Analysis of BER Performance with MMSE Detection Scheme for DSCDMA and MC-CDMA Systems and Compare them

Analysis of BER Performance with MMSE Detection Scheme for DSCDMA and MC-CDMA Systems and Compare them

Kamal A. Patel, Prof.

Electronics and Communication Department, GEC, Surat, Gujarat, India

Prof. Tejas S. Patel

Electronics and Communication Department, GEC, Surat, Gujarat, India

Abstract

In direct Sequence spread spectrum transmission, the user data is multiplied by a binary code sequence. And the receiver side, the coded signal is again multiplied by same code sequence to get original signal back. Multi-Carrier Code Division Multiple Access (MC-CDMA) is a multiple access scheme used in OF DM-based telecommunication systems, allowing the system to support multiple users at the same time. MC-CDMA system is orthogonal communication scheme. DS -CDMA system is on type of simple C DMA system. MC- CDMA is combination of OFDM and DS-C DMA. The performance measure is based on Rayleigh Fading Channel. The bit error rate performance of MC-CDMA and DS-C DMA systems with Minimum Mean Square Error (MMSE ) detection are carried out by MATLAB simulation. This paper represents BER performance for DS-CDMA and MC-CDMA systems and find out w hich C DMA system is good for communication among DS-C DMA and MC- CDMA. This paper also presents that what effect on BER, if no of bits per user is increased, if no of data sub-carriers increased and if no of users increased.

1. Introduction

   In a multi-user communication system, mult iple access capability is required to transmit mu lt iple signals simultaneously. The proble m with simp le non overlapping Frequency division mult iple xing (FDM ) system is that at least twice the bandwidth of the original data signal is required to transmit the mu lti-carrier modulated signal. A more e fficient and improved version of FDM is orthogonal frequency division multip le xing (OFDM). OFDM is multi- carrie r modulation scheme in which seria l data stream is divided into several parallel b it stream, which are transmitted using large number of orthogonal carriers. OFDM is bandwidth effic ient by allo wing its sub-channels to overlap.

   Code Division Multiple Access (CDMA) : Multiple users want to commun icate in co mmon geographical area. Now the proble m is, how should we share our recourses so that as many users as

   possible can communicate simultaneously. We can solve this problem by CDMA system. In CDMA system each user is allocated with different codes. Also, there is no proble m of interference because the codes are orthogonal to each other.

   CDMA issues:

   Tight synchronization is required to use orthogonal codes, which then break in a mu lti-path channel anyway.

   Quasi-orthogonal codes cause self- interference, which do minates the performance in most CDMA systems.

   Near-far p roble m is a serious hindrance, requiring fast and accurate power control (that uses up bits we could otherwise send informat ion with).

   And for all this, the required bandwidth is now J times larger than it was before, so there doesnt appear to be a capacity gain.

   There are three types of CDMA system.

   Direct sequence-CDMA (DS-CDMA) Multi carrie r (MC-CDMA)

   Multi carrier- direct sequence (MC-DS-

   CDMA).

   We are interested only in analysis of MC-CDMA and generate BER performance based on MMSE detection scheme for single user and multi user system and observe the waveform generated by MATLAB code.

2. Direct Sequence Code Division multiple Access (DS-CDMA)

   Direct Sequence spread spectrum transmission, the user data signal is mu ltiplied by a code sequence. Mostly, binary sequences are used. The duration of an element in the code is called the "chip time". The ratio between the user symbol time and the chip time is called the spread factor. The transmit signal occupies a bandwidth that equals the spread factor times the bandwidth of the user data. At the receiver, the received signal is again mu ltip lied by the same (synchronized) code. Th is operation re moves the code, so we recover the original user data. A ma jor d ifficu lty in Direct Sequence transmission is the Near-Fa r e ffect. That can be solved by power control system. There is also

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   another problem of inherent diversity in case of DS- CDMA system. In a mu lt i-path fading channel, direct signal may interfere with the delayed reflection signal. Hence, DS-CDMA signal is suffering fro m the multi-path dispersion, which can be detected by the rack receiver. These receivers only combines the signals received over mult iple paths. Multiple access capability in DS -CDMA is achieved by allocating distinct codes to different users of the system.

