The monthly workshop of WelComLab was held at No. 3 Conference Room, New Main Building, Beihang University, on July 3th, 2014.
The topic of this workshop was “5G”. Prof. Yang Chenyang first introduced some techniques of 5G wireless communication. Then the PhD student Liu Wenjia gave a report on the energy efficiency comparison of massive MIMO and small cell network. The master student Chen Zhilin presented his recent results about pilot decontamination for massive MIMO systems. Finally, the PhD student Liu Dong introduced his recent research on the caching at base station. The topics and abstracts were as follows.
Liu Wenjia: “Energy Efficiency Comparison of Massive MIMO and Small Cell Network”
Abstract: Massive MIMO and small cell network (SCN) are two technologies increasing the antenna density. In this report, we compare the average energy efficiency (EE) of massive MIMO system and SCN under the conditions of identical user density, antenna density and data rate requirements, where the uplink and downlink training overhead as well as multi-cell power control are taken into account. Then we analyze the average EE of the heterogeneous network (HetNet) comprising massive MIMO and small cells, which work in different frequencies. Our analysis shows that the average transmit power of SCN decreases faster than massive MIMO as the antenna density increases. Moreover, massive MIMO achieves a higher average EE than SCN if its power consumption except the power for transmitting data is very low or the data rate requirement is very high. Considering that in SCN with large number of base stations (BSs) the opportunity for turning some BSs into idle mode is higher, massive MIMO exhibits higher EE only if its circuit power of each active antenna is lower than that for each antenna at an idle small BSs. If antenna idling is allowed for massive MIMO, massive MIMO will be more energy efficient than SCN when the former has smaller circuit power for each idling antenna.
Chen Zhilin: “Pilot Decontamination for Massive MIMO Systems”
Abstract: This work deals with the pilot contamination problem for massive MIMO systems. Considering the sparse nature
of channel impulse response inherent in wideband systems, most paths of desired channel distinguishable from the paths of interference channels in time-domain. Based on this observation, we first estimate the power delay profile, from which we acquire the delay of each path. By extracting the corresponding channel components from the contaminated channel estimate, a clean channel estimate can be obtained. We propose a pilot assignment method among adjacent cells to randomize the interference. Simulation results demonstrate substantial sum rate gain of the proposed approach over existing methods.
Liu Dong: “Will Caching at Base Station Improve Energy Efficiency of Downlink Transmission?”
Abstract: Caching popular contents at the base station (BS) can reduce the end-to-end delay of service and the cost of cellular networks. Yet it is unknown whether introducing cache to BS can improve the energy efficiency (EE) of downlink systems. With BS caching, energy consumption for backhauling can be reduced, but caches will also consume energy. In this paper, we analyze the EE of two kinds of networks with a macro BS or multiple non-coordinated pico BSs with caches, where the impact of content popularity and request arrival density are considered. Numerical and simulation results show that introducing cache can improve the EE if the file catalog size is not too large and placing the caches at the pico BSs is more energy efficient.
