The evolution to denser radio access network (RAN) with small cells in cluttered urban environments increases the need for wireless non-line-of-sight (NLOS) backhaul. A novel NLOS backhaul solution is designed that is optimized for coexistence with TD-LTE radio access. The NLOS backhaul solution is based on TD-LTE air-interface for fast development but with critical modifications and improvements to meet all the technical requirements of wireless backhaul. The system is targeted for unprecedented 10 bits/s/Hz spectral efficiency through 256-QAM, concatenated turbo and Reed-Solomon (RS) coding, and 2x2 MIMO techniques. This paper discusses the key design concepts of the air-interface with simulation results in NLOS channels.

In this work, we study the design of polar codes for high-order constellations. We propose a new methodology for selecting polar codes rates in multilevel coding scenarios in order to optimize the overall error performance. We also propose a novel method for the design of polar codes based on the second order statistics of the bit error performance of the equivalent bit channels. This new code design is applied to the proposed multilevel coding scheme as well as to the standard quadrature amplitude modulation (QAM) constellations. We establish the effectiveness of the proposed algorithms through several practical examples.

We describe novel algorithms to enable the use of hexagonal constellations in practical high-rate communication systems. The first algorithm aims at optimizing the codewords assignment to the constellation points of a hexagonal grid to minimize the average Hamming distance between adjacent codewords. This codeword assignment is critical for the bit error rate performance that is usually more relevant than symbol error rate in coded schemes. We describe an efficient dynamic programming algorithm to solve this assignment problem. Then, we study the construction of the hexagonal grid that exploits the inherent redundancy in the hexagonal grid where the available constellation points for a given peak power are usually more than the grid size. Finally, we describe computationally efficient procedures for hard and soft decoding of hexagonal constellations. The proposed algorithms are shown to provide significant improvement for the bit error rate performance for both coded and uncoded schemes.

In this paper, we propose two efficient low-complexity quantization methods for multiple-input multiple-output (MIMO) systems with finite-rate feedback based on proper parameterization of the information to be fed back followed by quantization in the new parameter domain. For a MIMO channel which has multiple orthonormal vectors as channel spatial information, we exploit the geometrical structure of orthonormality while quantizing the spatial information matrix. The parameterization is of two types: one is in terms of a set of unit-norm vectors with different lengths, and the other is in terms of a minimal number of scalar parameters. These parameters are shown to be independent for the i.i.d. flat-fading Rayleigh channel, facilitating efficient quantization. In the first scheme, each of the unit-norm vectors is independently quantized with a finite number of bits using an optimal vector quantization (VQ) technique. Bit allocation is needed between the vectors, and the optimum bit allocation depends on the operating SNR of the system. In the second scheme, the scalar parameters are quantized. In slowly time-varying channels, the scalar parameters are also found to be smoothly changing over time, leading to the development of a simple quantization and feedback method using adaptive delta modulation. The results show that the proposed feedback scheme has a channel tracking feature and achieves a capacity very close to perfect feedback with a reasonable feedback rate

This paper investigates quantization methods for feeding back the channel information through a low-rate feedback channel in the context of multiple-input single-output (MISO) systems. We propose a new quantizer design criterion for capacity maximization and develop the corresponding iterative vector quantization (VQ) design algorithm. The criterion is based on maximizing the mean-squared weighted inner product (MSwIP) between the optimum and the quantized beamforming vector. The performance of systems with quantized beamforming is analyzed for the independent fading case. This requires finding the density of the squared inner product between the optimum and the quantized beamforming vector, which is obtained by considering a simple approximation of the quantization cell. The approximate density function is used to lower-bound the capacity loss due to quantization, the outage probability, and the bit error probability. The resulting expressions provide insight into the dependence of the performance of transmit beamforming MISO systems on the number of transmit antennas and feedback rate. Computer simulations support the analytical results and indicate that the lower bounds are quite tight

This paper investigates the problem of transmit beamforming in MIMO spatial multiplexing (SM) systems with a finite-rate feedback channel. Assuming a fixed number of spatial channels and equal power allocation, we propose a new design criterion for designing the codebook of beamforming matrices that is based on minimizing the capacity loss resulting from the limited rate in the feedback channel. Using the criterion, we develop an iterative design algorithm that converges to an optimum codebook. Under the i.i.d, channel and high SNR assumption, the effect on channel capacity of the finite-bit representation of beamforming matrix is analyzed. Central to this analysis is the complex multivariate beta distribution and tractable approximations to the Voronoi regions associated with the code points. Furthermore, to compensate for the degradation due to the equal power allocation assumption, we propose a multi-mode SM transmission strategy wherein the number of data streams is determined based on the average SNR. This approach is shown to allow for effective utilization of the feedback bits.

