Communication capacity of sectored CDMA systems

The use of smart or adaptive array antennas has attracted considerable interest in the mobile communications community. We investigate the combined use of an adaptive/smart array antenna and Rake receiver (space-time processing) to jointly mitigate the effect of multiple access interference (MAI) and multipath fading in a CDMA system, while taking angle spread into account. We provide simulation results concerning mean bit error rate (BER) performance of such a system for different numbers of antenna elements, numbers of users and angle spreads. The obtained results show that the investigated smart antenna system can effectively combat adverse MAI and fading effects and improve the CDMA system performance.

A double spreaded CDMA system employing Parity bit selected spreading sequences is proposed in this paper for improving the Bit error rate (BER) performance of Multiuser CDMA systems. In a CDMA system with parity bit selected spreading sequences (CDMA/PB), the transmitted data bits is used to select the spreading code from a set of mutually orthogonal spreading waveforms allocated to each user. After this the spreaded data is again undergone a second spreading using a unique PN sequence. Multiple Access Interference (MAI) caused in Multiuser CDMA/PB systems can be mitigated by this double spreading method. A new detection algorithm is also used at the receiver for the betterment of the BER performance. The complexity of this proposed method is less compared to Parallel Interference Cancellation (PIC) method in Multiuser CDMA systems. Hadamard codes are used for spreading in the proposed system in order to maintain the orthogonality between the spreading codes.

Multiple antenna arrays are promising architectures for overcoming the effects of multipath interference and increasing the spectrum efficiency. In order to be able to investigate these systems, in this article, we derive generalized spatial correlation equations of a circular antenna array for two types of angular energy distributions: a Gaussian angle distribution, and uniform angular distribution. The generalized spatial correlation equations and bit error rate (BER) performance is investigated. It has been reported that spatial correlation and BER performance using linear array antenna, but very few attempts have been made at circular array antenna. It was found from the result that BER of receiving arrival wave with uniform distribution is better than that with Gaussian distribution

In this paper, a multistage parallel interference canceller (MPIC) with multiple-beam reception for a DS-CDMA system is proposed to suppress multiple access interference (MAI) effectively. Its aim is to reduce the computational complexity of the conventional MPIC cascaded with an adaptive array antenna. It employs multiple fixed beams based on phased array and selects suitable beams to demodulate the transmitted signal of each user. Then it suppresses residual interference signals by the MPIC cascaded with multiple-beam receiver. Its bit error rate (BER) performance is evaluated by computer simulations assuming an uplink single-chip-rate multiple-spreading-factor DS-CDMA system over both exponentially decaying 5-path and equal average power 2-path Rayleigh distributed channels. When there are 16 users in an 120°-sectored single cell, the proposed receiver with 6-element array antenna and 2-stage MPIC shows better or comparable BER performance compared with that of the conventional receiver. Moreover, the proposed receiver with 8 beams can reduce the number of complex multiplications to about 40% of that of the complexity-reduced conventional receiver over 5-path channels.

In the Multi Carrier (MC)-CDMA system, the frequency diversity gain is obtained by its being spread in the frequency domain. The frequency interleaving technique can improve the frequency diversity gain. In this paper, the bit error rate (BER) performance in the MC-CDMA system which adopts the frequency interleaving scheme in the frequency selective fading channel is evaluated by computer simulation. In this simulation, orthogonal restoration combining (ORC) and minimum mean square error combining (MMSEC) are considered as frequency equalization combining techniques. This paper shows that BER performance with the frequency interleaver is better than without it in various environments.

Optical CDMA systems have received much attention in fiber optic networks where the high-speed, the huge band width, the secure and immune communication, asynchronous and simultaneous access for multiple users on the same channel. Those features nominate strongly Optical CDMA for future optical multimedia networks. However, multiple access interference (MAI), which is originated from other simultaneous users, severely limits the capacity of the system. In this paper, an efficient transceiver architecture has been proposed using MAI cancellation to improve the performance of the existence simple transceiver without MAI cancellation. The strategy of the proposed transceiver is based on an estimation and cancellation of MAI in Optical CDMA system with employing a new spreading code named double padded modified prime code (DPMPC) and an efficient pulse-position modulation (PPM) scheme. In addition, for further improvement of the bit-error rate (BER) performance for the proposed transceiver Manchester coding (source coding) is used in signaling the transmitted data. The MAI estimation is achieved by pre-reserving one of optical spreading code sequences at the receiver based on the correlation property of the DPMPC. The proposed receiver estimates the interference and cancels it out from the received signal after the photo-detection process. The performance of the proposed transceiver architecture and its improvement will be compared with the performance of simple transceiver without MAI cancellation. The performance of various codes like modified prime code (MPC), new modified prime code (NMPC) are applied and compared with the proposed DPMPC to demonstrate its potential. An upper bound on the BER for the proposed system is derived and compared with a lower bound on the BER from the system without cancellation. The simulated computer results reveal that the performance, in terms of the BER, of the proposed system is significance improved compared with that of the system without cancellation.

