Simple diversity scheme for IDMA communication system.
Shukla, M. ; Shukla, Aasheesh ; Kumar, Rohit 等
Introduction
Now-a-days the requirements of wireless communication are to have
high voice quality, high data rates, multimedia features, lightweight
communication devices etc. But the wireless communication channel
suffers from many impairments such as thermal noise, the path loss in
power (as in the case of radio signal propagation), the shadowing effect
(due to the presence of fixed obstacles in the radio signal path), the
fading which is due to the effect of multiple propagation paths, and the
rapid movement of mobile communication devices. Upon the signal
transmission, different signal copies undergo different attenuation,
distortion, delays and phase shifts. Due to this problem, the overall
system performance can be severely degraded[4].
In a typical wireless communication environment, multiple
propagation paths often exist from a transmitter to a receiver due to
scattering by different objects. Signal copies following different paths
can undergo different attenuation, distortions, delays and phase shifts.
Constructive and destructive interference can occur at the receiver.
When destructive interference occurs, the signal power can be
significantly diminished. This phenomenon is called fading. The
performance of a system (in terms of probability of error) can be
severely degraded by fading. [4]
The improvement in signal to noise ratio may be achieved by higher
transmit power or additional bandwidth, but this is the contradiction to
the requirements of wireless communication system. So, this is necessary
to reduce the problem of fading, but not at the cost of extra power or
additional bandwidth.
One effective solution is proposed for wireless system named
diversity, with out the requirement of power or extra bandwidth.
Time and frequency diversity can be effectively used to combat the
effect of fading. However, time interleaving produces large delays when
the channel is slowly varying. And spread spectrum techniques are
ineffective when the coherence bandwidth of the channel is larger than
the spreading bandwidth [2].
In most wireless channels, antenna diversity is a practical,
effective and widely used technique for reducing the effect of multipath
fading.
Since this is also known that one new multiple access technique
called IDMA is proposed for future wire less communication system, which
is the advanced version of CDMA, and removes the problems of MAI and
ISI.
The objective of this paper is to use diversity in IDMA
communication system to reduce the effect of fading. The paper is
organized as follows. Concept of IDMA is introduced in section I.
Section II deals with diversity techniques, followed by the classical
MRRC approach with IDMA in section III. Performance analysis is given in
section IV. Finally conclusion is given in section V.
Concept of Idma
The performance of conventional CDMA systems [1] is limited by
multiple access interference (MAI), as well as intersymbol interference
(ISI). Also, the complexity of CDMA multi-user detection has always been
a serious concern. The problem can be seen from the angle of
computational cost as well complexity of multi-user detection
algorithms. The use of signature sequences for user separation is a
characteristic feature for a conventional CDMA system. The possibility
of employing interleaving for user separation in CDMA systems is briefly
mentioned in [1] but the receiver complexity is considered as a main
obstacle.
Possible Solution for User Separation
Now the question arises that what should be the strategy for
distinguishing the different users. The possible solutions includes
narrow band coded-modulation scheme using trellis code structures, and
to employ chip-level interleavers [1].
Introduction to IDMA
IDMA stands for interleave-division multiple-access (IDMA) scheme
for spread spectrum mobile communication systems, in which users are
distinguished by different chip-level interleavers instead of by
different signatures as in a conventional CDMA system. The scheme
considered is a special case of CDMA in which bandwidth expansion is
entirely performed by low-rate coding. For convenience, it may be
referred as interleave-division multiple-access (IDMA). This scheme
inherits many advantages from CDMA such as dynamic channel sharing,
mitigation of crosscell interferences, asynchronous transmission, ease
of cell planning, and robustness against fading. It also allows a low
complexity multiple user detection techniques applicable to systems with
large numbers of users in multipath channels.
