Narendra Anand

Argos: practical many-antenna base stations

Multi-user multiple-input multiple-output theory predicts manyfold capacity gains by leveraging many antennas on wireless base stations to serve multiple clients simultaneously through multi-user beamforming (MUBF). However, realizing a base station with a large number antennas is non-trivial, and has yet to be achieved in the real-world. We present the design, realization, and evaluation of Argos, the first reported base station architecture that is capable of serving many terminals simultaneously through MUBF with a large number of antennas (M >> 10). Designed for extreme flexibility and scalability, Argos exploits hierarchical and modular design principles, properly partitions baseband processing, and holistically considers real-time requirements of MUBF. Argos employs a novel, completely distributed, beamforming technique, as well as an internal calibration procedure to enable implicit beamforming with channel estimation cost independent of the number of base station antennas. We report an Argos prototype with 64 antennas and capable of serving 15 clients simultaneously. We experimentally demonstrate that by scaling from 1 to 64 antennas the prototype can achieve up to 6.7 fold capacity gains while using a mere 1/64th of the transmission power.

Design and experimental evaluation of multi-user beamforming in wireless LANs

Multi-User MIMO promises to increase the spectral efficiency of next generation wireless systems and is currently being incorporated in future industry standards. Although a significant amount of research has focused on theoretical capacity analysis, little is known about the performance of such systems in practice. In this paper, we present the design and implementation of the first multi-user beamforming system and experimental framework for wireless LANs. Using extensive measurements in an indoor environment, we evaluate the impact of receiver separation distance, outdated channel information due to mobility and environmental variation, and the potential for increasing spatial reuse. For the measured indoor environment, our results reveal that two receivers achieve close to maximum performance with a minimum separation distance of a quarter of a wavelength. We also show that the required channel information update rate is dependent on environmental variation and user mobility as well as a per-link SNR requirement. Assuming that a link can tolerate an SNR decrease of 3 dB, the required channel update rate is equal to 100 and 10 ms for non-mobile receivers and mobile receivers with a pedestrian speed of 3 mph respectively. Our results also show that spatial reuse can be increased by efficiently eliminating interference at any desired location; however, this may come at the expense of a significant drop in the quality of the served users.

STROBE: Actively securing wireless communications using Zero-Forcing Beamforming

We present the design and experimental evaluation of Simultaneous TRansmission with Orthogonally Blinded Eavesdroppers (STROBE). STROBE is a cross-layer approach that exploits the multi-stream capabilities of existing technologies such as 802.11n and the upcoming 802.11ac standard where multi-antenna APs can construct simultaneous data streams using Zero-Forcing Beamforming (ZFBF). Instead of using this technique for simultaneous data stream generation, STROBE utilizes ZFBF by allowing an AP to use one stream to communicate with an intended user and the remaining streams to orthogonally “blind” (actively interfere with) any potential eavesdropper thereby preventing eavesdroppers from decoding nearby transmissions. Through extensive experimental evaluation, we show that STROBE consistently outperforms Omnidirectional, Single-User Beamforming (SUBF), and directional antenna based transmission methods by keeping the transmitted signal at the intended receiver and shielded from eavesdroppers. In an indoor Wireless LAN environment, STROBE consistently serves an intended user with an SINR 15 dB greater than an eavesdropper.

Mode and user selection for multi-user MIMO WLANs without CSI

A Multi-User MIMO (MU-MIMO) Access Point (AP) can obtain a capacity gain by simultaneously transmitting to multiple clients. This technique requires Channel State Information (CSI) at the transmitting AP to set antenna gains and phases to enable simultaneous reception through beamforming. The AP must also select both the mode (number of transmit and collective receive antennas) and the user set prior to transmission. While the ideal mode and user selection is a function of CSI, CSI must be estimated with an overhead intensive channel sounding process. We design, implement, and evaluate Pre-sounding User and Mode selection Algorithm (PUMA), a method for mode and user selection prior to channel sounding. We show that even without CSI, PUMA (i) exploits theoretical properties of MU-MIMO system scaling with respect to mode, (ii) characterizes the relative cost of each potential mode, and (iii) estimates per-stream transmission rate and aggregate throughput in each mode for a potential user set, all without CSI. Once PUMA has selected the appropriate mode and user group, the chosen protocols channel sounding method is used on the intended user subset to carry out the transmission. We show that, on average, PUMA selects the mode and group that achieves an aggregate rate within 3% of the saturation throughput of what would have been achieved by sounding all users (which would require significant additional overhead). Moreover, we show that PUMA obtains 30% higher aggregate throughput compared to the best fixed-mode policy that uses the maximum number of available transmit and receive antennas.