Code Division Multiple Access or CDMA has its roots in WWII era spread spectrum technology. In this technology, multiple users can transmit the data simultaneously in a channel using the same frequency band and each user is assigned a distinguished code for transmission. CDMA uses spread-spectrum technology to minimize the problems relating to interference in the channel. Different types of codes are used for this purpose and generating more efficient and secured spreading code that is immune to interference is an ongoing area of research in Wireless Technology. In this paper, we have proposed a generalised algorithm to generate sets of spreading codes that can be allocated to sufficiently large number of users while maintaining satisfactory performance in terms of channel interference and Bit Error Rate (BER). More extensively, the Bit Error Rate with varying SNR values in an AWGN channel for one of the generated sets has been incorporated as a measure of performance of CDMA system that is designed using the spreading codes generated from this proposed algorithm.

Multicarrier code division multiple access (MC-CDMA) is a transmission scheme that combines the robustness of orthogonal modulation with the flexibility of CDMA schemes. It can lower the symbol rate and is therefore suited to the bandwidth limited optical wireless indoor channel. In this paper, new work is presented for an optical wireless (OW) system that utilizes MC-CDMA to improve the bit error rate (BER) performance. Our results indicate that at a BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> a 7 dB SNR improvement is obtained by a 4 user MC-CDMA OW system compared to a single user OW OOK system. This improvement is obtained while enabling multi-user communication. The performance degrades gradually as the number of users increases due to the increase in multiple access interference (MAI). A consistent BER performance is achieved for all receiver positions in a line strip spot-diffusing transmitter system (LSMS) with an angle diversity receiver, whereas a fluctuating BER performance is obtained in a conventional diffuse optical wireless system (CDS).

Closed-form solutions for the average bit error rate (BER) performance of a direct-sequence code division multiple-access system with imperfect power control are derived for both coherent and noncoherent reception operating over a multipath Rayleigh fading channel. The RAKE structure receivers under consideration employ despreading sequences weighted by adjustable exponential chip waveforms optimized for multiple-access interference rejection. The chip-weighting waveforms employed are determined only by one parameter /spl gamma/ which leads to easy tuning of the waveforms in practice to achieve the best performance. The results indicate that the number of active users supported at a given BER for the case of /spl gamma/ tuned to maximize the average signal to interference plus noise ratio H/spl circ/ is much larger than the case of /spl gamma/=0 (fixed or rectangular despreading sequence). It is shown that imperfect power control affects the irreducible BER for the case of /spl gamma/=0. On the other hand, the effect of imperfect power control on the BER performance for the case of /spl gamma/ tuned to maximize H/spl circ/ is equivalent to a reduction in the average signal-to-noise ratio, and, hence, system performance can be compensated by increasing the transmitter power. It is further shown that the effect due to imperfect power control on BER performance is significant while that on the maximum value of H/spl circ/ obtained by tuning /spl gamma/ is rather insignificant.

Beamforming (BF) with multiple receiving antennas significantly improves the bit error rate (BER) performance for a DS-CDMA system in a flat Rayleigh fading environment. However, BF fails to suppress multiple access interference (MAI) completely, which results in an irreducible BER floor. Furthermore, soft-decision Viterbi decoding (VD) with BF further lowers the BER floor. The BER performance difference between the minimum mean square error (MMSE) linear multiuser detector (MUD) and zero forcing (ZF) MUD increases with the eigenvalue spread of the code correlation matrix. BF with MUD performs better than MUD alone, and BF+VD is much more effective at combating MAI than MUD+VD for low to moderate SNRs. In this regime, the performance of BF+VD approaches that of BF+MUD+VD, which clearly offers the best performance for all SNRs.

