In this paper, a novel channel sharing scheme for IEEE 802.22 systems is introduced and its performance is investigated. In order to minimizing the number of user to share one channel, the status of backup channels, not occupied by any adjacent users, is checked. In addition, the condition of channels occupied by neighboring users is investigated. Based on these channel statuses, the proposed channel sharing scheme selects the optimal channel, which includes a non-occupied backup channel and the channel shared by least users, in order to change the channel currently in use with other users. The simulation results illustrate that significant performance enhancement is achieved by applying the proposed channel sharing scheme to IEEE 802.22 systems.

In this paper, the antenna switching scheme is proposed for Time Switched Transmit Diversity (TSTD) with ARQ (automatic repeat request) in WCDMA LCR-TDD systems, and the system performances are also investigated. The proposed TSTD system employs the ARQ scheme in which the receiver sends the acknowledgement signal (ACK or NACK) to the transmitter in order to predict the condition of the channel. For the ARQ-aided TSTD system, the proposed antenna switching pattern triggers the transmit antenna switches. Simulation results demonstrate that the performances of the TSTD systems can be improved by applying the ARQ scheme. Furthermore, the performances of ARQ-aided TSTD systems may be significantly improved by applying the proposed antenna switching scheme, especially when the mobile speed is low.

3G Long Term Evolution, which aims for various mobile multimedia services provision by enhanced wireless performance, proposes the VoIP-based voice service through the PS domain. When delay and loss-sensitive VoIP traffic flows through the PS domain, more challenging technical difficulties are expected than in the existing 3G systems which provide the CS domain based voice service. Moreover, since 3G LTE, which adopts the OFDM as its physical layer, introduces Physical Resource Block (PRB) as the unit for the transmission resources, it becomes necessary to develop new types of resource management schemes. This paper proposes a MAC layer PRB scheduling algorithm for the efficient VoIP service in 3G LTE and shows the simulation results regarding its performance. The key idea of the algorithm consists of two parts; dynamic activation of a VoIP priority mode for the voice QoS satisfaction and adaptive adjustment of the VoIP priority mode duration in order to minimize the performance degradation induced by its priority mode application.

Burts-by-burst (BbB) adaptive high-speed down-link packet access (HSDPA) style multicarrier systems are reviewed, identifying their most critical design aspects. These systems exhibit numerous attractive features, rendering them eminently eligible for employment in next-generation wireless systems. It is argued that BbB-adaptive or symbol-by-symbol adaptive orthogonal frequency division multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput or robustness in comparison to their fixed-mode counterparts. Although they act quite differently, various diversity techniques, such as Rake receivers and space-time block coding (STBC) are also capable of mitigating the channel quality variations in their effort to reduce the bit error ratio (BER), provided that the individual antenna elements experience independent fading. By contrast, in the presence of correlated fading imposed by shadowing or time-variant multiuser interference, the benefits of space-time coding erode and it is unrealistic to expect that a fixed-mode space-time coded system remains capable of maintaining a near-constant BER. Following a basic portrayal of adaptive OFDM, the paper investigates a combined system constituted by a constant-power adaptive modem employing space-time coded diversity techniques in the context of both OFDM and multicarrier code division multiple access (MC-CDMA). The combined system is configured to produce a constant uncoded BER and exhibits virtually error-free performance when a turbo convolutional code is concatenated with a space-time block code. Again, it was found that the advantage of rendering the multiple-input multiple-output (MIMO)-aided modem BbB-adaptive erodes, when the affordable system complexity facilitates the implementation of multiple transmitters and receivers, if it can be ensured that no shadowing and no multiuser interference fluctuation is experienced. By contrast, in the presence of these impairments, only BbB-adaptive transceivers can provide a near-constant BER.

The crest factor properties of binary phase-shift keying modulated two- and four-code assisted multicarrier code-division multiple-access (MC-CDMA) employing complementary-sequence-based spreading sequences are characterized. More specifically, a complementary-sequence pair, a complementary-sequence-based subcomplementary code pair, and a Sivaswamy complementary code set are studied. It was found that the corresponding crest factors are bounded by 3 dB, which corresponds to the crest factor of a single sine wave. This low crest factor resulted in a lower power loss and a lower out-of-band power spectrum due to clipping when the time-domain signal was subjected to a typical nonlinear power amplifier, in comparison to those of Walsh code and orthogonal Gold code-spread MC-CDMA signals.

The envelope power of MC-CDMA signals usually exhibits wide fluctuations, leading to a high value of the associated crest factor. However, when Shapiro-Rudin based sequences, namely a Shapiro-Rudin (1959) pair, a Shapiro-Rudin based sub-complementary pair and a Shapiro-Rudin based Sivaswamy's (1978) complementary set are used for two- and four-code BPSK modulated MC-CDMA spreading sequences, the crest factor is found to be bounded by 3dB. These sequences belong to the so-called orthogonal complementary set and Walsh-Hadamard sequences constitute a special case of these sets, when the sequence length is equal to the number of codes. When a typical nonlinear power amplifier is used, the low crest factor of the Shapiro-Rudin sequence based multi-code MC-CDMA scheme results in reduced amplifier-induced signal clipping. Hence a reduced power loss is imposed due to clipping in comparison to OFDM and to multicode MC-CDMA schemes based on other spreading sequences, such as Walsh codes, orthogonal Gold codes, Frank (1962) codes and Zadoff-Chu (1972) codes.

A closed form bit error rate (BER) formula is derived for a fixed-mode quadrature amplitude modulation (QAM) scheme employing RAKE receivers and receiver antenna diversity. The analysis is extended to constant power adaptive QAM (AQAM) expressing the average BER and the average throughput as a closed form function of the modulation mode switching levels. Then, the switching levels are optimised so that the average throughput is maximised, while maintaining a given target BER. This results in a constant-BER, variable-throughput arrangement. The results show that our constant-power AQAM scheme exhibits an SNR gain of about 5dB in comparison to fixed-mode QAM, when operating over a wireless asynchronous transfer mode (W-ATM) channel employing one antenna. However, the achievable throughput gain of the system over conventional fixed-mode modems is substantially reduced, as the diversity order of the receiver is increased.