Sang Wu Kim

Combined rate and power adaptation in DS/CDMA communications over Nakagami fading channels

We consider combined rate and power adaptations in direct-sequence code-division multiple-access communications, where the transmission power and the data rate are adapted relative to channel variations. We discuss the power gain that the combined adaptations provide over power adaptation. Then, we consider an integrated voice and data transmission system that offers a constant bit rate voice service, using power adaptation and a variable bit rate data service with rate adaptation. We present an expression for the required average transmission power of each traffic type having different quality-of-service specifications and discuss the capacity gain over power adaptation for voice and data.

Optimum rate Reed-Solomon codes for frequency-hopped spread-spectrum multiple-access communication systems

The authors consider a multiple-access frequency-hopped spread-spectrum communication system with Reed-Solomon codes. The performance measures of interest are an achievable region and the channel throughput. The achievable rate region is the set of all pairs of code rate and number of users for which communication is possible with error probability below a fixed value. The throughput measures the expected number of successful codeword transmissions per unit bandwidth. Two models of interference are considered. For these two models, the authors determine the optimal number of users for a given bandwidth and the optimal rate Reed-Solomon code that maximize the throughput. They also determine the achievable region for these models. >

Truncated power control in code-division multiple-access communications

We analyze the performance of truncated power control in a code-division multiple-access (CDMA) communication system. This power control scheme compensates for propagation gain above a certain cutoff fade depth: below the cutoff level, data transmission is suspended. We assume a channel with fast Rayleigh fading and slow lognormal shadowing plus path loss, where truncated power control is applied to the shadowing plus path loss and a RAKE receiver combines the Rayleigh fading multipath components separated by more than a chip time. We find that truncated power control exhibits both a power and capacity gain. These performance improvements result from the fact that with truncated power control, less power is wasted compensating for deep fading, and mobiles on the cell boundaries cause less interference to adjacent cells. We find that truncated power control is most effective for channels with large power fluctuations or with large background noise.

Log-likelihood-ratio-based detection ordering in V-BLAST

We propose a new detection ordering based on the log-likelihood ratio (LLR) in the iterative nulling and cancellation process of vertical Bell Laboratories layered space-time (V-BLAST) decoding. The motivation for using the LLR is that it provides the reliability information on the maximum a posteriori probability decision. As a result, the error propagation associated with a wrong cancellation can be minimized. Simplified ordering schemes that require a much less computation, but provide a performance virtually identical to the LLR-based ordering, are also provided. The performance of the LLR ordering in the V-BLAST combined with space-time block codes is evaluated.

Frequency-hopped multiple-access communications with multicarrier on-off keying in Rayleigh fading channels

We propose the multicarrier on-off keying (MC-OOK) as a bandwidth-efficient modulation method for frequency-hopped multiple-access (FHMA) communications. The motivation for using MC-OOK is that a more bandwidth-efficient modulation scheme allows a larger number of frequency slots, and thus provides a higher immunity against multiple-access interference in FHMA systems. We analyze the average bit-error probability in slow frequency-nonselective Rayleigh fading channels with background noise. We find that the capacity gain that MC-OOK/FHMA system provides over the MFSK/FHMA system in an interference-limited region is more than 2.5 when the modulation alphabet size M is 8, and even a higher capacity gain can be obtained with a larger M.

RAKE reception with maximal-ratio and equal-gain combining for DS-CDMA systems in Nakagami fading

We study coherent binary direct-sequence code division multiple access (DS-CDMA) systems operating over frequency-selective Nakagami fading channels. In particular a performance comparison between maximal-ratio combining (MRC) and equal-gain combining (EGC) RAKE receivers is considered. For interference limited systems, perfect MRC RAKE receivers can accommodate about 5m/(5m-1) more users than EGC RAKE receivers, where m is the Nakagami fading parameter. MRC has a higher rate of improvement than EGC as the number of combined paths increases. However, imperfect or inaccurate channel fading estimation leads to serious degradation in the performance of MRC receivers and in that case EGC becomes superior to MRC.

Optimum receive antenna selection minimizing error probability

The optimum receive antenna selection combining rule that minimizes the bit error probability is presented. It is derived from a general relationship between the bit error probability and the log-likelihood ratio (LLR), and selects the receive antenna providing the largest LLR magnitude. The optimum selection combining rule is applied to single transmit (Tx) antenna and space-time block code (STBC) systems, with N/sub R/ receive (Rx) antennas, and the bit error probability is derived for BPSK signaling in Rayleigh flat fading channels. A suboptimum selection combining rule based on noncoherent envelope detection is also presented and analyzed. For STBC systems, the optimum generalized selection combining in which M(/spl les/ N/sub R/) Rx antennas providing the largest LLR magnitude are selected and combined is presented.

Performance limits of Reed-Solomon coded CDMA with orthogonal signaling in a Rayleigh-fading channel

The asymptotic performance of Reed-Solomon (RS)-coded M-ary orthogonal signaling with ratio-threshold test (RTT) type demodulation in a Rayleigh-fading channel is considered. We show that the minimum E~/sub b//N/sub 0/ needed for error-free communication is eln2 (2.75 dB) with RTT, and 4.79 (6.8 dB) with hard decisions. The optimum code rate that minimizes the required E~/sub b//N/sub 0/ is e/sup -1/ with RTT and 0.46 with hard decision, and the optimum ratio threshold approaches 1 for large M. Next, we investigate the fundamental limit in a direct-sequence spread-spectrum multiple-access (DS/SSMA) system employing an M-ary orthogonal code of length N=Mm, which is obtained by spreading every row of an M/spl times/M Hadamard matrix with a user-specific random sequence of length N. We derive the minimum E~/sub b//N/sub 0/ for error-free communication as a function of the number of users, the optimum code rate that minimizes E~/sub B//N/sub 0/, and the maximum limit on the total information transmission rate. Then, we consider a multirate DS/SSMA system, where a population of users simultaneously transmit at different power levels a variety of traffic types of different information rates. We derive the minimum required E~/sub B//n/sub 0/ and the optimum code rate for each traffic type.

Cooperative spatial multiplexing for high-rate wireless communications

We propose a new cooperative communication approach that simplifies the transmit and receive processing requirement on the relay node while providing significant savings in the transmission energy over the cooperative diversity technique, particularly in high spectral efficiency regime. The proposed approach is to make each relay node to detect only a fraction (sub-stream) of the source data stream and forward all sub-streams simultaneously over the same physical channel. Then, multiple receive antennas at the destination allow the sub-streams to be detected separately based on their spatial characteristics. The distinctive benefit of this approach is the reduction of transmit and receive processing requirements on each relay node. Therefore, the proposed approach is particularly attractive in sending high-rate data where each relay node can handle only low-rates due to limited resources in terms of energy, computation power, hardware, and space (size).

Log-likelihood ratio based detection ordering for the V-BLAST

We propose a new detection ordering for the V-BLAST. The main idea is to detect and cancel sub-streams in order of the magnitude of log-likelihood ratio (LLR), i.e. the symbol with the largest magnitude of LLR is detected first. The motivation is that the reliability of data decision increases with increasing magnitude of LLR. As a result, the error propagation associated with a wrong decision and the resulting error probability for the remaining sub-streams can be minimized. It is shown that the proposed LLR-based ordering significantly outperforms the conventional SNR-based ordering. Simplified LLR-based ordering and envelope-based ordering that require a much less computation, but provide a performance virtually identical to the LLR-based ordering, are also proposed.