The effects of multipath fading and additive white Gaussian noise (AWGN) on the bit error rate (BER) performance of direct-sequence code-division multiple-access (DS-CDMA) systems is studied in this paper. The expression for the conditional BER, conditioned on a given level of fading is derived and then the unconditional BER is evaluated. The BER performance results of the system are evaluated in a fading channel for different magnitude of fading. The proposed approach is also extended to estimate the BER performance of the CDMA system having multiple receiving antennas. The effectiveness of the selection method and maximal ratio combining (MRC) method as diversity combining schemes are investigated and compared. To enhance the system performance channel coding is also applied to the system.

The paper evaluates the performance of a CDMA based wireless sensor networks (WSN) with layered architecture in terms of bit error rate (BER) and energy consumption for successful transmission of data packets. Correlation amongst signal and interferers in shadowed environment has been considered. Two kinds of interference namely multiple access interference (MAI) and node interference (NI) are considered. The bit error rate (BER) and packet error rate (PER) of the received signal are evaluated and energy required for successful delivery of information to the sink considering single hop communication is estimated. The effects of node density, correlation amongst interferers and power control error on BER, PER, average number of retransmission and energy consumption for successful reception of a packet at sink in a single hop are investigated. Analytical results are verified through simulations.

Multi-carrier code division multiple access (MC-CDMA)has been considered as a strong candidate for next generation wireless communication system due to its excellent performance in multi-path fading channel and simple receiver structure. However, like all the multi-carrier transmission technologies such as OFDM, the inter-carrier interference (ICI) produced by the frequency offset between the transmitter and receiver local oscillators or by Doppler shift due to high mobility causes significant BER (bit error rate) performance degradation in MC-CDMA system. Many ICI cancellation methods such as windowing and frequency domain coding have been proposed in the literature to cancel ICI and improve the BER performance for multi-carrier transmission technologies. However, existing ICI cancellation methods do not cancel ICI entirely and the BER performance after ICI cancellation is still much worse than the BER performance of original system without ICI. Moreover, popular ICI cancellation methods like ICI self-cancellation reduce ICI at the price of lowering the transmission rate and reducing the bandwidth efficiency. Other frequency-domain coding methods do not reduce the data rate, but produce less reduction in ICI as well. In this paper, we propose a novel ICI cancellation scheme that can eliminate the ICI entirely and offer a MC-CDMA mobile system with the same BER performance of a MC-CDMA system without ICI. More importantly, the proposed ICI cancellation scheme (namely Total ICI Cancellation) does not lower the transmission rate or reduce the bandwidth efficiency. Specifically, by exploiting frequency offset quantization, the proposed scheme takes advantage of the orthogonality of the ICI matrix and offers perfect ICI cancellation and significant BER improvement at linearly growing cost. Simulation results in AWGN channel and multi-path fading channel confirm the excellent performance of the proposed Total ICI Cancellation scheme in the presence of frequency offset or time variations in the channel, outperforming existing ICI cancellation methods.

In Wavelet Packets Based Multicarrier Multicode CDMA system, the multicode (MCD) part ensures the transmission for high speed and flexible data rate, the multicarrier (MC) part ensures the flexibility of handling multiple data rates, and wavelet packets modulation technique contributes to the mitigation of the interference problems. The CDMA system can suppress a given amount of interference. In this paper, the receiver employs suppression filter (SF) to mitigate the effect of narrow-band jammer interference and diversity techniques to reduce multiple access interference. The framework for the system and the performance evaluation are presented in terms of bit error rate (BER) over a Nakagami fading channel. Also, we investigate how the performance is influenced by various parameters, such as the number of taps of the SF, the ratio of narrow-band interference bandwidth to the spread-spectrum bandwidth, the diversity order, the fading parameter and so on. Finally, the performance of the system is compared with the performance Sinusoidal (Sin) based MC/MCD CDMA system.

In this paper, we tackle the problem of theoretical evaluation for the multistage parallel interference cancellation (PIC) scheme in a DS-CDMA system with orthogonal modulation and long scrambling codes. The studied system operates on the reverse link in a time-varying multipath Rayleigh fading channel. By applying the Central Limit Theorem to multiple access interference (MAI) and intersymbol interference (ISI), as well as identically distributed chips from a single interferer, the bit error rate (BER) performance of the PIC scheme at any stage can be recursively computed from the signal-to-noise ratio, number of users, the number of paths per user, processing gain of the CDMA system, as well as the average received power of each path. The proposed approximative analysis is validated by the Monte-Carlo simulations and proved to be accurate, and it gives insight into the performance and capacity one can expect from the PIC based receivers under different situations.