Joseph C. Liberti

Experimental results using a MIMO test bed for wideband, high spectral efficiency tactical communications

This paper presents results from a MIMO measurement campaign designed to demonstrate spectral efficiencies of 13.7-26.0 information bits per second per Hz (bps/Hz) using an 8times8 test bed over a range of outdoor wireless channels with low antenna heights at each end of the link. Links included line-of-sight and non-line-of-sight channels obstructed by foliage and buildings. Measurements were performed at 456, 904, and 2177 MHz using a variety of array configurations including multiple polarization configurations. The transmitted MIMO OFDM signal occupied a 5 MHz bandwidth and used several error control coding and interleaving strategies. Signal processing at the receiver employed iterative detection with spatial filtering, SISO demappers, BCJR-based decoding (for turbo codes), and soft-cancellation of self-interference. Results demonstrated successful link closure at spectral efficiencies up to 26 bps/Hz using real-world synchronization, channel estimation, and carrier-frequency offset tracking

A low complexity iterative receiver for high spectral-efficiency battlefield MIMO communications

For multiple-input multiple-output (MIMO) systems, space-time bit-interleaved coded modulation (STBICM) using iterative detection has been recognized as a method to achieve near-capacity performance. However, the a posteriori probability (APP) detector required for this near-optimal performance exhibits prohibitive implementation complexity at high rates (ges16 coded bps/Hz) due to its exponential complexity in rate. With a view to enabling practical implementations of high spectral-efficiency wireless communications, this paper introduces a single-stream detection approach using spatial-filtering (or nulling) and soft-cancellation. Relative to the conventional APP detector, the proposed method is shown to have a superior performance-complexity trade. This paper also demonstrates the robustness of the proposed single-stream detection approach in overloaded conditions where there are fewer receive antenna elements devoted to signal separation than there are transmitted streams

Measurements of Multiple-Input Multiple-Output (MIMO) performance under army operational conditions

This paper presents the results from a field measurement campaign that was conducted to provide an understanding of Multiple-Input Multiple-Output (MIMO) performance relative to that of a Single-Input Single-Output (SISO) system in military-type environments. The Space & Terrestrial Communications Directorate (STCD) MIMO system was used to conduct these experiments. The system has two antennas and operates at transmit frequencies of 430 and 1380MHz, which fall within military frequency bands. A variety of operational environments with many scenarios were considered. The experiments were conducted at C4ISR OTM testing facility, Fort Dix, New Jersey for transmissions along a wide road, a narrow road and through heavy foliage. These experiments measured the throughput gain of MIMO over SISO given the same transmit power and channel usage. The gain in throughput was corroborated by information-theoretic capacity calculations using channel estimates collected during the experimental campaign. In addition, the impact of the antenna spacing on throughput gain was also studied. Depending on the multipath-richness of the environment, the experimental results show that the 2-antenna system provides a throughput of 1.3 to 2.0 times that of a SISO system. On average, a range extension of 1.5 times could be realized for all the considered scenarios and transmit frequencies. The results suggest that antennas in MIMO systems should be placed at least a half of carrier wavelength apart, as indicated in open literature.

A new low-complexity demapper for high-performance iterative MIMO: information-theoretic and BER analyses

Space-time bit interleaved coded modulation with iterative detection has been recognized as a method for achieving near-capacity performance using multiple-input multiple-output (MIMO) systems. However, the maximum-likelihood/joint-detection, required for this method, exhibits prohibitive implementation complexity at high rates (/spl ges/15 bps/Hz). With a view to enabling practical implementations of high spectral-efficiency wireless communications, this paper introduces a new single-stream detection approach using spatial-filtering and soft-cancellation. This method also exhibits superior performance relative to the conventional multi-stream detection approach as long as there are as many receive elements as there are transmit streams. The proposed method is evaluated in terms of bit error rate performance and information-theoretic considerations using an EXIT (extrinsic information transfer) chart method.

