Keum Chan Whang

Capacity analysis of macro/microcellular CDMA with power ratio control and tilted antenna

The reverse link capacity is obtained analytically for macro/microcellular code-division multiple-access (CDMA) systems operating in the same frequency band. The focuses are on the ratio of required receive power of the macrocell base station (BS) to that of the microcell BS and the tilt angle of the microcell antenna to increase the system capacity. The microcell-to-macrocell interference is derived in closed form by geometric approximation, and the macrocell-to-microcell interference is calculated on the divided regions of macrocell. The optimal tilt angle is obtained by defining the minimum interference tradeoff factor that maximizes the system capacity. It is shown that the system capacity increases remarkably with power ratio control in macro/microcellular environments. Also, the properly chosen antenna tilt angle adds more capacity, and enables the microcell users to save on the transmit power.

Effect of tap spacing on the performance of direct-sequence spread-spectrum RAKE receiver

The effect of tap spacing on the performance of a RAKE receiver is analyzed analytically in a frequency-selective fading channel. A continuous time multipath fading channel model is used for the analysis, and the expression of the correlation between the desired signals, interference signals, and noise signals at the output of each branch of the RAKE receiver is derived for various chip waveforms. Since the noise components of each branch signal are correlated to each other, an optimum combining rule based on the maximum-likelihood criterion is derived to gain utmost performance. It is shown that the performance of the system can be improved by setting the tap spacing of the RARE receiver below the chip duration when the bandwidth of the transmitted signal is larger than the inverse of the chip duration. Also, it is shown that the normalized capacity of the system can be increased by using a chip waveform occupying wider bandwidth, which takes advantage of the increased diversity gain merits of a wide-band code-division multiple-access system at the same chip rate. It is noted that the derived combining rule gives diversity gain against the fading process as well as noise whitening processing gain against multiple-access interference at the same time.

Performance evaluation of DS-CDMA system with M-ary orthogonal signaling

The effects of signature sequence on the performance of direct-sequence code-division multiple-access (DS-CDMA) systems employing Walsh M-ary orthogonal signaling are considered. The mean-squared value of multiple access interference (MAI) is derived by treating the signature sequence as deterministic. The numerical results show that Walsh orthogonal signaling changes the original properties of signature sequences, and a different choice of signature sequence results in significant performance difference. These results can be used as guides in selecting signature sequences for systems that employ Walsh M-ary orthogonal signaling.

Correction to "Comparison of diversity combining techniques for Rayleigh-fading channels"

The authors comment that the probability density function (PDF) of the signal-to-noise ratio (SNR) for third-order selection combining (SC3) given by Eng, Kong and Milstein (see ibid., vol.44, p.1117-29, 1996) is incorrect. A correction is given by presenting a more efficient method of deriving the generalized PDF of the SNR essential in the analysis of Nth-order selection combining.

Other-cell interference with power control in macro/microcell CDMA networks

The effects of power control on the other-cell interference of cellular CDMA networks are examined and quantified. Assuming that the path loss follows log-normal statistics, different propagation models for macrocells and microcells are adopted. Based on the Monte Carlo simulation, the other-cell interference has been evaluated in a number of situations of interest. Our results show that other-cell interference increases as the degree of imperfection of power control. The cross interference factors are shown in a mixed environment of macrocells and microcells.

Structured puncturing for rate-compatible B-LDPC codes with dual-diagonal parity structure

In this paper, we propose a generalized formula for generating puncturing patterns for block-type low-density parity check (B-LDPC) codes with dual-diagonal parity structure. The proposed formula distributes punctured bits uniformly in the zigzag edge connections, as well as maximizes the minimum recovery speed and the reliability of each punctured node. Also, the proposed puncturing can be applied to any B-LDPC code with dual-diagonal parity structure and can provide efficient bitwise puncturing patterns even when the number of puncturing bits is not equal to an integer multiple of the block size. Simulation results show that the proposed punctured B-LDPC codes are better than existing punctured B-LDPC codes and even dedicated B-LDPC codes used in commercial standards.

