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

[1] M. Moher and P. Guinand, "An iterative algorithm for asynchronous coded multi-user detection," IEEE Commun. Lett., vol. 2, pp. 229-231, Aug. 1998.

[2] S.M.Alamouti. "A Simple transmitter diversity scheme for wireless communications" IEEE J. Select. Areas Commun., vol.16, pp. 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.