Seyeong Choi

Soft handover overhead reduction by RAKE reception with finger reassignment

We propose and analyze in this paper a new finger assignment technique that is applicable for RAKE receivers when they operate in the soft handover (SHO) region. This scheme employs a new version of generalized selection combining (GSC). More specifically, in the SHO region, the receiver uses by default only the strongest paths from the serving base station (BS) and only when the combined signal-to-noise ratio (SNR) falls below a certain pre-determined threshold, the receiver uses more resolvable paths from the target BS to improve the performance. Hence, relying on some recent results on order statistics we attack the statistics of two correlated GSC stages and provide the approximate but accurate closed-form expressions for the statistics of the output SNR. By investigating the tradeoff among the error performance, the path estimation load, and the SHO overhead, we show through numerical examples that the new scheme offers commensurate performance in comparison with more complicated GSC-based diversity systems while requiring a smaller estimation load and SHO overhead.

Finger Replacement Method for Rake Receivers in the Soft Handover Region

We propose and analyze a new finger replacement technique that is applicable for RAKE receivers in the soft handover (SHO) region. More specifically, the receiver uses in the SHO region by default the strongest paths from the serving base station (BS) and only when the combined signal-to-noise ratio falls below a certain pre-determined threshold, the receiver uses more resolvable paths from the target BS to improve the performance. Instead of changing the configuration for all fingers, the receiver just compares the sum of the weakest paths out of the currently connected paths from the serving BS with the sum of the strongest paths from the target BS and selects the better group. Using accurate statistical analysis, we investigate in this letter the tradeoff between error performance, average number of required path comparisons, and SHO overhead offered by this newly proposed scheme.

Optimization of Switched MIMO Systems Over Rayleigh Fading Channels

This paper proposes a new switched MIMO system where the MIMO includes both diversity-based MIMO and spatial multiplexing MIMO (SM-MIMO). The proposed scheme is a closed-loop system where the feedback bits are sent back to the transmitter to indicate the channel quality. The switching operation is employed among groups of a certain number of transmit antennas. For diversity-based MIMO systems, the switching operation aims to improve the link performance, while the goal of the switching for the SM-MIMO systems is to increase the data rate. The authors analyze and optimize the proposed systems and show that there is considerable benefit in terms of performance and practical use

Comparison of Relaying Strategies for Cooperative Diversity Systems with Adaptive Modulation

In this paper, we propose several relay selection schemes for cooperative diversity systems with adaptive trans- mission. The proposed schemes are based on dual-hop relaying with multiple relays. We analyze the performance of the proposed schemes by quantifying the average spectral efficiency and the outage probability. We also investigate the tradeoff of perfor- mance and complexity among proposed schemes by evaluating the average number of active relays, path estimations, and signal- to-noise ratio comparisons. Our proposed schemes and analysis are also applicable to single relay systems with multiple antennas. the tradeoff involved in such relay systems by examining the performance and the complexity of several relay selection strategies. We consider a two-hop relay system with adaptive transmission. The relays perform decode and forward type of relaying and to ensure that the instantaneous error rate performance of both hops are satisfactory, the transmission mode is selected based on the quality of the worst hop. For the resulting system, we study the performance mea- sures, such as the average spectral efficiency, the outage probability, and other complexity measures, including the average number of active relays, the average number of path estimations, and the average number of signal-to-noise ratio (SNR) comparisons, of the three different relay selection schemes through analysis whenever feasible in a simple fash- ion. Several selected numerical examples are presented and discussed to illustrate the advantage of different relay selection schemes. We observe that the performance of relay system can considerably benefit from more active relays at the cost of additional complexity of relay selection and coordination. We would like to note that while this paper is presented in the context of multiple relays selection while assuming that high- speed links exists between relays for information exchange and coordination, the formulation and analysis can also apply to the scenario of antenna subset selection at a multiple-antenna relay (8). In this case, saving in terms of the average number of active relays will be equivalent to saving in terms of the active antennas/RF chains in the relay, which implies the reduction in the processing power at the relay.

Joint adaptive modulation and diversity combining with feedback error compensation

This letter investigates the effect of feedback error on the performance of the joint adaptive modulation and diversity combining (AMDC) scheme which was previously studied with an assumption of error-free feedback channels. We also propose to utilize adaptive diversity to compensate for the performance degradation due to feedback error. We accurately quantify the performance of the joint AMDC scheme in the presence of feedback error, in terms of the average number of combined paths, the average spectral efficiency, and the average bit error rate. Selected numerical examples are presented and discussed to illustrate the effectiveness of the proposed feedback error compensation strategy with adaptive combining. It is observed that the proposed compensation strategy can offer considerable error performance improvement with little loss in processing power and spectral efficiency in comparison with the no compensation case.