   Figure 1.DS-CDM A Signal

   Fro m this waveform we can generate the BER performance MATLAB code for the DS_CDMA system. All signals are overlap and interfere with each other. Receiver can recover individual signal by correlating the combined signal with codes of desired users. If the cods are mutually orthogonal, then the original signal can be regenerated without any interference. Th is can be achieved by using orthogonal PN sequenced codes, and then it is the combination of DS-CDMA and OFDM. Th is combined system is called M C-CDMA system

3. Multi-carrier Code Division Multiple Access (MC-CDMA)

   The future wire less mobile co mmun ication systems will be required to support high speed transmission rate. The high data rate requires broad frequency bands. Unfortunately in broadband wireless channel, due to the more number of resolvable mu ltiple paths fading degrade the BER performance. MC-CDMA is mult iple access scheme that allowing system to support mult iple users at the same time with good BER using standard receiver techniques. MC CDMA spreads each user symbol in the frequency domain. That is, each user symbol is carried over mult iple para llel sub-carrie rs, but it is phase shifted (typically 0 or 180 degrees) according to a code value. The code values differ per sub – carrie rs and per user. The receiver combines all sub- carrie rs signals, by weighing these to compensate varying signal strengths and undo the code shift. The receiver can separate signals of different users, because these have different (e.g. orthogonal) code values. Since each data symbol occupies a much wider bandwidth (in hertz) than the data rate (in bit/s), a signal-to-noise-plus interference ratio (if

   defined as signal power divided by total noise plus interference power in the entire transmission band) of less than 0 dB is feasible.

   Unlike DS-CDMA, MC_ CDMA applies spreading sequences in frequency domain. The original info rmation becomes spreaded in frequency domain direct ly. After this spreading, the highly successful OFDM transmitter structure is borrowed and we hope that it will show all the benefits of DS- CDMA and OFDM.

   Features:

   Frequency domain spreading.

   The resulting spectrum of each subcarrier can satisfy the orthogonality condition with the minimu m frequency separation. So, spectrum can be utilized mo re efficiently.

   In MC-CDMA, the code sequence is the Fourier transform of Walsh Hadamard sequence.

   It can be imple mented via OFDM technique.

   In MC-CDMA, a fter spreading FFT is performed.

   Advantages of MC-CDMA :

   As compared to DS-CDM A:

   DS-CDMA is a method to share spectrum among mu ltiple simu ltaneous users. Moreover, it can exploit frequency diversity, using a RAKE receiver. However, in a d ispersive mult ipath channel, DS-CDMA with a spread factor N can accommodate N simultaneous users only if highly comp le x interferene cancellation techniques are used. In practice this is difficult to imple ment. MC- CDMA can handle N simultaneous users with good BER using standard receiver techniques.

   As compared to simple OFDM:

   To avoid excessive bit errors on subcarriers that are in a deep fade, OFDM typically applies coding. Hence, the number of subcarriers needed is larger than the number of bits or symbols transmitted simu ltaneously. MC-CDMA replaces this encoder by an N×N matrix operation. Our initia l results reveal an imp roved BER.

   Therefore, theoretically it is proved that MC- CDMA is more robust and identical to communicat ion system than DS-CDMA. Now, to prove it practically, we have to generate MATLAB code for DS-CDMA and MC-CDMA with MMSE detection scheme.

4. Mc-CDMA Trans mitter and Receiver Block Diagram

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   The stronger amplitudes usually have less noise component in them. M RC is optimu m for the single user case, but in the presence of multip le users, the distraction of orthogonality of the WH codes is increased which results in an increase in the effective mult iple access interference (MAI). The gain e xpression is given by:

   W *=h . l = 1, 2 L. (1)

   l l

   Figure 2. MC-CDMA Transmitter

   Figure 3. MC-CDMA Transmitter

   At the receiver side inverse process is done as shown in figure 3. According to these two block diagra m generate MATLA B code for MC-CDMA system.

