Minnie Ho

Indoor multiple-antenna channel characterization from 2 to 8 GHz

In multiple-antenna channels, the optimality of a transmission scheme hinges on a detailed characterization of the channel. In this paper, we will present measurement results from 2 to 8 GHz in both LOS and NLOS scenarios, at both office and residential environments. Data processing methods are briefly outlined and a statistical characterization of the channel is presented as well.

Analog combining of multiple receive antennas with OFDM

Optimal signal processing of M receiver antennas requires digitization of each of the M RF signals. For example, MRC (maximal ratio combining) of M receiver antennas requires M ADCs (analog to digital converters). Since the ADC is a highly power-consumptive component in current receiver chain architectures, we investigate suboptimal signal processing schemes that combine M analog signals before digitization, thus enabling a much cheaper implementation with a single ADC. We focus on IEEE 802.11a like OFDM LAN systems with one transmit and multiple receiver antennas (SIMO - single input multiple output), with perfect channel knowledge at the receiver. Our scheme uses scalar weights to combine the receiver antennas in the time-domain. These weights are a function of the spatio-temporal channel matrix. The results are very promising: 4-antenna linear combining (LC) outperforms 2-antenna selection diversity by 5-7 dB at an uncoded BER of 0.01, and this margin is even wider at lower BERs.

The Effects of Co-channel Interference on Spatial Diversity Techniques

This paper investigates the effects of co-channel interference on spatial diversity techniques. By analyzing distributions of post-processing signal to noise plus interference ratio (SINR) after matched filtering, we capture the performance characteristics of spatial diversity techniques in an interference limited environment. Using intuition from the theoretical analysis of a single carrier system, the performance of spatial diversity techniques in an OFDMA system with co-channel interference is also characterized. Our analytical and simulation results show that space time block code (STBC) schemes are more sensitive to co-channel interference than other spatial diversity techniques so their performance gain in a noise-limited environment can be lost in an interference-limited environment.

High rate space frequency block codes for next generation 802.11 WLANs

Achieving higher link rates in support of newer services and increasing range are the two main goals for next generation wireless LAN networks. This work primarily contributes to a high rate solution, based on a novel extension to the rate-1 space-frequency block code (SFBC) design in L. Shao et al. (Dec. 2003) to full rate case. To further improve performance without increasing complexity, we propose unequal power allocation at the transmitter with no requirement of channel knowledge. Compared with standard 802.11a transmitter which uses convolutional code with a single transmit antenna and antenna selection diversity among 2 receive antennas with soft Viterbi decoding, we show that our HR-SFBC code with 2 transmit antenna not only doubles the throughput (as expected) but also provides additional link margin (and hence improves range).

MIMO Broadcast Channels with Channel Estimation

We consider a multi-user MIMO downlink where the transmitter has only estimates of the channel while the receivers have perfect channel information. The impact of channel estimation error on the sum rate is studied. It is shown that the sum rate saturates due to the inaccurate channel information. In order to maintain full multiplexing gain, the channel estimation quality has to increase with at least the square root of data SNR but there is no need to increase more than linearly with the SNR. With user scheduling, the sum rate scales at least M/2 log log K, where K is the number of users and M is the number of transmit antennas.