Finger assignment schemes for RAKE receivers with multiple-way soft handover

The authors propose and analyze new finger assignment techniques that are applicable for rake receivers in the soft handover (SHO) region. More specifically, in the SHO region, the receiver uses by default only the strongest paths from the serving base station (BS) and only when the combined signal-to-noise ratio (SNR) falls below a certain pre-determined threshold, the receiver uses more resolvable paths from the target BSs to improve the performance. Relying on the previous results for the case of two-BS case, the authors consider the multi-BS situation by attacking the statistics of several correlated generalized selection combining (GSC) stages and provide closed-form expressions for the statistics of the output SNR. By investigating the tradeoff among the error performance, the path estimation load, and the SHO overhead, the authors show through numerical examples that the new schemes offer commensurate performance in comparison with more complicated GSC-based diversity systems while requiring a smaller estimation load and SHO overhead.

Soft Handover Overhead Reduction by RAKE Reception with Finger Replacement

We propose and analyze a new finger replacement technique that is applicable for Rake receivers when they operate in the soft handover (SHO) region. In the SHO region, the receiver uses by default the strongest paths from the serving base station (BS) and only when the combined signal-to-noise ratio falls below a certain pre-determined threshold, the receiver uses more resolvable paths from the target BS to improve the performance. Instead of changing the configuration for all fingers, the receiver just compares the sum of the weakest paths from the serving BS with that of other paths from the target BS and select the better group. Applying some recent results on order statistics, we attack the statistics of two partial sums of order statistics and provide an analytical framework for the performance evaluation of the proposed scheme. By investigating the tradeoff between the error performance and the SHO overhead, we show through numerical examples that the new scheme can save some SHO overhead load

Joint Adaptive Modulation and Diversity Combining with Feedback Error Compensation

This paper investigates the effect of feedback error on the performance of the joint adaptive modulation and diversity combining (AMDC) scheme which was previously studied with an assumption of error-free feedback channels. We also propose to utilize adaptive diversity to compensate for the performance degradation due to feedback error. We accurately quantify the performance of the joint AMDC scheme in the presence of feedback error, in terms of the average number of combined paths, the average spectral efficiency, and the average bit error rate. Selected numerical examples are presented and discussed to illustrate the effectiveness of the proposed feedback error compensation strategy with adaptive combining. It is observed that the proposed compensation strategy can offer considerable error performance improvement with little loss in processing power and spectral efficiency in comparison with the no compensation case.

Performance analysis of maximal-ratio combining with transmit antenna selection for generalized selection criterion

We investigate the performance of transmit antenna selection in multi-input multi-output systems where only a single transmit antenna is selected for the transmission and multiple receive antennas are employed for maximal-ratio combining. Antenna selection is performed by a generalized selection criterion based on the ordinal number of the strength of the received signal-to-noise ratio (SNR). Starting with the statistics such as the cumulative distribution function, the probability density function, and the moment-generating function, we derive an exact closed-from expression for the performance such as the average output SNR and the average bit-error rate. By considering Nakagami fading models, we show the dependence of the performance on the fading severity. received signal-to-noise ratio (SNR) in selecting one antenna out of multiple transmit antennas is proportional to the ordinal number of the selected antenna, have been presented in terms of the bit-error rate (BER) for binary phase-sift keying (BPSK) over Rayleigh fading channels in an approximated form. As noted in (2), the exact calculation of the performance is important for the system designer to apply transmit antenna selection. However, no-closed form expressions for the perfor- mance of transmit antenna selection with a generalized selec- tion criterion over Nakagami fading channels (even Rayleigh fading channels) have been presented due to the mathematical difficulties in finding the statistics. In this paper, we present the exact closed-form expression for the performance of transmit antenna selection with a generalized selection criterion by obtaining statistics such as the cumulative distribution func- tion (CDF), the probability density function (PDF), and the moment-generating function (MGF). The channels are gen- eralized as Nakagami-m fading models with the assumption of statistically independent and identical conditions among multiple fading paths. We then verify our results by showing that our generic formulae can be further simplified for the special cases of Rayleigh fading, conventional MRC, and selection combining (SC), which are well-known results.

Finger Assignment Schemes for RAKE Receivers with Multi-Way Soft Handover

The authors propose and analyze new finger assignment techniques that are applicable for rake receivers in the soft handover (SHO) region. More specifically, in the SHO region, the receiver uses by default only the strongest paths from the serving base station (BS) and only when the combined signal-to-noise ratio (SNR) falls below a certain pre-determined threshold, the receiver uses more resolvable paths from the target BSs to improve the performance. Relying on the previous results for the case of two-BS case, the authors consider the multi-BS situation by attacking the statistics of several correlated generalized selection combining (GSC) stages and provide closed-form expressions for the statistics of the output SNR. By investigating the tradeoff among the error performance, the path estimation load, and the SHO overhead, the authors show through numerical examples that the new schemes offer commensurate performance in comparison with more complicated GSC-based diversity systems while requiring a smaller estimation load and SHO overhead.