   Here, we use Walsh-Hadamard code, which are orthogonal for a specific delay. They are not orthogonal in general. So, spreading is necessary. For a e fficient co mmunicat ion BPSK modulation required. It converts 0 level into -1 level. At receiver side, received signal is filtered and noise corrupted version of transmitted signal.

   As signal to noise ratio is higher, the system performance is better. Now, we have to imple ment BER versus Eb/N0 graph. The transmitter block diagra m fo r M C-CDMA is shown in figure 2.

5. Detection Techniques

   Various detection methods have been investigated for M C-CDMA. Since each sub-carrier conveys a narrowband waveform, coherent detection schemes are the most common. For the downlink systems, single user based detection approaches such as MRC and MMSE are considered. In these schemes, the spreading code and channel coefficients of the user of interest are only utilized fo r the detection process.

   1. Maximal Ratio Combiner (MRC):

      In ma xima l ratio co mbin ing, (M RC) the amp litudes of the received signal are actually squared, so that it enhances the detection decision.

      Where L = no of users.

   2. Minimum Mean Square Error (MMSE):

      In MMSE detection technique, the mean square error between transmitted signal Sl and the equalize r output Xl is given by

      E={|Sl-Xl|2}. (2)

      l

      l

      0

      s

      Wl=h */(|h |2+(LN /(2KE ))). (3)

      Where Es denotes the average energy of the received data bit, K is the number of total users and No/2 is a two-sided noise spectral density. And hl is Received signal b it. Thus, MMSEC detection requires finding the MMSE gains at each carrier using above eq. and then correlating the equalize r outputs signal with spreading code of the desired user.

6. Simulation and Results

   Now, by using MATLAB software, we can generate our code of both CDMA systems for BER performance. For that there are some theoretical and practical BER e xpressions which are used in code, those are as below.

   1. Theoretical expression

      (4)

      Theoretical BER for AW GN channel:

      (5)

      Theoretical BER for Rayle igh fading channel:

      (6)

   2. Practical expression

      (7)

      (8)

   3. Simulation Results for DS-CDMA

      Figure 4. Theoretical BER for AWGN and Rayleigh channel.

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      Figure 5. BER performance of DS-CDM A sysyem with MM SE detection scheme with no of bits per user is 10, no of data subcarriers is 4 for the no of users is 2

      Figure 6. BER performance of DS-CDM A sysyem with MM SE detection scheme with no of bits per user is 10^4, no of data subcarriers is 4 for the no of users is 2

      Figure 4 shows the theoretical BER perfo rmance for AW GN and Ray leigh fading channel. Figure 5 shows the BER performance of M C-CDMA for MMSE detection techniques with no of bits per user is 10 and no of data sub-carriers is 4. If we increase the no of bits per user from 10 to 10^4, we get better performance than in Figure 5.

      The increased bits per us er result is shown in figure 6. The increased no of data sub-carriers and increased no of users results for DS-CDMA are shown in figure 6 and figure 7 respectively.

      The results of figure 5, 6, 7 and 8 are compare with Theoretica l result in Figure 4. One can note that the results are not good for DS-CDMA system. Therefore we are going for M C-CDMA system and doing same process.

      Figure 7. BER performance of DS-CDM A sysyem with MM SE detection scheme with no of bits per user is 10^4, no of data subcarriers is 8 for the no of users is 2

      Figure 8. BER performance of DS-CDM A sysyem with MM SE detection scheme with no of bits per user is 10^4, no of data subcarriers is 8 for the no of users is 3

      6.3. Simulation Results for DM-CDMA

      The Figure 9 shows the BER performance for MC-CDMA system for MMSE detection technique. As we increased the no of bits per user from 10 to 10^4, we get better performance of MC-CDMA system as shown in figure 10.

      Now, if we increased the no of data-sub-carriers, we get much better result as shown in figure 11 than in figure 10. And if we increased no of users then the result is like figure 12. No w, co mpare MC- CDMA result with theoretical result in figure 4 and see the difference between the results of DS-CDMA and MC-CDMA. Also compare result of figure 8 and figure 12.

