Electrical and Computer Engineering

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Browsing Electrical and Computer Engineering by Subject "802.11ac"

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    Client Beamforming for Rate Scalability of MU-MIMO Networks
    (2015-04-24) Yu, Hang; Zhong, Lin; Knightly, Edward W; Sabharwal, Ashutosh; Johnson, David B
    The multi-user MIMO (MU-MIMO) technology allows an AP with multiple antennas to simultaneously serve multiple clients to improve the network capacity. To achieve this, the AP leverages zero-forcing beamforming (ZFBF) to eliminate the intra-cell interference between served clients. However, current MU-MIMO networks suffer from two fundamental problems that limit the network capacity. First, for a single MU-MIMO cell, as the number of clients approaches the number of antennas on the AP, the cell capacity often flattens and may even drop. Second, for multiple MU-MIMO cells, the multiple APs cannot simultaneously serve their clients due to inter-cell interference, so that the concurrent streams are constrained to a single cell with limited network capacity. Our unique perspective to tackle these two problems is that modern mobile clients can be equipped with multiple antennas for beamforming. We have proposed two solutions that leverage the client antennas. For the capacity scalability problem in a single MU-MIMO cell, we use multiple client antennas to improve the orthogonality between the channel vectors of the clients. The orthogonality between clients’ channels determines the SNR reduction from the zero-forcing beamforming by the AP, and is therefore critical for the capacity of a MU-MIMO cell to become more scalable to the number of clients. We have devised a 802.11ac-based protocol called MACCO, in which each client locally optimizes its beamforming weights based on the channel knowledge obtained from overhearing other clients’ channel reports. For the inter-cell interference problem in multiple MU-MIMO cells, we leverage multiple client antennas to assist the interfering APs to coordinately cancel the inter-cell interference between them. To achieve such coordinated interference cancellation in a practical way, We have proposed a two-step optimization including antenna usage optimization and beamforming weight optimization. We have devised another 802.11ac-based protocol called CoaCa, which integrates this two-step optimization into 802.11ac with small modifications and negligible overhead, allowing each AP and client to locally identify the optimal beamforming weights. We have implemented both MACCO and CoaCa on the WARP SDR platform leveraging the WARPLab framework, and experimentally evaluated their performance under real-world indoor wireless channels. The results have demonstrated the effectiveness of MACCO and CoaCa toward solving the capacity scalability and inter-cell interference problems of MU-MIMO networks. First, on average MACCO can increase the capacity of a single MU-MIMO cell with eight AP antennas and eight clients by 35%, compared to existing solutions that use client antennas differently. Second, for a MU-MIMO network with two cells, by cancelling the inter-cell interference CoaCa can convert the majority of the number of streams increase (50%-67%) into network capacity improvement (41%-52%).
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    Protocol Design and Experimental Evaluation for Efficient Multi-User MIMO Wireless Networks
    (2015-04-24) Bejarano Chavez, Oscar; Knightly, Edward W.; Sabharwal, Ashutosh; Aazhang, Behnaam; Ng, T.S. Eugene
    Information theoretic results on Multi-User MIMO (MU-MIMO) have demonstrated a many-fold increase in capacity compared to Single-Input Single-Output. By leveraging multiple antennas at the Access Point (AP) and beamforming techniques, MU-MIMO enables simultaneous transmissions of multiple independent streams on the downlink. Ideally, with sufficient antennas at the AP, MU-MIMO can attain capacity gains proportional to the number of streams. However, the cost required to enable efficient and robust multi-stream transmissions is much higher than that for the single-stream case and worsens with increasing number of streams. More specifically, two key factors hinder the potential gains that can be attained via MU-MIMO: (i) To serve multiple users simultaneously, the AP needs to collect Channel State Information (CSI) from all users to be served (i.e., sounding). Sounding overhead reduces the effective data airtime utilization of the overall system. (ii) Multi- stream transmissions are highly susceptible to inter-stream interference originated due to inaccurate or outdated CSI, thereby reducing packet reception performance. I demonstrate that in practice, the costs of MU-MIMO not only decrease the gains demonstrated by theory but can completely outweigh the benefits. I identify those adverse situations and propose several techniques that alleviate the negative impact caused by sounding overhead and CSI inaccuracies. First, I design CUiC and MUTE, two protocols that address MU-MIMO sounding overhead by performing overhead compression along spatial and temporal domains, respectively. CUiC exploits the available Degrees-of-Freedom (DoF) at the AP to allow multiple users to reply with their control messages (e.g., channel estimates and acknowledgements) simultaneously, therefore reducing the time required for users to reply, to a constant. MUTE exploits epochs characterized by slowly moving channels to reduce the frequency of channel sounding. Second, I design CHRoME, a protocol that addresses interference-leakage caused by outdated and inaccurate CSI as well as out-of-cell interference. CHRoME proposes a bit rate selection strategy that re-tunes the selection according to current channel and interference conditions. Additionally, if necessary, CHRoME realizes a fast soundless retransmission that exploits liberated DoF at the AP to minimize retransmission overhead. I implement and evaluate all three schemes using a combination of WARP FPGA-based transceivers, and custom emulation platforms.