IDMA Mechanism
In conventional CDMA, signature sequences are used for user
separation while in IDMA, every user is separated with individual
interleaver, orthogonal in nature. The block diagram of IDMA scheme is
shown for K users in figure 1. Data from user k is first encoded by a
rate-R binary forward error control (FEC) code followed by spreader.
Each user has been assigned common signature sequence [s.sub.k] with
length S. The elements in [s.sub.k] are called chips. The spreader for
user k spreads a coded bit to a chip sequence (i.e., it transmits either
[s.sub.k] or--[s.sub.k] to represent one bit). The spreading operation
results bandwidth expansion since a single chip alone can carry one bit
of information. Then interleaving is done on each of the data related to
individual user. The redundancy from interleaving is introduced mainly
to distinguish different users. From a coding theory point of view,
however, this is a good choice since it introduces redundancy with
coding gain.
The principle of iterative multi user detection (MUD) which is a
promising technique for multiple access problems (MAI), is illustrated
in the lower part of Fig. 1. The turbo processor involves an elementary
multi-user detector (EMUD) and a bank of K decoders (DECs). The EMUD
partially resolves MAI without considering FEC coding. The outputs of
the EMUD are then passed to the DECs for further refinement using the
FEC coding constraint. The DECs outputs are fed back to the EMUD to
improve its estimates in the next iteration. This iterative procedure is
repeated a preset number of times (or terminated if a certain stopping
criterion is fulfilled). After the final iteration, the DECs produce
hard decisions on the information bits.
[FIGURE 1 OMITTED]
In the turbo processor, each DEC handles the data for a particular
user only and ignores the others. Therefore, the DEC complexity per user
is independent of the user number K. The task of the EMUD, on the other
hand, is to find a joint solution considering all users. The complexity
involved (mainly for solving a size KxK correlation matrix) is
O([K.sup.2]) per user by the well-known iterative minimum mean square
error (MMSE) technique in CDMA. while in IDMA, it is independent of
user. This can be a major benefit when K is large.
Diversity Techniques
Diversity techniques can be used to improve system performance in
fading channels without the requirement of extra power or bandwidth.
Instead of transmitting and receiving the desired signal through one
channel, N copies of the desired signal through M different channels.
The idea is that while some copies may undergo deep fades, others may
not. So, enough energy can be obtained at the receiver to make the
correct decision on the transmitted symbol. Introduction of of diversity
techniques are given below, which are commonly use in wireless
communication systems.
Frequency Diversity
One approach to achieve diversity is to modulate the information
signal through M different carriers. Each carrier should be separated
from the others by at least the coherence bandwidth. So that all the
copies of the signal undergo independent fading. At the receiver, the N
independently faded copies are "optimally" combined to give a
true decision. Frequency diversity can be used to reduce frequency
selective fading.
Time Diversity
One more approach to achieve diversity is to transmit the desired
signal in M different periods of time, i.e., each symbol is transmitted
M times. The intervals between transmissions of the same symbol should
be at least the coherence time so that different copies of the same
symbol undergo independent fading. Optimal combining can also be
obtained with the maximum ratio combiner.
Space Diversity
Another approach to achieve diversity is to use M antennas to
receive M copies of the transmitted signal. The antennae should be
spaced far enough apart so that different received copies of the signal
undergo independent fading. In this type of diversity, no additional
work is required on the transmission end, and no additional bandwidth or
transmission time is required.
This paper also focuses on the space (antenna) diversity, because
this is widely used technique in wire less communication system However,
physical constraints may limit its applications. Sometimes, several
transmission antennae are also employed to send out several copies of
the transmitted signal. That is known as transmit diversity.
