Masashi Fushiki

Analysis on the effect of transmission power applied to small cells in LTE-advanced downlink

In order to cope with huge mobile data traffic, 3GPP (3rd Generation Partnership Project) has started to discuss the architecture and optimization of a network that introduces a large amount of small base stations (small cells) with higher frequency bands into the area covered by macro base stations (macro cells) with lower frequency bands. In such a network, spectral efficiency will be degraded because of the inter-cell interference among small cells. Therefore, it is important to clarify how much the transmission power of small cells is required to decrease inter-cell interference. This paper presents throughput performances as a function of small cell transmission power for two deployment scenarios. The first is the “co-channel scenario,” where the same frequency bands are shared by both macro and small cells. The other is the “dedicated channel scenario,” where the frequency band of macro cells is different from that of small cells. The numerical results showed that especially the cell-edge user throughput decreases significantly as the transmission power of small cells decreases. In addition, the co-channel scenario outperforms the dedicated channel scenario regardless of the transmission power of small cells, however, higher cell-edge user throughput can be achieved in the dedicated channel scenario compared to that in the co-channel scenario when the traffic offloading technique is introduced.

Throughput Gain of Fractional Frequency Reuse with Frequency Selective Scheduling in SC-FDMA Uplink Cellular System

SC-FDMA is a multiple access technique adopted in LTE uplink transmission. SC-FDMA, as well as OFDMA, can improve system throughput by assigning different frequency portions of system bandwidth to different user equipments (UEs) adaptively based on their channel quality information. In SC-FDMA uplink cellular system, interference arising from UEs in neighboring cells degrades system performance, especially the performance of the cell-edge UE. To overcome this drawback, many papers propose the Fractional Frequency Reuse (FFR) technique and analyze its influence. However, these studies have different conclusions on the effectiveness of FFR, and the throughput gain of FFR depends on the evaluating conditions. In this paper, we reveal the conditions where FFR is effective by demonstrating the throughput gain of FFR in SC-FDMA uplink cellular system. In order to analyze the throughput gain of FFR, we focus on the performance relationship between FFR and frequency selective scheduling (FSS). From the analysis, the throughput gain of FFR is small because of the degradation of the FSS gain, and FFR is effective when the following conditions are met; (i) the number of UEs is small, (ii) the multipath delay spread is large or equal to 0.

Interference coordination of the downlink control channel under macro-femto deployment in long term evolution system

This paper proposes a simple interference coordination method for the downlink control channel in a long term evolution system. The proposed method works well on a heterogeneous network coexisting with macro and closed access femto base stations (BSs). When a user without authorization to access the femto BS comes close to it, the user suffers from serious interference from the femto BS. The user mentioned above is called a “victim user” hereafter. In order to resolve this problem, the proposed method aims to reduce the collision probability in control channel reception at the victim user. The proposed method applied to the femto BS reduces radio resources for the control channel with the aid of transmission power boosting. The system level simulation demonstrates that the proposed method is always superior to conventional resource assignment without interference coordination in the outage probability of the victim user. The outage probability with the proposed method is 1.2% and substantially less than that of the conventional method, which is 8.6%. In addition, when the proposed method is compared with the alternative, which reduces the transmission power by permitting an increase in the collision probability, it is obvious that the proposed method is better for reducing the outage probability of the victim users, especially when the number of users per hotspot is 10 or less. Therefore, the proposed method is more applicable to macro-femto deployment than the alternative method.

Experimental evaluations of multiuser MIMO with two-dimensional antenna configuration.

This paper presents experimental results for multiuser-multiple input multiple output (MU-MIMO) with a two-dimensional antenna configuration. MU-MIMO can increase spectral efficiency significantly by employing many antennas at the base station (BS). However, the performance is upper-bounded when many antennas are arranged in a line because antenna spacing has to be small to maintain the required span for antenna arrangement. In order to overcome the problem, this paper evaluates a two-dimensional antenna configuration, where multiple antennas are set in both the horizontal and vertical directions using the testbed developed by the authors. The experimental results show that the spectral efficiency with a two-dimensional antenna configuration is 12.8 % higher than that with conventional linear array configuration when the span for the antenna arrangement in the horizontal direction is constrained to 7λ, where λ indicates the wavelength of carrier frequency.