sub-band. A common
In traditional UWB systems, a short pulse-like signal is receiver is designed to receive and decode signals from
used to form a single-band signal which occupies a wide multiple users, transmitting independently from each
spectrum. This design has the advantage of implementing other.
simple receivers and transmitters, but lacks the flexibility The rest of the paper is organized as follows. In section
to manage radio resources efficiently. In addition, the II, we present the system model and describe the
system requires extremely complex signal processing transmitter, the receiver, and the channel. The section III
resources to deal with the requirements of ultra short formally describes the proposed multiuser receiver for
pulse signals [5]. In order to overcome the problem of multiband DS-UWB system. In section N, simulation
processing wide spectrum signals and reducing the results for the proposed system with the description of

978-1-61284-941-6/11/$26.00 ©2011 IEEE

65
different parameters are presented. Finally, Section V where aim. ! are the multipath gain coefficients, TI is the
concludes this paper.
h h
delay of the l cluster, r/:'.! is the delay of the m1 multipath
component relative to i' cluster arrival time and Xi is the
II. SYSTEM M OD EL lognormal shadowing for lh realization.
We consider a multi band DS-UWB system which
C. Receiver
consists of multiple transmitters and a single receiver or a
The receiver structure for the multiuser multiband DS­
common access point. The receiver simultaneously
UWB systems is shown in Fig. 2. The received signal is
receives a composite signal which is the sum of
the sum of all the transmitted signals from different
independent signals from all the transmitters. The PIC
independent users and is represented as,
technique, along with Rake receiver, is employed at the
receiver in order to separate the data of an individual K

user. The PIC scheme is used to cancel multiuser r(t)= LSk(t)*hk(t)

interference (MUI) while Rake receiver is used to
k=1
mitigate the multipath propagation effects.
In this equation, hk(t) is the impulse response of the
802.15.3a channel for the transmitted signal Sk(t) of user k.
A. Transmitter
The block diagram for a single user multiband DS­
UWB transmitter is shown in Fig. 1. The data from user k
is spread with Walsh-Hadamard codes and the resulting
User2
sequence is modulated by a modulation function. The
Pulse Amplitude Modulation (PAM) is used as a
modulation scheme. The modulated symbols are hlJDandRaIo:e I
1
converted into parallel signals used by Inverse Fast Userl(·1

Fourier Transform (IFFT) implementation. A cyclic prefix

is added to the outputs from IFFT block. At this stage, the
parallel signals are converted back into a serial stream,
which is finally transmitted on the channel.
The output of the IFFT block for a sub-band is written
as [15], Fig. 2 Common DS-UWB receiver.

N-I The first step at the receiver is the separation of user's

xk(t)= Ldk(n)exp(j21D1Afl) signals by implementing parallel interference cancellation
n=l technique (PIC). In addition, Rake receiver is used to
where Xk(t) is the OFDM symbol of user If., dk(n) is the data remove multipath interference from the received signal of
of user k in subcarrier n and N is the total number of each user. The cyclic prefix is removed for each individual
subcarriers. The transmitted signal for user k after up­ user's signals and the operation of despreading is
conversion is given as, performed. This signal is converted from serial to parallel
format, which is then processed by the FFT algorithm.
S k (t) = LRe{xk (t - kTSYM )exp(j21ifkt)} The demodulation is performed after FFT and the data bits
k
are available after hard decision.

where fk specifies the carrier frequency of a sub-band for III. MULTIUSER RECEIVERF OR MULTIBAND DS-UWB
f(h OFDM symbol. The duration of a transmitted symbol The performance of multiuser multiband DS-UWB
is given as, TsThFTFFT+Tcp+TGI. systems is influenced by the number of users that are
sharing the bandwidth. In order to increase the capacity of
-,
,
multiuser systems, the proposed receiver implements
,
signal processing techniques for interference cancellation.
Therefore, the receiver design is dominated by PIC and
J
Cyelk
1m Prefix
SI Rake receivers. The block diagram of the proposed
receiver is shown in Fig. 3.
Initially, the received signal is processed by a matched
filter bank. The outputs from these matched filters are
Fig. I Transmitter for user k used to extract useful information about each user. The
signal of an individual user is obtained by subtracting
B. Channel other users matched filter's outputs from the received
In this work, we use IEEE 802.l5.3a channel model for signal.
multiband DS-UWB system. The impulse response of the For example, the output for user 1 is obtained by
channel is given as, subtracting the matched filter's outputs of user 2 through
user K from the composite received signal. The Rake
L M receivers are used to further process the signal for each
hi(t)= Xi L L d",,10(t - T/ - r�,1) user. The outputs from these Rake receivers are fed to
1=1 m=1 cyclic prefix removers, as shown in Fig. 2.

66
 The interference due to multiuser and multipath
propagation has been reduced at this state. The signal is
further processed by the remaining portion of common
DS-UWB receiver as shown in Fig. 2.