A Collaborative Cooperation Scheme using Hierarchical Modulation

In this paper, a novel forward link collaborative cooperation scheme using hierarchical modulation is proposed. Relay stations in a cell are assumed to operate in half-duplex mode. This leads to the degradation of throughput [bits/sec/Hz] performance due to the effect of additionally required resource for relaying. To overcome this limitation, we define a cooperative unit cell as a unit which includes base-station (BS), fixed relay- station (FRS) and two types of mobile-station (MS) being the relay-assisted mobile-station (RAMS) and the non-relay-assisted mobile-station (NRAMS). For collaboration between BS-FRS- RAMS link and BS-NRAMS link in the cooperative unit cell, hierarchical modulation is exploited. By using hierarchical modulation which enables simultaneous transmission of two data streams and provides multi-level protections, the proposed strategy yields an improved throughput reducing total transmission time in the cooperative unit cell. From the exact BER (bit error rate) performance of hierarchical modulation, throughput performance of the proposed strategy is deduced. It is shown that the proposed strategy recovers the throughput performance loss of half-duplex relay and outperforms the conventional cooperative and non-cooperative communication strategies with respect to the throughput performance.

Multistage decision-directed channel estimation scheme for DS-CDMA system with M-ary orthogonal signaling

A multistage decision-directed channel estimation scheme is proposed for a coherent direct-sequence code-division multiple-access (DS-CDMA) system with M-ary orthogonal signaling. The receiver with a multistage decision-directed channel estimator makes alternate use of symbol detection and channel estimation in an iterative manner, resulting in a multistage structure where each stage consists of an estimator and a detector. The estimator obtains the estimates of the channel by using the decisions from the detector in the previous stage, and the detector uses these estimates for coherent detection of the transmitted symbols. Modifications to the process of generating the channel estimates and the decision variables are made to mitigate the error propagation in successive stages. It is shown that the performance of the proposed estimation scheme is better than the conventional scheme, especially for channels with high diversity order. The gain is mostly achieved by two stages for channels with low diversity order and by five stages for channels with high diversity order. The proposed scheme can be used to increase the capacity of DS-CDMA systems currently operating in such environments.

Comparison of coherent and noncoherent CDMA system in time varying fading channel

The direct-sequence code-division multiple access (DS-CDMA) system has received much attention for digital mobile cellular telephone and personal communication network (PCN) applications. The performance of a coherent CDMA system is compared with that of a noncoherent CDMA system according to the extent of the channel variation and offset frequency. A pilot signal is used for coherent detection. Using a RAKE receiver, we evaluate the BER of the uncoded system analytically considering the estimation error of the carrier phase. Also the power ratio of the pilot signal to the data signal and the integration length of the channel estimator of the coherent BPSK system is optimized according to the fading bandwidth and offset frequency. The required SIR of the BPSK system is compared with that of a DPSK system. Additionally we determine the extent of performance improvement which can be achieved by convolutional coding. The results show that the optimum performance of BPSK is at least 1 dB better in terms of the required SIR than that of DPSK in practical ranges.

Blind Carrier Frequency Synchronization for the Uplink of Interleaved OFDMA Systems

In this letter, a novel blind CFO estimation algorithm for the uplink of an OFDMA system is proposed. The proposed method exploits the inherent redundant information in OFDMA symbols and does not require additional pilot or preamble overhead. Since it is a post-FFT estimator, it does not use filter banks to separate the desired users signal from the others in the time domain. Hence, the subcarriers of a certain user are not restricted to be clustered in the frequency domain. Therefore, the proposed estimator can be applied to OFDMA systems with an arbitrary subcarrier assignment over the entire bandwidth, including IEEE 802.16e, to obtain sufficient frequency diversity in a frequency selective fading channel. The proposed method can be efficiently used for continuous tracking of all active users CFOs only with two FFT windows within a single OFDM symbol. From simulation results, the performance of the proposed scheme is shown better than that uses preamble symbols.