Shadi Abu-Surra

Spatially Sparse Precoding in Millimeter Wave MIMO Systems

Millimeter wave (mmWave) signals experience orders-of-magnitude more pathloss than the microwave signals currently used in most wireless applications and all cellular systems. MmWave systems must therefore leverage large antenna arrays, made possible by the decrease in wavelength, to combat pathloss with beamforming gain. Beamforming with multiple data streams, known as precoding, can be used to further improve mmWave spectral efficiency. Both beamforming and precoding are done digitally at baseband in traditional multi-antenna systems. The high cost and power consumption of mixed-signal devices in mmWave systems, however, make analog processing in the RF domain more attractive. This hardware limitation restricts the feasible set of precoders and combiners that can be applied by practical mmWave transceivers. In this paper, we consider transmit precoding and receiver combining in mmWave systems with large antenna arrays. We exploit the spatial structure of mmWave channels to formulate the precoding/combining problem as a sparse reconstruction problem. Using the principle of basis pursuit, we develop algorithms that accurately approximate optimal unconstrained precoders and combiners such that they can be implemented in low-cost RF hardware. We present numerical results on the performance of the proposed algorithms and show that they allow mmWave systems to approach their unconstrained performance limits, even when transceiver hardware constraints are considered.

Low complexity precoding for large millimeter wave MIMO systems

Millimeter wave (mmWave) systems must overcome heavy signal attenuation to support high-throughput wireless communication links. The small wavelength in mmWave systems enables beamforming using large antenna arrays to combat path loss with directional transmission. Beamforming with multiple data streams, known as precoding, can be used to achieve even higher performance. Both beamforming and precoding are done at baseband in traditional microwave systems. In mmWave systems, however, the high cost of mixed-signal and radio frequency chains (RF) makes operating in the passband and analog domains attractive. This hardware limitation places additional constraints on precoder design. In this paper, we consider single user beamforming and precoding in mmWave systems with large arrays. We exploit the structure of mmWave channels to formulate the precoder design problem as a sparsity constrained least squares problem. Using the principle of basis pursuit, we develop a precoding algorithm that approximates the optimal unconstrained precoder using a low dimensional basis representation that can be efficiently implemented in RF hardware. We present numerical results on the performance of the proposed algorithm and show that it allows mmWave systems to approach waterfilling capacity.

The capacity optimality of beam steering in large millimeter wave MIMO systems

Millimeter wave (mmWave) systems must overcome the heavy attenuation at high frequency to support high-throughput wireless communication. The small wavelength in mmWave systems enables beamforming using large antenna arrays to combat path loss with large array gain. Beamforming in traditional microwave systems is often done at baseband for maximum flexibility. Such baseband processing requires a dedicated transceiver chain per antenna element. The high cost of radio frequency (RF) chains in mmWave systems, however, makes supporting each antenna with a dedicated RF chain expensive. This mismatch between the number of antennas and transceiver chains makes baseband processing infeasible; thus mmWave systems typically rely on a traditional approach known as beam steering which can be done at RF using inexpensive phase shifters. Unlike baseband precoding, however, traditional beam steering is not explicitly designed to achieve the capacity of the mmWave channel. In this paper, we consider both beamforming and multi-stream precoding in single user systems with large mmWave antenna arrays at both transmitter and receiver. Using a realistic channel model, we show that the unconstrained capacity-achieving precoding solutions converge to simple beam steering solutions. Therefore, in large mmWave systems, no rate loss is incurred by adopting the traditional lower-complexity solution.

Antenna Array Design for Multi-Gbps mmWave Mobile Broadband Communication

Almost all cellular mobile communications including first generation analog systems, second generation digital systems, third generation WCDMA, and fourth generation OFDMA systems use Ultra High Frequency (UHF) band of radio spectrum with frequencies in the range of 300MHz-3GHz. This band of spectrum is becoming increasingly crowded due to spectacular growth in mobile data and other related services. More recently, there have been proposals to explore mmWave spectrum (3-300GHz) for commercial mobile applications due to its unique advantages such as spectrum availability and small component sizes. In this paper, we discuss system design aspects such as antenna array design, base station and mobile station requirements. We also provide system performance and SINR geometry results to demonstrate the feasibility of an outdoor mmWave mobile broadband communication system. We note that with adaptive antenna array beamforming, multi-Gbps data rates can be supported for mobile cellular deployments.