[FIGURE 2 OMITTED]
Maximal Ratio Combining Scheme
In this method, the diversity branches are weighted for maximum
SNR. As shown in block diagram dk is data of kth user,after encoding and
spreading the data is randomly interleaved and termed as
'chips'.Now this chip Signal [x.sub.k] is sent from the
transmit antenna, which will propagate from both the channel. If we
consider 1 transmit and 2 receive antenna, then channel between transmit
antenna and the received antenna zero is [h.sub.0] and between the
transmit antenna and receive antenna one is denoted by h1. the channel
can be modeled having magnitude and phase response. So,
[h.sub.0] = [[alpha].sub.0][e.sup.i[theta]0]
[h.sub.1] = [[alpha].sub.0][e.sup.i[theta]1] (1)
Noise can be added at both the receiver. The resulting received
signals are
[R.sub.0] = [h.sub.0][x.sub.k]+[n.sub.0]
[R.sub.1] = [h.sub.1][x.sub.k]+[n.sub.1] (2)
Where [n.sub.0] and [n.sub.1] represents the noise and interference
at both the receiver separately.
Now the Receiver combining scheme for two branches MRRC can be
written as
[[bar.X].sub.K] = [h.sub.0] * [R.sub.0] + [h.sub.1] * [R.sub.1] (3)
Now this output of maximal ratio combiner can fed to the detector
for the proper estimation of transmitted signal [x.sub.k].
[FIGURE 3 OMITTED]
Performance Analysis of Proposed Scheme
The BER performance of IDMA scheme was already verified, that it
shows better performance than conventional CDMA scheme. But in wireless
communication fading is major impairment and it has been already shown
that diversity scheme is the only solution of this problem without any
extra consumption of bandwidth and power.
So, in this section, we present some simulation result to
demonstrate the performance of the proposed scheme. Here we refer the
channel as slow fading Rayleigh channel. The interleavers used in
simulations are generated randomly and independently. Following
notations are also adopted;
[R.sub.message] = Number of message bits.
[S.sub.l] = Spread length
It = No. of iterations
[FIGURE 4 OMITTED]
Figure 4 illustrates the BER performance of IDMA with and without
diversity. The channel used for both the simulation is slow fading
Rayleigh channel. The no. of information bits Rmessage=500. Spread
length=16. No. of Iterations It=10.From this figure we can see that the
performance with diversity is far better than without diversity. Here
two branches maximal ratio combining scheme is used for diversity.
Degree of complexity remains lower as in the case of simple IDMA.
In figure 5 again simulation result are plotted for different data
lengths. Although as data length increases the performance of IDMA with
diversity improves as without diversity. Here simulations are taken at
data lengths 100,500,1000 bits.
[FIGURE 5 OMITTED]
Conclusions
Although concept of IDMA can generate fruitful results in the area
of wireless communication. Since IDMA is just a special form of DS-CDMA.
As a consequence, existing CDMA systems may be enhanced by IDMA as well.
But multipath fading limits the performance of these advance techniques.
So we proposed this scheme so that fading problem can be reduced in the
case of IDMA. Simulation Results verifies that IDMA with diversity
performs better. We have used here classical maximal ratio receiver
combining scheme transmit diversity can also be used for the purpose of
reducing the size and cost of mobile receiving units.
References
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2, pp. 229-231, Aug. 1998.
[2] S.M.Alamouti. "A Simple transmitter diversity scheme for
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1451-1458, Oct.1998.
[3] V.Tarokh, H.Jafarkhani, and A.R. Calderbank, "Space-time
block coding for wireless communications: performance result"
IEEEJ.selecte.Areas Commun., vol 17, pp. 451-460, Mar. 1999.
[4] Hafeth Hourani "An Overview of diversity techniques in
wireless communication systems" S-72.333 Postgraduate course in
radio communication,2004
[5] K.Y.Wu, W.K. leung and Li Ping " Iterative detection of
interleaver based space time codes" IEICE TRANS. Commun., 2004
M. Shukla *, Aasheesh Shukla *, Rohit Kumar *, V.K. Srivastava **
and S. Tiwari **
* Department of Electronics Engineering, HBTI. Kanpur. India
** Department of Electronics & Comm. Engineering, MNNIT.
Allahabad, India.