FDD Massive MIMO Uplink and Downlink Channel Reciprocity Properties: Full or Partial Reciprocity?

One challenge for FDD massive MIMO communication system is how to obtain the downlink channel state information (CSI) at the base station. Except for traditional codebook feedback through uplink pilot transmission, some channel reciprocity properties can be utilized through uplink channel estimation and channel parameter estimation algorithms. In this paper, the uplink and downlink channel reciprocity properties are analyzed. It is theoretically proved that not all multipath parameters for FDD downlink and uplink channels are equivalent. Therefore, the so called full reciprocity property does not hold while the partial reciprocity property holds. Moreover, the channel measurement campaign is conducted to verify our theoretical analysis. Finally, in order to support the partial reciprocity property, the revision for the standardization 5G channel model is proposed as well. With the contribution of this paper, the FDD massive MIMO system transmission scheme design could be led to the right direction.

I. Introduction

Massive multiple-input multiple-output (MIMO) will be an essential part of 5G systems. Knowledge of channel state information (CSI) at the transmitter (CSIT) is a fundamental prerequisite for operation of massive MIMO systems. However, a massive MIMO base station (BS) has much larger number of antennas than users'. In time division duplex (TDD) systems, the BS can obtain the downlink CSIT through uplink pilot transmission from the user equipment (UE), since the channel reciprocity holds as long as uplink and downlink transmissions occur within the channel coherence time. However, in frequency division duplex (FDD) system, the uplink (UL) and downlink (DL) channels have no such reciprocity as TDD system, since the uplink and downlink in FDD system are usually separated by more than a coherence frequency bandwidth. The current 5G NR system uses downlink CSI-RS transmission, and Type I or Type II codebook feedback from the UE, to get downlink CSIT [1]. It leads to considerable feedback overhead, and performance loss due to quantized error and channel aging problem [2]. Recently, one alternative method to get CSIT is to use some FDD UL & DL channel intrinsic reciprocity properties. There are two kinds of assumption on such FDD channel reciprocity. One assumes that the channel consists of numerous multipath, and all the parameters of each multipath, (including phase, amplitude, delay, angle of arrival, departure, etc.) are equivalent to UL and DL channels. Such assumption is referred to as full reciprocity property. Based on the full reciprocity property, one method to obtain the DL CSIT is the extrapolation of the complex, instantaneous channel frequency response by UL CSI without any feedback in [3]–[7]. However, In[8], [9], some measurement results and theoretical investigations show that the full reciprocity does not hold, and it points out that the phase relationship between different multipath components is not reciprocal for FDD DL and UL channels. Therefore, another assumption assumes that only part of channel characteristics have such reciprocity property, such as, the angular power spectrum, channel covariance matrix, delay and angle of each multipath, etc, which is called partial reciprocity property. Some literatures make use of such partial reciprocity property with limited feedback to improve DL massive MIMO transmission performance [10]–[16].