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      Figure 9. BER performance of M C-CDMA sysyem with MM SE detection scheme with no of bits per user is 10, no of data subcarriers is 4 for the no of users is 2

      Figure 10. BER performance of M C-CDMA sysyem with MM SE detection scheme with no of bits per user is 10^4, no of data subcarriers is 4 for the no of users is 2

      Figure 11. BER performance of M C-CDMA sysyem with MM SE detection scheme with no of bits per user is 10^4, no of data subcarriers is 8 for the no of users is 2.

      By compa ring the results of DS-CDMA and MC- CDMA in figure 8 and 12, one can say that, the MC- CDMA shows better performance than DS-CDMA, as no of users increasing.

      Figure 12. BER performance of M C-CDMA sysyem with MM SE detection scheme with no of bits per user is 10^4, no of data subcarriers is 8 for the no of users is 3.

      MC-CDMA has good BER performance than DS- CDMA. So we can say that MC-CDMA is more robust and better performing fo r co mmunication system than DS-CDMA.

7. Conclusion

MC-CDMA system is more robust and well performed in co mmunicat ion system than DS – CDMA and OFDM system, because of its unique advantages. The BER performances for MC-CDMA are better for increasing no users. The waveform for the multi user shows that as the no of bits per user increasing, the overall BER performance is increasing for M C-CDMA. So this system with MMSE detection technique provides a good system. Also as no of data subcarriers is increase, the BER performance is better for MC-CDMA system.

References

1. Khairy El_Barbary1& Hamad M . Alneyadi, Comparison of behavior of MM SE detection technique for DS-CDM A and M C-CDMA Wireless and Optical Communications Networks, WOCN, Second IFIP International Conference, 490- 495, ISBN: 0-7803-9019- 9, on 6-8 June, 2005.

2. Tamer A. Kadous & Akbar M . Sayeed Equivalence of Linear MM SE Detection in DS-CDM A and M C- CDM A Systems over Time and Frequency Selective Channels, IEEEEURASIP Journal on Applied Signal Processing 2002:12, 13351354 @2002 Hindawi Publishing Corporation M ay 2002.

3. S. Kaiser, OFDM -CDM A versus DSCDMA: Performance evaluation for fading channels, Proc. of IEEE ICC95, pp. 1722- 1726, 1995.

4. Shinsuke Hara and Ramjee Prasd, Overview of M C- CDM A system,126-133, IEEE Communication M agazines, December 1997.

5. Bernard sklar, Rayleigh fading Channel in mobile

   digital Communication systems-Channel Equalization, 90-100, July 1997.

6. Tao LUO, Gang WU, and Shaogian LI, Yong Liang GAUN, Choi LOOK LAW, DS-CDM A and M C-CDM A with per-user MM SE Frequency Domain Equalization, International Journal of Hybrid Information Technology, 11-20, Vol 1.1, No. 3, July 2008.

7. M ohamed Slim Alouini and Andrea J. Goldsmith, Capacity of Rayleigh Fading channel under Different Adaptive transmission and Diversity -Combining Technology, 1165-1181, Vol 4.8, No 4, July 1999.

8. Jorg Habetha and Ling Liu, M C-CDMA Based IEEE

   802.11 wireless LAN, 2004.

9. A. C. M ccornick and E. A. Al-susa, M ulticarrier CDM A for future generation M obile Communication, 52-60,Electronics and Communication Engineering Journal, April-2002.

10. J. G. Proakis, Digital Communications, Code division multiple access, Third Edition, M cGraw-Hill Inc, 1995.

11. J. G. Proakis, Digital Communications, OFDM , M cGraw-Hill, New York, NY, USA, 3rd edition, 1995.

12. B. P. lathi, M odern digital and analog communication system., third edition, channel equilization-pg-323-325.

13. L. Hanzo, M . M unster, B. J. Choi, T. Keller, OFDM and M C-CDMA for Broadband M ultiuser Communication, WLANs and Broadcasting, 1-21,705- 828, 869-890, 2002.

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