IV. S IMU LATI ON RESULTS

We simulate a multiuser multiband DS-UWB system

for IEEE 802.15.3a channel using MatLab [17]. Walsh­
Hadamard codes are used for spreading the information
bits. The system is simulated for three different types of
channel models (CM). Two types of Rake receivers,
namely Selective Rake (S Rake) and Partial Rake (P
Rake), are used in the system. The PIC is used as MUI
cancellation technique. The simulation parameters are
summarized in Table I.
Fig. 3 The internal block diagram of two-stage MUD and Rake receiver. TABLEI
SIMULAnON PARAMETERS
The output from the f(h matched filter can be expressed
as [16], Parameters Simulation Values
1 TSYM Total number of OFDM

Yk(t) f r(t)ak(t)cos(OJi)dt
128
subcarriers
=-
Subcarrier frequency
TSYM 0 soacin2 (ill
4.125 MHz

The signal after subtraction from the received signal is TFFT 242.42 ns
represented as, Tep 60.61 ns

K TGI 9.47 ns
Yk, s(t) r(t)- � >j
=

j=l TsYM 312.5 ns

j�k
Total number of Sub-
\0
At this point, the effect of multiuser interference are Bands
suppressed and removed from the received signal. The next
step is to mitigate the distortion caused by multipath The simulations are performed for three different
propagation. The objective of the Rake receiver is to resolve instances of the IEEE 802.15.3a channel. The different
multipath distortions due to channel impairments. The cases and parameters of this channel are given in Table II.
multipath components in the signal of f(h user are given as,
TABLEII
M
CHANNEL PARAMETERS
Lamsk(t-'rj)ak(t-'rm) + net)
m=l A I.. 0, 0, 0,
Type r r
where ak(t)
is the spreading waveform. The decision
L OS
(llnsec) (llnsec) (dB) (dB) (dB)

statistics which results from the Rake receiver can be (Q.4m) 0.0233 2.5 7.1 4.3 3.3941 3.3941 3
expressed as,
NL OS
(Q.4m) 0.4 0.5 5.5 6.7 3.3941 3.3941 3

NL OS 0.0667 2.1 14 7.9 3.3941 3.3941 3

(4-lOm)

where a: is the complex conjugate of multipath gain and

Zm is given as, Fig. 4 presents the performance comparison of the
multiuser multiband DS-UWB system simulated for CM 1
1 TSYM with hybrid PIC (one-stage and two-stage) Rake receivers.

Zm fY k,s (t)ak(t-'rm)dt
•

=--
The simulation results show that the two-stage PIC with S
TSYM 0 Rake receiver has the best performance among all other
combinations.

67
 ---------------
====== == == ==

= = =1= = = c = = c = = 1: = =
: : : I: : : c : : [ : : I : :
__ � __ ___L __ L __ L __ L _ _

= = i= = = + = = =i = = � = = =1 = = = 1= = = � = -
: : [ : : I : : J : : J : : :1: : : I: : : c : : [ _
1 - - -1- - - I- - - 1- - -
- �SRakew.h StagePIC
2
=
�SRakew.h1StagePIC
: ====== : � == � == � _
= � PRakew.h2StagePIC 1 = = = 1= = = � = = F = =
= �PRakew.h1Sta ePIC 1====
;: = _ --+-- �---�-- ... -- �--�---� --+--�---
I ==�==t===t===
- - r-- - - "1 - - "l - --, ---,---- , - - r-- r --
- - -- T - - 1- - - - - - - - - - - - - - - - - --
I T I 1 1 1 I T
10 '�--�--�L- --�-- --
1��--�--L---�--�--��-- �S �1�---�
12�� 1 4�� 1�
--
�1S��
1O �1�
2
--�� �6�-f1S��20
14 ---1 O O 6 20
O
SNR (dB) Number of Users

Fig. 4 Performance comparison of the system for CM I Fig. 7 Performance comparison of the system with different number of
users.

Fig. 5 and Fig. 6 show the performance comparison of The simulation results for both cases depict that two­
the system for CM 2 (0-4 m) and CM 3 (4-10 m), stage PIC with S Rake receiver, is the optimum scheme in
respectively. These two models are for Non Line of Sight terms of system performance. The degradation in
(NLOS) scenarios. multiuser multiband DS-UWB system performance by
increasing the number of users is shown in Fig. 7.

V. C ONC LUSI ONS

In this paper, we have studied the performance of

multiuser multiband DS-UWB system for IEEE 802.15.3a
channel model. A hybrid PIC and Rake receiver is
proposed for multiuser detection. PIC is used for MUI
1 1 1 1 1 cancellation while Rake receiver is used to mitigate the
= = f = = � = = 3 = = =,= = = E = = f =_

::- ::- ±+ ::- ::- j-+ -

:: ::- j = :: =1= :: :: t :: :: t :: :: :± _ multipath propagation effects. The results show that the
-i - - -1- - - I- - - + - - -
-r - - T- - -
-+ -
performance of the system is enhanced by increasing the
-
r-"'�����=
... Rake w�h 2Stage PIC
....S
- - �-�'-=-i
- - "T -
r - - T - - "T -
- -,
-, -- stages of PIC receiver. The combination of two-stage PIC
_ -0-S Rake with
1 Stage PIC L __ .L __ 1 __ J ___1___

� :::: : ::�: ::� � �::: ::� � � n � � � � n � � �: � � � receiver with S Rake receiver has better performance as
compared to P Rake for all channel models.
= = I _ = J = = =1 = = = 1= = = C = = I = = J: = = J = = =1 = = =
1 1 1 1 1 1 1 1 1
10 �--�--�--�--�-- -
� -1�-- f 12��14�� 1� 6 ---1�S--� 20
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O
SNR (dB) "
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