B.J. Choi

Optimum mode-switching-assisted constant-power single- and multicarrier adaptive modulation

A set of optimum mode-switching levels is derived for a generic constant-power adaptive-modulation scheme based on a closed-form expression of the average bit error ratio (BER) and the average bits-per-symbol (BPS) throughput of the adaptive-modulation scheme. This results in a constant BER, variable-throughput arrangement. The corresponding BPS throughput performance and the achievable signal-to-noise ratio (SNR) gain are investigated for the optimum mode-switching assisted constant-power adaptive-modulation schemes employing various diversity schemes, including maximal ratio combining (MRC) receive-antenna diversity, a two-dimensional RAKE receiver, as well as transmit-diversity aided space-time (ST) coding, when communicating over various fading scenarios. The BPS throughput of our constant-power adaptive quadrature amplitude modulation (AQAM) scheme approaches the throughput of variable-power variable-rate AQAM within 1 dB. However, the achievable throughput gain of the adaptive-modulation scheme, in comparison to conventional fixed-mode modems, is substantially reduced as the diversity order of the receiver is increased. Hence, adaptive modulation constitutes a lower complexity alternative to multiple-transmitter and receiver-based systems when considering the range of techniques that can be used for mitigating the effects of the channel-quality fluctuations imposed by wireless channels.

On the design of LAS spreading codes

A family of large area synchronised (LAS) codes is studied, which exhibits a so-called interference free window (IFW), where both the intersymbol interference (ISI) and the multiple user interference (MAI) are suppressed, provided that the relative time offset of the codes is within the IFW. Hence, LAS codes have the potential of increasing the capacity of CDMA networks. However, a specific drawback of this family of sequences is their relatively low duty ratio. A modified algorithm is proposed for determining the constituent LA codes pulse positions for the sake of improving the duty ratio of the corresponding LAS codes. It is shown that the modified LAS codes exhibit an 18.6% higher duty ratio, than that of the original LAS codes, when considering a minimum LA code pulse interval of 38. The LAS code design examples provided revealed that the system employing the proposed approach improved the data throughput by 40% for the scenario, when the required IFW is /spl plusmn/4-chips and the minimum pulse interval is 19. The availability of the LAS codes is also investigated in conjunction with for the various LS and LA code lengths, employing an extended LS code construction scheme for polyphase complementary sequences and orthogonal LA pulse codes. It is shown that these modified spreading code construction schemes can be combined for generating a family of LAS codes in order to meet various design requirements.

MAC Scheduling Scheme for VoIP Traffic Service in 3G LTE

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.

Crest factors of complementary-sequence-based multicode MC-CDMA signals

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.

Near-Instantaneously Adaptive HSDPA-Style OFDM Versus MC-CDMA Transceivers for WIFI, WIMAX, and Next-Generation Cellular Systems

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.

Comparative study of concatenated turbo coded and space-time block coded as well as space-time trellis coded OFDM

Space-time block codes provide substantial diversity advantages for multiple transmit antenna systems at a low decoding complexity. In this study, we concatenate both space-time block and trellis codes with turbo convolutional (TC) coded orthogonal frequency division multiplexing (OFDM) for achieving a high coding gain. The associated performance and complexity of the coded OFDM schemes is compared.

Concatenated space-time block coded and turbo coded symbol-by-symbol adaptive OFDM and multi-carrier CDMA systems

Symbol-by-symbol adaptive orthogonal frequency division multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput in comparison to their fixed-mode counterparts. By contrast, various diversity techniques, such as RAKE receivers and space-time coding mitigate the channel quality variations in their effort to obtain a reduced BER. This 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 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. It is found that the advantage of the adaptive modem erodes, as the overall diversity-order increases.

Optimum mode-switching assisted adaptive modulation

Adaptive modulation techniques combat fading by employing a suitable modulation mode depending on the instantaneous channel conditions for improving the Bit Error Rate (BER) performance or the average throughput. Based on a generic model of constant-power adaptive modulation, exact closed form expressions of the average BER and the average throughput are derived, when employing square Quadrature Amplitude Modulation (QAM) or Phase Shift Keying (PSK) as the constituent modulation modes, operating over a Nakagami fading channel. The optimum modulation-mode switching levels, achieving the highest possible throughput under the constraint of the average BER, are obtained using the Lagrangian optimisation method. Adaptive modulation employing the optimum switching levels shows a superior performance, while maintaining a constant average BER.

Crest factors of Shapiro-Rudin sequence based multi-code 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 Sivaswamys (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.

RAKE receiver detection of adaptive modulation aided CDMA over frequency selective channels

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.