Jiann-An Tsai

Analysis of optimal use of CQI channels for OFDMA systems

In this paper we analyzed the optimal use of CQI (Channel Quality Indicator) channels for OFDMA systems. CQI is considered as an overhead that each mobile terminal (MT) is required to report its observed channel quality back to the base station (BS) so that the usage of forward link subchannels can be optimally assigned. Here, we are interested in finding the minimum number of CQI channels required to achieve certain usage of forward link subchannels for OFDMA systems. We analyzed the trade off between reverse link overhead and usage of forward link subchannels. The problems are solved based on probability analysis, which is consistent with our simulation results. Take IEEE 802.16e as an example, in order to guarantee with a probability of 90% that at least 15 of 16 subchannels are reported by users and hence scheduled by the BS, at least 9 CQI channels are needed. However, if all the 16 subchannels need be reported with a probability of 90%, as many as 14 CQI channels are needed. On the other hand, only 6 CQI channels are needed if one focuses on a certain subchannel and wants it to be reported by users with a probability of 90%. The results presented in this paper is substantial in OFDMA implementation for high spectrum utilization without undue reverse link overhead.

Performance of low-complexity MMSE beamforming for WLAN systems

In this paper the use of adaptive antenna arrays for OFDM-based WLAN systems in indoor wireless environment is investigated. We present the performance of a new proposed low-complexity MMSE beamforming for OFDM systems in a frequency selective Rayleigh fading channel. The proposed MMSE beamforming algorithm is based upon hybrid frequency-time domain processing, where the dominant computation of the beamforming algorithm is carried out in the time domain while the minor part of the beamforming computation is done in the frequency domain. This approach provides significant computational complexity reduction over the conventional frequency domain beamforming for OFDM-based systems. We evaluate the system performance of the hybrid frequency-time domain beamforming algorithms through the use of computer simulation. The preliminary simulation results show that the proposed hybrid frequency-time domain beamforming outperforms time domain beamforming. Furthermore it provides acceptable system performance with significant computation reduction as compared to the conventional frequency domain beamforming.

Uplink Power Control for Next Generation Mobile Broadband Wireless Access Systems: System-Level Simulation Results

In uplink OFDM/A systems that support link adaptation, e.g., via adaptive modulation and coding (AMC), and/or HARQ schemes, there have been suggestions that fast, closed-loop power control (PC) may not be necessary. It might be sufficient to use open-loop, slow power control method as a countermeasure to the effects of slow fading, i.e., propagation path loss and shadow fading. Here we will compare the performance of uplink open-loop, closed-loop power control with a fixed SINR target or AMC, based on some simulation results. Although it is necessary to emphasize the high throughput aspect of the technology evolution, the support of various traffic types that require medium to low data rates, e.g., VoIP and video conferencing applications should not be overlooked. As transmission at high power often introduces additional interference to users in the other sectors or cells, and increases battery power consumption, the user terminal (UT) should only transmit at the minimum required power to support the data rate as required by the application. We will show that in a typical cellular environment, closed-loop power control can be used to limit the UTs transmit power. This leads to a longer battery life and lower intercell interference.