Weijun Zhu

Multi-antenna testbeds for research and education in wireless communications

Wireless communication systems present unique challenges and trade-offs at various levels of the system design process. Since a variety of performance measures are important in wireless communications, a family of testbeds becomes essential to validate the gains reported by the theory. Wireless testbeds also play a very important role in academia for training students and enabling research. In this article we discuss a classification scheme for wireless testbeds and present an example of the testbeds developed at UCLA for each of these cases. We present the unique capabilities of these testbeds, provide the results of the experiments, and discuss the role they play in an educational environment.

A practical, hardware friendly MMSE detector for MIMO-OFDM-based systems

Design and implementation of a highly optimized MIMO (multiple-input multiple-output) detector requires cooptimization of the algorithm with the underlying hardware architecture. Special attention must be paid to application requirements such as throughput, latency, and resource constraints. In this work, we focus on a highly optimized matrix inversion free MMSE (minimum mean square error) MIMO detector implementation. The work has resulted in a real-time field-programmable gate array-based implementation (FPGA-) on a Xilinx Virtex-2 6000 using only 9003 logic slices, 66 multipliers, and 24 Block RAMs (less than 33% of the overall resources of this part). The design delivers over 420 Mbps sustained throughput with a small 2.77-microsecond latency. The designed linear MMSE MIMO detector is capable of complying with the proposed IEEE 802.11n standard.

An open access wideband multiantenna wireless testbed with remote control capability

This paper introduces an open access wideband multiantenna wireless testbed. The testbed is configured as a four transmit antenna by four receive antenna system based on software defined radio technology. It operates in the 2.4 GHz ISM band and supports an RF bandwidth compatible to IEEE 802.11a/g standard. A robotic positioning system has been developed to automatically control the position and orientation of the antenna array, which makes automated testing possible and greatly reduces engineering time. The testbed is configured such that it can be controlled by any computer on the Internet. This makes remote debugging and testing possible. This characteristic of test automation and remote access through Internet was designed into the testbed to make it a valuable resource to the other researchers. A set of software has been developed to support research using the testbed, with the current focus on MIMO channel characterization and MIMO-OFDM packet communications. A data acquisition system and an offline processing system have been developed for the testbed and have been used for over the air testing to produce results for the ongoing IEEE 802.11n standardization activity.

A real time MIMO OFDM testbed for cognitive radio & networking research

A real time, 2 Mbps to 200 Mbps portable radio unit with MIMO and sensing capability which exposes all the PHY parameters to the higher layers will help advance experimental cognitive radio (CR), and wireless networking research. Collaboration between Silvus Communication Systems and the UCLA WISR group has resulted in the first generation radio specifically designed to meet the needs of the CR and wireless networking community. The current platform is based on a COTS FPGA platform with dual-band RF capabilities. It implements a slight variant of the 802.11n draft spec. It is a fully self contained PHY solution with over 100 unique modes of operation. Moreover it features a robust API to the MAC through which all PHY parameters can be controlled on a per-packet basis. The same API will allow the PHY to communicate channel state information, SNR, and RSSI measurements to the MAC. A 16 micro-second packet decode latency ensures that the PHY processing does not inhibit the systems fast response to changing channel and interference condition.

Field test results for space-time coding

Multiple antenna radios are recently the subject of much research due to the significant improvements they offer in throughput and reliability in wireless communications. Research in the field of space-time coding has led to the design of modulation techniques offering improved performance. Their performance, although promising in simulations, needs to be studied in real wireless channels due to the inability to accurately model all possible wireless channel conditions that might be seen in practice. With this motivation, a range of experiments were conducted to measure the performance of some of the well-known space-time codes in literature over real channels and under implementation impairments. This paper discusses the testbed setup for the experiments and present the results of field trials conducted at the University of California, Los Angeles.

