June Chul Roh

Efficient feedback methods for MIMO channels based on parameterization

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

NLOS Backhaul PHY Optimized for Small Cells

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.