Simulation of a Coded Spread Spectrum System for

IETE Journal of Research

ISSN: 0377-2063 (Print) 0974-780X (Online) Journal homepage: http://www.tandfonline.com/loi/tijr20

Simulation of a Coded Spread Spectrum System for the Impulsive and Multipath Channel Sakuntala S Pillai FIETE & M Harisankar FIETE To cite this article: Sakuntala S Pillai FIETE & M Harisankar FIETE (1990) Simulation of a Coded Spread Spectrum System for the Impulsive and Multipath Channel, IETE Journal of Research, 36:5-6, 542-549, DOI: 10.1080/03772063.1990.11436926 To link to this article: http://dx.doi.org/10.1080/03772063.1990.11436926

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J. INSTN. ELECIRONICS &. TELECOM. ENORS.• Vol. 36. Nos. 5 &. '· 1990

sequences described in this paper is less than this upper bound of [6].

REFERENCES 1. M B Pursley, Performance evaluation for phase coc

;pread spectrum multiple access communication-Part I : System analysis, IEEE Trans Commun, vol COM-25, pp 795-799, 1977.

Therefore the new class of sequences are inferior to Gold sequences only in terms of periodic ACF/CCF. However as discussed in [1,2,6] in asynchronous spread spectrum systems average correlation parameters are more important compared to the peak periodic correlation parameters.

2. C S, Goswami & M Beale, Correlation properties of dual BCH, Kasami and other sequences for spread spectrum multiple access systems, lEE Proc, Pt F, vol 135, pp 114-118, 1988. 3. S Prasad, P V A Narasimham & L C Quynh, Some further results on a class of sequences with good auto-correlation and high linear complexity, lEE Electron Lett, vol 24, pp 1447-1449, Nov 1988.

CONCLUSIONS

4. D V Sarwate & M B Pursley, Corss-correlation properties of pseudorandom and related sequences, Proc IEEE, vol 68, pp 593· 619, 1980.

In this paper a new class of small set of sequences and another class of large set of sequences are introduced. The number of sequences in the small set is equal to the number of sequences in the small set of Kasami sequences. The peak correlation properties are almost similar to those of the set of sequences sets introduced in {2] but they are inferior to those of the small set of Kasami sequences. However the average normalised interference parameters of all the three classes are shown to be comparable. Similar results are shown to hold for the large set of sequences in comparison with the Gold sequences.

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5. S Prasad & L C Quynh, Equivalent linear span analysis of binary sequences having an interleaved structure, lEE Proc, Pt. F, vol 133, pp 288-292, 1986.

6. M B Pursley & D V Sarwate, Performance evaluation for phase coded spread spectrum multiple access communications-Part II: Code sequence analysis, IEEE Trans Commun, vol COM-25, pp 800-803, 1977. 7. S Prasad & L C Quyhn, Class of binary cipher sequences with best possible auto-correlation function, lEE Proc, Pt F, pp 576580, 1985.

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Simulation of a Coded Spread Spectrum System for the Impulsive and Multipath Channel SAKUNTALA S PILLA!, FIETI! State Committee on Science, Technology & Environment, Trivandrum 695 037, India AND

M HARISANKAR, FIET1! College of Engineering, Trivandrum 695 016, India Performance of a direct sequence spread spectrum communication system operating in an impulsive noise and multipath cbannel is evaluated using computer simulation techniques. The results indicate tbat the direct sequence spread spectrum receiver tends to convert tbe block error pattern In to random error pattern and further application of convolutional coding with Viterbi decoding gives substantial improvement in performance. Balanced Gold codes are found to perform better than Gold codes and m-sequenees. Indexing terms : Spread spectrum communication, Eerror correcting code, Digital computer simulation, Viterji decoding, Soft decision decoding, Multipath channel, Impulsive noise channel, Atmospheric noise

WI11-I the unprecedented developments in VLSI

techno~

logy, spread spectrum modulat-ion techniques are finding increasing applications in cilvilian areas as well. This paper presents the simulation studies in a coded Direct Sequence spread spectrum system operating in the HF band in tropics. Multipath fading and impulsive atmos~ pheric noise are the major sources of interference to digtal communication services in thls frequency band. Paper No. 145-D; Copyright

C

1990 by IETE.

