Performance Evaluation of a Variable Processing Gain DS/CDMA System
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Multicarrier direct-sequence code-division multiple access (MC-DS-CDMA) becomes an attractive technique for the future fourth-generation (4G) wireless system because it can flexibly adapt transmission rates by changing both time and frequency spreading factors and possesses many physical-layer advantages in dispersive fading channels. However, power control errors (PCE) and the complete multiple access interference (MAI) from all the intersubcarriers may significantly degrade the performance of the MC-DS-CDMA system. In this paper, we propose an analytical method to evaluate the joint effects of the PCE and the complete MAI on the multirate MC-DS-CDMA system. From analysis and simulation, we obtain some important insights into the performance issues of the MC-DS-CDMA system. First, the effect of PCE can exacerbate the impact of the complete MAI on the MC-DS-CDMA system, or vice versa . For BER=10/sup -3/ in a considered case, the joint effect of the complete MAI and PCE further degrades the performance by 2.1 dB compared with the sum of the degradation from the complete MAI and the PCE individually. Second, increasing frequency or time-domain spreading gain can improve the performance of the MC-DS-CDMA system, but the system also becomes more sensitive to power control errors. Third, a larger PCE can possibly make the frequency-domain diversity diminish faster than the gain obtained from the time-domain spreading although an MC-DS-CDMA system with a larger frequency-domain spreading gain (M) is usually better than that with a larger time-domain spreading gain (G/sub o/). In our example, for the standard deviation of PCE (/spl sigma//sub e/) equal to 0 dB, the BERs with (M,G/sub o/)= (4, 16) and (16, 4) are 9.3/spl times/10/sup -4/ and 3.7/spl times/10/sup -5/, respectively, while for /spl sigma//sub e/=4 dB, the BER performances of the two cases are all in the order of 10/sup -3/.
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In this paper, the performance of two FFH-CDMA multirate schemes are studied and compared. The first one is the classical variable processing gain fast frequency hopping CDMA (VPG FFH-CDMA) and the second is the newly proposed overlapped fast frequency hopping CDMA (OFFH-CDMA). Specifically, we analyze the signal to interference ratio for the VPG FFH-CDMA and the OFFH-CDMA systems. The bit error rate for the two systems is shown in the numerical results. We show that the newly proposed OFFH-CDMA always outperforms the classical VPG FFH-CDMA for different system scenarios.
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A multi-carrier direct sequence code division multiple access (MC-DS-CDMA) system combined with frequency spread coding is proposed and investigated for the downlink. The proposed system exploits frequency diversity without additional redundancy by using frequency spread coding in a frequency-selective fading channel. Computer simulation results show its effectiveness in terms of average bit error rate and bandwidth efficiency. Furthermore, the proposed system is compared with a multi-carrier (MC)-CDMA system and a single-carrier (SC)-DS-CDMA system using a RAKE receiver.
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<p>This paper follows the study of bit error rate evolution in a mobile communications system using DS – CDMA (Direct Sequence – Code Division Multiple Access) technology. We have assessed the bit error rate (BER) based on the signal/noise ratio, Eb/N0, and the number of users in the system. For this purpose, we have used M sequence and Orthogonal Gold sequence and the AWGN (Additive white Gaussian noise) transmission medium<strong>.</strong></p>
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In this contribution the family of generalized multicarrier DS-CDMA (MC DS-CDMA) schemes is investigated, when considering time-domain half-sine and raised-cosine chip waveforms, in addition to the rectangular chip waveform. Our results show that for a given subcarrier spacing the particular choice of the chip waveform has a substantial influence on the system's performance. However, in the context of MC DS-CDMA using the optimum subcarrier spacing, all chip waveforms may provide a similar bit error rate (BER) performance.
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Bandlimited direct-sequence code division multiple access (DS-CDMA) attracts much attention for its compact spectrum and the ability to suppress inter-symbol interference. Among the various bandlimited DS-CDMA systems available, minimum-bandwidth DS-CDMA (MB-DS-CDMA) is the only realizable Nyquist rate transmission system. But, MB-DS-CDMA only applies to certain kinds of spreading codes. Accordingly, this study proposes a modified DS-CDMA structure which extends the application of MB-DS-CDMA to all common spreading codes at the expense of a negligible reduction in the transmission rate. Additionally, the bit error rate of the proposed schemes adopting either single-user or multi-user detection receiver is analyzed and compared with that of the commonly-used raised-cosine-pulsed DS-CDMA over multipath fading channels. The numerical results show that given a sufficiently large number of users, the bit error rate performance of modified MB-DS-CDMA is comparable to that of the raised-cosine-pulsed DS-CDMA scheme; meanwhile, the realizable modified MB-DS-CDMA approaches the ultimate transmission rate.
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Multi-code CDMA is among the new features proposed for application in W-CDMA. The concept of multi-code CDMA is based on the idea of splitting a single sequence (at high symbol rate) that uses low processing gain into a number of sequences (at lower symbol rate) that require higher processing gain. However, using conventional matched filter detectors the gain from the increase in the processing gain is neutralised by the introduction of interference by the generated sequences. In this study we examine the application of interference cancellation in multi-code CDMA in order to reduce/eliminate the generated interference and thus improve the BER performance of the system.
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In this paper, we compare two signal designs for uplink quasi-synchronous code division multiple access (CDMA) channels in order to optimize the trade-off between bandwidth efficiency and power efficiency. The design we call band-limited DS/CDMA design, is based on the time-domain assignment of Gold sequences, just as in the ordinary DS/CDMA, but with band-constrained cyclic chip interpolation functions, which is unlike the ordinary DS/CDMA. The other design, MC/CDMA design, is based on frequency-domain assignment of the sequences, as in the ordinary MC/CDMA. In both designs, we assume insertion of guard intervals at the transmitter and frequency-domain processing in reception. Assuming quasi-synchronous arrival of CDMA signals at the CDMA base station and FFT in the effective symbol interval, the intersymbol interference is evaded in both designs. First we identified the signal parameters that optimize bandwidth efficiency in each of the band-limited DS design and MC design. Second, we clarified the signal parameters that optimize the power efficiency as functions of frequency efficiency in each of the two designs. Finally, we derived and compared the trade-off between the bandwidth efficiency and power efficiency of band-limited DS and MC designs. We found a superiority of band-limited DS design over MC design with respect to the optimized trade-off.
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