MIMO performance evaluation for airborne wireless communication systems

This paper presents a characterization of the practical performance gains of a multiple-input/multiple-output (MIMO) system with respect to a traditional single-input/single-output (SISO) system in an air-to-ground environment. Analysis of these performance gains is based on actual data throughput and channel-state measurements of an airborne 4×4 MIMO-enabled orthogonal frequency division multiplexing (OFDM) system. Practical realization of these performance gains with a coded-OFDM wideband system was accomplished through airborne field trials across multiple flight profiles. We show through experimental validation that average MIMO throughput gains in excess of 2x can be achieved relative to a SISO system. These field trials also indicate that MIMO processing can deliver a 1.6x increase in the range of the link. Additionally, if channel-state information (CSI) is available at the transmitter - which enables MIMO eigen beamforming - the average throughput gain can be further increased to 4.5x and the link range can be increased by as much as 3.6x. It is also shown that for a 4×4 MIMO system to deliver the same throughput performance as a conventional SISO system in an air-to-ground environment, a relative TX power savings of up to 21 dB can be realized.

A signaling scheme and estimation algorithm for characterizing frequency selective MIMO channels

This paper presents a signaling scheme and channel estimation algorithm designed to characterize frequency selective MIMO channels. This scheme has been implemented in a wideband MIMO radio testbed and used to measure the achievable channel capacity in an indoor office environment. Also, we report the achievable capacity for a real hardware platform at a particular combination of channel, SNR, antenna separation and location. This is repeated over a large number of spatially unique antenna placements and locations to give a statistically representative set measurements for the environment. The data sets generated in the measurement campaign gives an indication of the variability of achievable capacity and not just the average channel capacity. This allows future studies to investigate the characteristics of a typical poor channel and how they cause a degradation in system performance.

Performance of a Concurrent Link SDMA MAC Under Practical PHY Operating Conditions

Space division multiple access (SDMA)-based medium access control (MAC) protocols have been proposed to enable concurrent communications and improve link throughput in multi-input-multi-output (MIMO) ad hoc networks. For the most part, the works appearing in the literature make idealized and simplifying assumptions about the underlying physical layer and some aspects of the link adaptation protocol. The result is that the performance predicted by such works may not necessarily be a good predictor of the actual performance in a fully deployed system. In this paper, we look to introduce elements into the SDMA-MAC concept that would allow us to better predict their performance under realistic operating conditions. Using a generic SDMA MAC, we look at how the network sum throughput changes with the introduction of the following: (1) use of the more practical MMSE algorithm, instead of the zero-forcing or singular-value-decomposition-based nulling algorithms used for receive beamnulling; (2) impact of channel estimation errors; (3) introduction of link adaptation mechanism specifically designed for concurrent SDMA MACs; and (4) incorporation of TX beamforming along with RX beamnulling. Following on the transmission window during which concurrent transmissions are allowed by the MAC, we qualify the impact of each of these four elements in isolation. At the conclusion, the performance of a system that incorporates elements 1-4 is presented and compared against the baseline system, showing an improvement of up to five times in the overall network sum throughput.

Super-orthogonal space-time block code using a unitary expansion

The paper introduces a super-orthogonal space-time block coding scheme. The code construction is based on the expansion of an orthogonal block code via a unitary matrix transformation. By expanding the orthogonal block code, we can both increase the code rate and improve the performance in terms of E/sub b//N/sub 0/, with a moderate increase of the receiver complexity. The performance is compared with orthogonal and quasi-orthogonal block codes in computer simulation.

MIMO Systems for Military Communications

Since the seminal work of Foschini, Gans and Teletar, the research community has generated a large body of work dealing with various aspects and benefits of MIMO for wireless data communications in civilian and commercial systems. Here we focus on the use of MIMO for military communication. In particular the effectiveness of MIMO for: (a) communications under very high mobility such as UAV based communication and, (b) the use of multi antenna techniques for covert (LPD) communications. Our results show the ability to operate at speeds of up to 200 mph, and a 17 dB reduction in the required TX power for covert, LPD communications, in addition to an interference/jammer mitigation technique based on MIMO eigen beam-nulling