The chief motivation for considering spread spectrum modulation is that it possesses certain capabilities which cannot be obtained through conventional modulation techniques. Spread spectrum systems have the capability to reject both deliberate and unintentional interferences [1]. Fuxther, the demodulator of the Direct Sequence system makes use of correlation detection which causes cancellation of the large amplitude noise spikes that severely degrade the performance of HF links. Due to these reasons, spread spectrum modulation has been

PILLAI & HARISANKAR : CooED SPREAD SPEcnnn.t Svsnw

found to perform well in impulsive noise environments [2-7]. The wide bandwidth of the spread spectrum system also yields a multipath tolerance capability. In the frequency domain, the improvement stems from the frequency diversity inherent in the spread signal. Viewed in the time domain, the correlation detection makes it possible to iso'~ e those portions of the signal arriving with different delays. This provides the ability not only to avoid the destructive effects of multi path but also to benrfit fwm the significant energy in the multipath returns by using them in the bit detection process. Turin [8] has conducted exhaustive computer simulation studies of digital signal transmission through urban/suburban multipath channels havng Gaussian noise distribution and has reported that spread spectrum systrms perform better than the conventional systems in the multipath environment. The simulation results presented in this paper highlight the importance of using spread spectrum techniques to combat the problems of impulsive noise interference as well as multipath fading. Error correction coding schemes are reported to exhibit poor error rates in burst noise channels. In this work it has been brought out that spread spectrum modulat:on helps to convert the burst errors caused by the impulsive noise spikes to random errors. These are subsequently overcome by the application of a random error correcting code like convolutional code. Soft decision decoding doubles the correction power of the code without further redundancy penalty, thereby improving the correction power per bit. In the work presented here the performance of the system incorporating 1/2 rate convolutional codes of constraint lengths 3 and 4 arc evaluated for both hard decision and soft decision Viterbi decoding. Evaluation of a digital communication system performance using prototype hardware experimentation is expensive, unweildy and inflexible. As an alternate course, it is possible to build a set of software modules which can act as simulation packages. If the software modules are carefully designed, it will be possible to test and evaluate a wide variety of configurations. But simulation of a communication system requires large amount of computet memory. The high data rates associated with the communication systems tend to result in computation times which exceed reasonable limits. In the present work, this is reduced to a great extent by selecting the sampling rate independent of the carrier frequency. To facilitate this the signals are generated as discrete time samples at baseband using complex low pass equivalents. Whenever the bandwidths vary over a wide range, the sampling rate is varied proportional to the bandwidth. Further, the programs developed simulate an abstraction of the system rather than the actual hardware system. This also helps to reduce the computer time and memory requirements to a great extent.

The software used in the simulation program is written in FORTRAN IV language. It is extremely important that the simulator be capable of handling the different systrm configurations. For this reason, the overall simulation package is made up of a mainframe and an extensible set of modules. These modules can be combined in a wide variety of ways to simulate the different situations. The interthces between the program modules are carefully defined to fzcilitate implementation of new modules and their integration into the system. Since signals and system parameters contain random variations, the performance is evaluated using Monte Carlo simulation techniques implemented on CDC CYBER 170/730 computer. SYSTEM MODEL

The functional blocks of the baseband spread spectrum system that is being simulated is as shown in Fig 1. The information source generates a sequence of binary random numbers using a pseudorandom binary sequence generator. Since the transmitted symbolr 0 and I are affected differently by the channel and the number of symbols used for transmission is limited, an all zero sequence is used in the present work to represent the input sequence. The channel encoder is used to encode the input data by adding redundant bits so as to accentuate the uniqueness of each message. In the present model, option is provided to select 1/2 rate convolutional codes of constraint lengths 3 or 4 as required. The encoder is implemented as a shift register of length K into which information bits are shifted one at a time. The shift register tappings correspond to the generator polynomials for the optimal codes suggested by Odenwalder. Convolutional code of contsraint length K = 3 has already been described in great detail [9]. Figure 2 shows the encoder for K = 4 with the code generators represented in binary, octal and polynomial forms. The trellis diagram describing the convolutional encoder for K = 4 is given in Fig 3. The direct sequence spread spectrum modulator modulates the binary encoded signals by a high speed PN code. The importance of the PN code sequence in a spread spectrum system is difficult to over emphasize, since the type of code used, its length and its bit rate set bounds on the capaci1y of the system tl1..at can be c}l..anged only by changing the code. In the present model, option is provide.t to select one of the following three sequences as the spreading code: (i) m-sequence, (ii) Gold code and (iii) Balanced Gold code. The m-scqucnces generated from irreducible polynomials have ideal auto l/Tm

(2)

Here Tm is the minimum resolvable multi path delay that is expected. The normal value of multipath delay vs.ries from 1 to JO msec for the HF band (13]. Energy in the multipath signals will be spread by the PN code in the Fig 2 Encoder for the 1/2 rate K=4 convolutional code correlator demodulator. However, their power spectral densities will be very f.mall. Instead of causing fading of 2