We investigate the quantization of multiple antenna channel to feed back through a low-rate feedback channel. Specifically, for multiple-input single-output (MISO) systems, we propose a new design criterion and the corresponding design algorithm for quantization of the random beamforming vector. For multiple-input multiple-output (MIMO) channels, which have multiple orthonormal vectors as channel spatial information for quantization, a matrix factorization method is proposed which provides a way to exploit the geometrical structure of orthonormality while quantizing the spatial information matrix. Results show that the quantization bit allocation over multiple spatial channels has a critical effect on the performance, and that the optimum bit allocation depends on the operating transmit power of the system.

The capacity of a multiple-input multiple-output (MIMO) channel can be improved if the transmitter has a knowledge of the channel. In this paper, we propose an efficient and practical feedback method based on parameterization and quantization of channel parameters. The spatial information of the channel at the transmitter, which is represented as a matrix with orthonormal columns, has a geometrical structure. In parameterization, the geometrical structure is exploited to extract a set of parameters that has a one-to-one mapping to the original matrix. In slowly time-varying channels, the parameters are also found to he smoothly changing in time. We employ adaptive delta modulation to quantize and feed back each parameter. The results show that the proposed feedback scheme has a channel tracking feature and achieves a capacity very close to the perfect feedback case with a reasonable feedback rate.

We investigate transmission strategies for flat-fading multiple antenna channels with t transmit and r receive antennas, and with channel state information (CSI) partially known to the transmitter. We start with an assumption that the first n eigenvectors of H/sup /spl dagger//H, where 0/spl les/n/spl les/min(t,r) and H is the channel matrix in /spl Copf//sup r/spl times/t/, are available at the transmitter as partial spatial information of the channel. A beamforming method is proposed in which a beamforming matrix is determined from the n eigenvectors in some predefined way; as a result, the receiver also knows the beamforming matrix. With this beamforming scheme, we develop a new multiple antenna system concept that provides a mechanism to reduce the amount of channel feedback information. This paper focuses on deriving the channel capacity of the multiple antenna channels employing the proposed beamforming and feedback methods. An important task for achieving capacity is the solution of interesting optimization problems for the optimal power allocation over the transmit symbols. The results show that the proposed methods lead to systems wherein the amount of feedback information can be significantly reduced with a minor sacrifice of achievable transmission rate.

We consider flat-fading multiple antenna channels with t transmit and r receive antennas, which is modeled by an r /spl times/ t complex matrix H. The first n eigenvectors of H/sup /spl dagger//H, where 0/spl les/n/spl les/min(t,r) are assumed to be available at the transmitter as partial spatial information of channel. A transmission method was proposed which enables better use of the multiple-input multiple output (MIMO) channels. By using the transmission scheme, the MIMO channel can be decomposed into two parts: n parallel channels and a new small coupled MIMO channel. One reasonable coding strategy is to employ conventional time-domain only code for each of the parallel channels and a space-time code for the small MIMO channel. This paper focuses on deriving the channel capacity of the multiple antenna channels employing such a coding strategy. The results show that the proposed methods lead to systems wherein the amount of feedback information can be significantly reduced with a minor sacrifice of achievable transmission rate.

A slotted asynchronous (SA) code-division multiple-access communication scheme controlling the time of arrival is proposed for distributed spread-spectrum packet radio networks where the transmission range is limited as in an indoor wireless system. In this scheme, each terminal can send its packet randomly at any one of N/sub w/, possible time instants, equally spaced over one period of direct-sequence spread-spectrum signals. Such transmissions initiated at different time instants can be resolved because of the high time resolution of wide-band signals if the channel delays associated with multipath are small due to limited transmission range. Quasi-synchronous distributed networks are considered to allow timing drift among terminals and also reflect wireless multiple-access channels, in which common-transmitter-based (C-T) and receiver-transmitter-based (R-T) spreading code assignments are adopted to permit a contention mode only for the header portion. The throughput is evaluated under the spread ALOHA assumption on collision events and also by reflecting the effect of the MAI in the header detection process. Theoretical results show that the combination of the SA scheme with C-T assignment results in more significant improvement than the case of R-T assignment, and also the former provides the benefit of the efficient usage of spreading codes in a code-limited environment.

We address an issue of channel sharing among users by using a random assignment/transmitter-oriented (RA/T) code scheme which permits the contention mode only in the transmission of a header while avoiding collision during the data packet transmission. Once the header is successfully received, the data packet is ready for reception by switching to one of the programmable matched-filters. But the reception may be blocked due to a limited number of matched-filters so that this effect is taken into account in our analysis. We also consider an acknowledgment scheme to notify whether the header is correctly detected and the data packet can be processed continuously, which aims at reducing the interference caused by unwanted data transmission. For a realistic analysis, we integrate the detection performance at the physical level with the channel activity at the link level through a Markov chain model. It is shown that compared to classical code-division multiple-access (CDMA) systems, a reduction in the receiver complexity of a half is allowed by choosing a proper number of RA/T codes without losing the performance quality in view of the normalized throughput.