The case for UHF-band MU-MIMO

While the UHF band exhibits superior propagation characteristics compared to other frequency bands used for broadband communications, limited spectral availability in time and space necessitates high spectral efficiency techniques such as Multi-user MIMO (MU-MIMO). In this paper we design and implement the first open MU-MIMO Software-Defined Radio (SDR) platform that operates on an order of magnitude frequency range, from 300 MHz to 5.8 GHz. We perform a comprehensive set of over-the-air experiments to evaluate the potential of UHF-band MU-MIMO in comparison to 2.4 and 5.8 GHz WiFi bands encompassing a range of operating environments. We evaluate MU-MIMO performance in both outdoor, indoor, line-of-sight (LOS), and non-line-of-sight (NLOS) environments, and demonstrate that while the temporal correlation of the measured UHF environment is increased, it does not come at the cost of increased spatial correlation as measured by the Demmel condition number, thus proving highly attractive for MU-MIMO. This evaluation demonstrates the effectiveness of MU-MIMO transmission techniques in UHF bands for high spectral efficiency and low-overhead wireless access.

Practical performance of MU-MIMO precoding in many-antenna base stations

Many-antenna base stations promise manyfold spectral capacity increases in theory. However, our recent experimental work has shown a significant performance gap between the traditional MU-MIMO linear precoding method, zero-forcing, and the method proposed for many-antenna base stations, conjugate. Thus, a critical question in the field of many-antenna base stations is: Under what scenarios,if any, does conjugate precoding outperform zero-forcing in real systems? Towards answering this question, we leverage our experience in building many-antenna base stations to derive a model for the performance of linear precoders in real-world systems. We isolate the primary factors which discrepantly affect these linear precoders, then capture their complex interactions in an analytical model. By combining our real-world capacity results with this analytical model, we find new insight in to the tradeoffs between conjugate and zero-forcing precoding. Our results suggest that conjugate will outperform zero-forcing when there are many simultaneous users, the users have high mobility, or the implementation employs less-capable hardware. We find that our model is not only useful for guiding the hardware design of base stations, but can also facilitate dynamically switching to the optimal linear precoding algorithm in realtime, through adaptive precoding.

WARPlab: a flexible framework for rapid physical layer design

In this paper, we present WARPLab, a framework for the rapid prototyping and implementation of physical layer algorithms for wireless LANs. WARPLab is based on WARP, an FPGA-based, wireless experimental platform. We first give an overview of WARPLab and then present an example of a physical layer algorithm that we implemented on the framework for our work. We then discuss the features, limitations, and future modifications as they pertain to our research.

Opportunistic Channel Estimation for Implicit 802.11af MU-MIMO

Multi-User MIMO (MU-MIMO) linear channel coding can greatly increase wireless system capacity when Stations (STAs) have fewer antennas than the Access Point (AP), but it comes at the cost of significant Channel State Information (CSI) estimation overhead. Previous work has suggested that 802.11af MU-MIMO systems might benefit from long channel coherence time, extending the useful duration of CSI. In this paper, we propose and analyze an opportunistic channel sounding policy that avoids sounding overhead in wireless channels by gathering implicit CSI opportunistically. This policy not only avoids CSI overhead, but also has the potential to enable efficient interoperability of multi-user APs with legacy single-stream STAs. To investigate the performance of this new policy, we implement a new mobile channel sounding framework on a custom 802.11af Software-Defined Radio (SDR) system designed for UHF-band experimentation and evaluate channel sounding performance in indoor and outdoor environments under various mobility modes. Additional protocol analysis shows that in UHF channels with sufficient channel coherence time, an opportunistic channel sounding policy offers significant protocol optimization while improving the scalability of next-generation MU-MIMO systems.

Demo: an open-source development platform for long-range UHF-connected wifi hotspots

We present a real-time software-defined radio (SDR) platform for prototyping and measuring the performance of broadband UHF radio networks operating over long distances with point-to-multipoint (PTMP) non-line-of-sight (NLOS) networks. Enabled by the Wideband UHF Radio Card (WURC), a custom high-power and frequency-flexible radio transceiver daughter-card for FPGA-based digital basebands, the 802.11 DCF-like MAC and PHY implementation is completely open source. We demonstrate a long-range PTMP NLOS network bonding several of the white space television channels available in Maui, Hawaii. Off-the-shelf client devices can use this network via 802.11a/g links implemented with the same SDR framework. The multi-carrier channel estimates and real-time MAC statistics of connected nodes in UHF and 2.4 GHz are recorded and displayed in real-time, demonstrating an unprecedented amount of flexibility in unlicensed frequency bands, enabling real-time TV-band cognitive networks and small-cell research deployments. A demonstration video is available at the following link: http://youtu.be/dTOJHRICklQ