Abstract
We consider multiple-input multiple output (MIMO) non-orthogonal multiple access (NOMA) downlink channels with zeroforcing beamforming transmission. The base station has multiple transmit antennas while all mobile devices have single receive antenna. With a limited feedback rate, channel direction information (CDI) needs to be quantized and fed back from mobile users to a base station. Thus, the accuracy of the quantized channel state information at the transmitter (CSIT) will depend on the feedback rate. To increase spectral efficiency, 2 active users in some clusters share the same beamforming vector and thus, will interfere fully with each other. Given a total-feedback rate, we analyze the feedback allocation for clusters with single active user and 2 active users. The objective is to either minimize the maximum outage probability or maximize the minimum rate among all active users in a cell. We show that the proposed feedback allocation is close to the optimum. Some numerical examples show that the resulting rate performance with the proposed feedback allocation is increased by 100% over that with the uniform feedback allocation.
Highlights
N ON-ORTHOGONAL multiple access (NOMA) has greater potential in supporting the increased demand for wireless transmission than does orthogonal multiple access (OMA) since NOMA can support more users, achieve higher spectral efficiency, and offer a better user fairness [2]–[9]
In our previous work [24], we modeled the time evolution of multiple-input multiple-output (MIMO) channels by a Gauss-Markov process parameterized by a temporal-correlation coefficient, and proposed the feedback scheme that maximizes the spectral efficiency for a given feedback rate
We show the outage probability obtained from numerical simulation and see that the simulation results are close to the analytical ones
Summary
N ON-ORTHOGONAL multiple access (NOMA) has greater potential in supporting the increased demand for wireless transmission than does orthogonal multiple access (OMA) since NOMA can support more users, achieve higher spectral efficiency, and offer a better user fairness [2]–[9]. We assume that clusters with 1 or 2 active users are allocated different feedback rates for quantizing theirs CDI. We find the optimal feedback allocation for both types of clusters that minimizes the maximum outage probability over all active users, subject to a total-feedback rate constraint. We derive the approximate outage probability for all users in a cell when CDI is quantized by the RVQ codebook with a limited number of feedback bits. The results allow us to approximate the optimal feedback rates allocated for both 1-active-user and 2-active-user clusters. The derived feedback allocation is shown to be close to the optimum in a high-SNR regime for a full-load cell (where the number of clusters for transmission equals the number of transmit antennas of the base station) and in a high-SNR large-feedback regime for a partial-load cell.
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