Multiple Antenna Techniques with Interfering Links in Cooperative Settings

The motivation is the fact that wireless networks have been deployed so densely nowadays. It causes a severe mutual interference problem that limits network capacity dramatically. Observing the great capability of multiple antenna techniques in interference suppression, we believe that it would mitigate the interferences and therefore improve network capacity. My research basically includes three parts as listed below.

1. Terminal embedded beamforming

For most techniques that have gained market success, the low cost and low complexity design plays very important role. Therefore, the first step of my research focused on the low cost beamforming technique. The beamforming is able to focus its transmission and reception to the related direction. In the environment with directional proprogation property, it can improve the link capacity from the array gain and interference suppression. Unlike the other researches that focus on using it on access point side, we put it on the terminal side and make it compliant with the current standards. It would have better market penetration as customers would like to have this feature on their laptops to have a better access experience to the existing widespread wireless networks. To demonstrate our idea, a beamforming testbed (a beamforming embedded laptop) was developed based on phased array technique and the experimental results showed averagely over 100% throughput improvement than the single antenna setting in an interference limited environment.

2. Cooperative spatial reuse with MISO links

Basically, the terminal embedded beamforming is an autonomous design that the beamforming is optimized for each terminal itself only. The joint optimization involving with multiple beamforming links is expected to further improve network capacity. Therefore, I started to investigate this to enable spatial reuse for MISO networks. As a result, I proposed a novel cooperative framework to enable spatial reuse in multi-rate network. My research has shown that it has the great potential to further improve network capacity and energy efficiency dramatically than the TDMA case. Over 70% capacity improvement and over 35% energy saving at receivers have been seen in a simulated two-links example with zeroforcing technique.

3. Multi-user diversity with interfering MIMO links

In recent years, MIMO techniques have attracted a lot of attentions in both academia and industries. Even though MIMO techniques have shown great capabilities to improve single link capacity, they are subject to the mutual interference. In this work, I focused on the closed-loop MIMO technique. Based on the observation of the selectivity of the eigenmodes, a cooperative scheme with zeroforcing is proposed to explore multi-user diversity in spatial domain among multiple links. It is basically to replace the weak spatial subchannels on one link with the stronger spatial subchannels on other links. The results show its significant potential on further improve the network capacity in low and medium SNR regime over the TDMA MIMO.