Throughput performance for multiple-input multiple-output networks

Signal processing techniques have been developed to implement multiple-input multiple-output (MIMO) systems, which exploit antenna arrays at each end of the link to achieve extremely high data rates. MIMO receivers always incorporate multiple antenna elements and signal processing techniques for interference suppression, which is required to extract multiple parallel channels between the transmitter and receiver. Therefore, these systems are inherently equipped to mitigate the effects of cochannel interferers in a multiuser environment through increased collision robustness. Consequently, these systems offer better trades between throughput and energy efficiency, both of which are critical for tactical radio networks. We investigate the throughput of a network comprised of MIMO capable nodes using the slotted ALOHA protocol.

Iterative MIMO detector using a group-wise approach

For multiple-input multiple-output (MIMO) systems, space-time bit-interleaved coded modulation (ST-BICM) using iterative detection has been recognized as a method for achieving near-capacity performance. However, the a posteriori probability detector required for the conventional ST-BICM receiver is not amenable to practical implementation for high-rate MIMO due to its exponential complexity in rate. This motivated the development of a single-stream detector exhibiting polynomial complexity and capable of delivering performance comparable to the conventional detector under near-ideal conditions. However, under realistic operating scenarios, with possibly low-rank channel conditions and/or employing fewer receive antennas than transmitted streams, the performance gap between the single-stream detector and the conventional detector widens. With a view to enabling practical implementations of high-rate near-capacity MIMO, we propose a novel detector based on a group-wise approach that can be flexibly configured to deliver the best possible combination of performance and complexity over a wide range of realistic operating conditions.

OFDM space-time trellis coded MIMO systems with experimental results

Multi-input multi-output (MIMO) systems have been shown to provide extremely high capacity in non-line-of-sight channels, and are thus well suited for mobile tactical communications. Using space-time trellis coded vector transmission, the paper shows that maximum likelihood joint detection (MLJD) receivers provide substantial improvements over layered space time (LST) approaches. It is shown that in full-rank channels, the MLJD approach provides significant SNR improvement over LST techniques. It is also shown that an MLJD receiver is much more robust in reduced rank channels. Unfortunately, these improvements come at the cost of high computational complexity at the receiver. One way to reduce the computational complexity is to employ a hybrid approach, in which the MLJD receiver is applied to groups of transmit antennas at a time, combined with an outer group-wise LST approach. Methods of performing this grouping based on channel conditions is presented and shown to provide a means of smoothly trading complexity for performance. Simulations and over-the-air measurements are presented that compare various MIMO receiver techniques.

MIMO adaptivity for tactical communications in dynamic multipath and mobility environments

In this paper we investigate practical adaptivity methods for tactical multiple-input multiple-output (MIMO) applications that need to operate on-the-move in variable multipath environments. The key features of these methods include the ability to estimate in real-time the MIMO channel between the transmitter and receiver in a dynamic multipath environment, select the highest performance spatial operating mode, and adapt the transmit waveform and receive/transmit processing. As channel dynamics vary, the proposed adaptive MIMO system determines whether to operate in Uninformed Transmitter (without feedback) or Informed Transmitter mode based on both spatial and temporal metrics that characterize the channel conditions. The adaptivity methods investigated in this study will be used in a real-time testbed designed to demonstrate the key features of an adaptive MIMO link and assess performance in a real-world environment. In this paper we also describe the adaptive MIMO testbed, the frame transmission structure, and channel sounding waveform used for the spatial adaptivity tests.

OFDM space-time trellis-coded MIMO systems with experimental results

Multi-Input Multi-Output (MIMO) systems have been shown to provide extremely high capacity in non-line-of-sight channels. Using Space-Time Trellis Coded vector transmission, this paper shows that Maximum Likelihood Joint Detection (MLJD) receivers provide substantial improvements over Layered Space Time (LST) approaches. It is shown that in full-rank channels, the MLJD approach provides significant SNR improvement over LST techniques. It is also shown that an MLJD receiver is much more robust in reduced rank channels. These improvements come at the cost of high computational complexity at the receiver. Hybrid approaches allow complexity to be reduced by applying ML techniques to groups of transmit antennas, rather than all transmit antennas. Methods of performing this grouping based on channel conditions is presented and shown to provide a means of smoothly trading complexity for performance. Simulations and over-the-air measurements are presented that compare various MIMO receiver techniques.