Channel Feasibility for Outdoor Non-Line-of-Sight mmWave Mobile Communication

Due to the scarcity of spectrum below 3 GHz for wireless communications, there have been proposals to explore millimeter wave (mmWave) spectrum (3- 300 GHz) for commercial mobile applications. MmWave spectrum provides unique advantages such as availability of GHz bandwidth and use of antenna arrays with beamforming to compensate for path loss. While there exist well-established models for mmWave indoor non-line-of-sight (60 GHz) and mmWave outdoor line-of-sight (backhaul) communication, mmWave channels for outdoor, non-line-of-sight, mobile communication have not been explored sufficiently. We present penetration and reflection measurements for different materials and line-of-sight and non- line-of-sight measurements for outdoor mmWave mobile communication. We find that while well-known lossy objects such as human body and concrete can have poor penetration, they are good reflectors at these frequencies, enabling the receiver to capture secondary reflections for non-line-of-sight communication. We also show that a wide beam width, low gain antenna at the mobile receiver can capture more energy in scattered non-line-of-sight environments and thus, can provide more gain than a narrow beam, high gain antenna for mobile communication. Our initial measurements motivate utilizing mmWave frequencies for outdoor non-line- of-sight mobile communication.

Trapping set enumerators for specific LDPC codes

In this paper, a method is presented for enumerating the trapping sets of a specific LDPC code given its Tanner graph. The technique involves augmenting the original Tanner graph with additional variable nodes, and then applying a weight-enumeration algorithm to the augmented Tanner graph. The proposed method is used to find trapping set enumerators for several LDPC codes in communication standards. The complexity of the proposed algorithm is discussed.

Hybrid Architectures With Few-Bit ADC Receivers: Achievable Rates and Energy-Rate Tradeoffs

Hybrid analog/digital architectures and receivers with low-resolution analog-to-digital converters (ADCs) are two low power solutions for wireless systems with large antenna arrays, such as millimeter wave and massive multiple-input multiple-output systems. Most prior work represents two extreme cases in which either a small number of radio frequency (RF) chains with full-resolution ADCs, or low-resolution ADC with a number of RF chains equal to the number of antennas is assumed. In this paper, a generalized hybrid architecture with a small number of RF chains and a finite number of ADC bits is proposed. For this architecture, achievable rates with channel inversion and singular value decomposition-based transmission methods are derived. Results show that the achievable rate is comparable to that obtained by full-precision ADC receivers at low and medium SNRs. A trade-off between the achievable rate and power consumption for the different numbers of bits and RF chains is devised. This enables us to draw some conclusions on the number of ADC bits needed to maximize the system energy efficiency. Numerical simulations show that coarse ADC quantization is optimal under various system configurations. This means that hybrid combining with coarse quantization achieves better energy-rate trade-off compared with both hybrid combining with full-resolutions ADCs and 1-bit ADC combining.

On the existence of typical minimum distance for protograph-based LDPC codes

In this paper we prove that, for a certain class of protograph-based LDPC codes with degree-two variable nodes, a typical minimum distance exists. We obtain a tight bound on the sum of weight enumerators, up to some weight d∗, for the ensemble of finite-length protograph LDPC codes. Then we prove that this sum goes to zero as the block length goes to infinity. Finally, we prove that Pr(d < d∗) goes to zero as the block length goes to infinity. This typical minimum distance exists if degree-two nodes have certain connections to the check nodes. This is also important in practice since it identifies a certain class of protograph LDPC codes that have typical minimum distances.

Gigabit rate mobile connectivity through visible light communication

Visible Light Communication (VLC) is a short range wireless transmission of visible light through free space. The light source can be based on Light-Emitting Diodes (LEDs), that are becoming common use in our future connected home where they can be simultaneously used for both illumination (i.e. LED TVs, Light bulbs, etc.) and communication with appropriate photodiodes in the receiver side. VLC can be adapted to mobile devices enabling new types of short range applications such as HD video streaming. However, many of the new emerging mobile applications require increasingly high data rates; some can even exceed 1 Gbps. In this paper, we propose new optical/electrical front-end and baseband systems that achieve the required bit rate of 1 Gbps over free space. We also detail the implementation challenges of the low power VLC communication system for a mobile device. Specifically we focus on the new VLC baseband system and propose new baseband scheme with state-of-the-art LDPC channel coder. Finally, we will present the results of the new proposed gigabit baseband system as well as lab test results of a single channel VLC front-end running at 540 Mbps and 1080 Mbps.

High-throughput beamforming receiver for millimeter wave mobile communication

In this paper, we present a novel FPGA-based high-throughput beamforming MIMO receiver for millimeter wave mobile communication. With vast spectrum and small antenna element size, millimeter wave communication becomes very attractive and promising to support next generation mobile communication (5G). However, the high data rate requirement challenges both algorithm and architecture. In order to support the high data rate and to reduce the overhead of selecting the best beam pair, we propose a novel beamforming synchronization scheme more suitable for mobile communication. By further optimizing the algorithm and the architecture, we present a complete mobile receiver based on FPGA, which includes RF frontend, ADC, beamforming control, synchronization, channel estimator, soft MAP detector, and channel decoder. The design operates at 28 GHz carrier frequency with 500 MHz bandwidth. The throughput can reach 1.52 Gbps. We also performed the indoor and outdoor over-the-air transmission field tests. This work provides a platform for future millimeter wave mobile communication research.