Abstract
An out-of-cell user (referred to UE2) is usually required to receive or send signals from or to a base station (BS) that is connected to wider area networks. In order to provide a wireless connection between BS and UE2 and hence extend their radio coverage, putting a relay between BS and UE2 is widely accepted. The relayed UE2 signal occupies a part of bandwidth that is originally allocated to an in-cell user (referred to UE1) and then UE2 shares UE1's capacity. Especially for single-antenna users, the asymptotic capacity that can be achieved when signal-to-noise power ratio (SNR) ρ goes to infinity is given by log <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (ρ) for UE1. By using the in-cell relay, the capacity is divided into β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> log <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (ρ) and β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> log <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (ρ) for UE1 and UE2, respectively, where β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> + β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ≤ 1, 0 ≤ β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> , β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ≤ 1. A certain portion of the capacity is sometimes lost and then β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> + β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> <; 1. In order to keep the sum-capacity growth rate (i.e., keeping β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> + β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> = 1), a coordinated direct and relay transmission (CDRT) with non-orthogonal multiple access (NOMA) principles is devised in the downlink, but β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> is just fixed at either 0 or 1/2. In the uplink, however, the full rate 1 is not yet restored. Regardless of whether in the downlink or in the uplink, so far there exists no way to manipulate β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> while keeping β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> + β <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> = 1. Motivated by designing the UE2 capacity flexibly, this article proposes a power allocation method used for CDRT-NOMA, which allocates a target positive capacity growth rate to UE2 and the asymptotic capacity accordingly. With the proposed power allocation, we derive exact or approximate expressions for the ergodic capacities, and provide the capacity scaling law with asymptotic analysis. The analysis proves that the capacity growth rate as well as the asymptotic capacity can be manipulated by a power allocation parameter. Moreover, in the uplink, the full rate 1 is restored with a full-duplexing relay by the proposed power allocation. Numerical investigation is also provided to verify the approximation and analysis used in the paper and shows that the proposed power allocation can control UE2 capacity as desired in the high SNR regime.
Highlights
Non-orthogonal multiple access (NOMA) is regarded as one of the key technologies for higher spectral efficiency for future radio access networks [1], [2]
We provide a power allocation method that flexibly allocates the asymptotic capacity between UE1 and UE2 without degrading the sum capacity growth rate in NOMA coordinated direct and relay transmission (CDRT)
We have shown that the proposed power allocation can increase the sum capacity growth rate in half-duplex relaying (HDR) NOMA CDRT uplink, compared to the rate given in the literature
Summary
Non-orthogonal multiple access (NOMA) is regarded as one of the key technologies for higher spectral efficiency for future radio access networks [1], [2]. With the proposed power allocation, the capacity growth rates of UE1 and UE2 in HDR are shown to be θ1 = 1 − β/2 and θ2 = β/2 in downlink, respectively, and θ1 = 1 − β and θ2 = β/2 in uplink, respectively. A fairness issue of the capacity allocation between UE1 and UE2 is treated in [13] for both HDR and FDR NOMA downlinks, in which BS switches between direct and relayed transmission to cell-edge UE2. We provide a power allocation method that flexibly allocates the asymptotic capacity between UE1 and UE2 without degrading the sum capacity growth rate in NOMA CDRT. We have shown that the proposed power allocation can increase the sum capacity growth rate in HDR NOMA CDRT uplink, compared to the rate given in the literature.
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