Byung moo Lee

An energy efficient antenna selection for large scale green MIMO systems

This paper proposes an energy efficient antenna selection for large scale (LS) MIMO in green base station (BS). For the maximum energy efficiency (EE) operation of LS-MIMO, antenna selection is necessary, because the RF chains which are connected with antennas are expensive, and consumes a lot of power. However, the excessive amount of antennas in LS-MIMO hinder the efficient antenna selection. We determine optimum antenna number in various circumstances, and show that with LS-MIMO, simple random antenna selection can provide significant EE gain. Moreover, we also show if the EE optimum number of antenna is larger than a certain threshold, the random antenna selection is already very close to the optimum antenna selection. The proposed technique is verified by both mathematical analysis and numerical simulations.

Minimizing transmit power for cooperative multicell system with massive MIMO

We consider the problem of designing transmit beamformer and power for downlink cooperative base-station (BS) system with a large antenna arrays. Since the design of the beamforming vector at the transmitter requires high computational complexity, in a large antenna arrays, we utilize the zero-forcing transmit beamformer, which is the simplest form and the optimal performance in a large antenna arrays. Therefore, this paper focuses on the design of power allocation with fixed transmit beamformer for minimizing the transmit power while meeting target signal-to-interference-and-noise-ratio (SINR) of each user and power constraints. We consider two scenarios according to the power constraints of cooperative BSs. One scenario is the sum power constraint on the cooperative base-stations. In this case, the cooperative BSs share the total available transmit power. However, each BS exists a maximum available transmit power in practical implementations. Thus, we consider a more realistic per BS power constraints.We proposed the solution strategies for both scenarios: For the sum power constraint case, a simple intuitive solution, where the power is allocated without regard to the power constraint until the SINR constraints is satisfied, is presented. For the per BS power constraints case, we use the properties of a large antenna arrays to find the solution of closed form. We also demonstrate, via numerical simulation, the performance of proposed strategy is convergent to the optimal performance which is achieved by using the iterative algorithm.

Energy efficient basestation operation with traffic-specific energy consumption

Basestation on-off algorithms for mobile-network energy saving beyond 4 G can be simplified due to absence of interference in multi-user MIMO systems. We discuss an on-off algorithm based on specific energy and its energy savings performance.

An energy efficient PAPR reduction technique for mobile terminals

An energy efficient PAPR reduction technique using adaptive clipping and filtering (CAF) for OFDM uplink transmission is proposed. Even though SC-FDMA is one of good choices for uplink transmission, there are several disadvantages which makes it less attractive to use in practical systems, such as high pilot overhead, link-level performance degradation, receiver complexity increment especially in MIMO mode, etc. There are various alternative PAPR reduction techniques to improve energy efficiency, and the CAF is the simplest and effective one, but oversampling and iteration requirements can be burdens. The proposed technique adaptively adjusts CR (Clipping Ratio) as a function of modulation, and has less burden of oversampling and iteration comparing with conventional Base Station (BS) CAF technique. Also, the effect of NI (Noise Interference) among users could be negligible owing to context-awareness and adaptive characteristics. Moreover it does not require receiver modification, thus improving energy efficiency of both BS and mobile terminals is possible. Simulation results show that the proposed technique presents excellent performance comparing with SC-FDMA.

Compact ridged substrate integrated waveguide cavity backed slot antenna

A compact ridged substrate integrated waveguide (SIW) cavity backed slot antenna (CBSA) is proposed and implemented for a massive MIMO base station antenna system with very high spectral efficiency. The ridge of the SIW cavity is adopted to reduce the volume of the CBSA and the transverse slot length on the cavity. The size of the cavity is about 35 mm × 25 mm × 2.12 mm (corresponding to 0.31 λ₀ × 0.22 λ₀ × 0.019 λ₀). The measured results are in very good agreement with the simulated data. The proposed compact antenna is suitable for the massive MIMO array system by providing high efficiency and low mutual coupling array performance.