A linear mesh network is considered in which a single user per cell communicates to a local base station via a dedicated relay (two-hop communication). Exploiting the possibly relevant inter-cell channel gains, rate splitting with successive cancellation in both hops is investigated as a promising solution to improve the rate of basic single-rate communications. Then, an alternative solution is proposed that attempts to improve the performance of the second hop (from the relays to base stations) by cooperative transmission among the relay stations. The cooperative scheme leverages the common information obtained by the relays as a by-product of the use of rate splitting in the first hop. Numerical results bring insight into the conditions (network topology and power constraints) under which rate splitting, with possible relay cooperation, is beneficial. Multi-cell processing (joint decoding at the base stations) is also considered for reference.
Wireless mesh networks are currently being investigated for their potential to resolve the performance limitations of both infrastructure (cellular) and multi-hop (ad hoc) networks in terms of quality-of-service and coverage [1]. Basically, mesh networks prescribe the combination of communication via direct transmission to infrastructure nodes (base stations) and via multi-hop transmission through intermediate nodes (relay stations). The latter can generally be mobile terminals, or fixed stations appropriately located by the service provider. The assessment of the performance of such networks is an open problem that has attracted interest from different communities and fields, especially information-theory [2] [3] and networking [4]. Recently, there has also been considerable interest in further enhancing the performance of infrastructure or mesh networks by endowing the system with a central processor able to pool the signals received by the base stations and perform joint processing (this scenario is usually referred to as distributed antennas or multi-cell processing) [5].
In this paper, we focus on a linear mesh network as sketched in Fig. 1. It is assumed that one mobile terminal (MT) is active in each cell in a given time-frequency resource (as for intracell TDMA or FDMA) and that each active MT communicates with the same-cell base station (BS) via a dedicated relay station (RS) (two-hop transmission). In order to allow meaningful analysis and insight, this scenario is modelled as illustrated in Fig. 2, where symmetry is assumed in the channel gains, i.e., every cell is characterized by identical intra-and inter-cell propagation conditions. This framework follows the approach of [6] (see also [5]), which extends the model of [7] to mesh networks.
The basic premise of this work is that the model in Fig. 2 can be seen as the cascade of two interference channels, one for each hop, with many sources and corresponding receivers (border effects are neglected). Therefore, from the literature on interference channels, a promising approach is that of employing rate splitting with successive interference cancellation at the receivers [10] [11]. It is recalled that the rationale of rate splitting is that joint decoding of (at least part of) the transmitted signals at the receivers has the potential to improve the achievable rates with respect to single-user decoders that treat signals other than the intended as noise. The main contributions of this work concerning the analysis of a mesh network modelled as in Fig. 2 are: • derivation of the performance of rate splitting applied to both hops with decode-and-forward relaying (Sec. III); • proposal of a cooperative transmission scheme for the RSs that leverages the common information obtained by the relays as a by-product of the use of rate splitting in the first hop (Sec. IV); • analysis of the cooperative transmission scheme above in the presence of multi-cell processing (Sec. IV); and • performance evaluation of rate splitting, with possible relay cooperation in the second hop, via numerical simulations; comparison with the reference cases of single-rate transmission and multi-cell processing is provided as well (Sec. V). Related work was recently reported in [6] [8] [9], where a cellular model similar to the one in Fig. 2 was addressed under the assumption of amplify-and-forward [6] [8] or decode-andforward (DF) relaying [9] with single-rate transmission.
We study the abstraction of the two-hop mesh network of Fig. 1 as sketched in Fig. 2. Cells are arranged in a linear fashion, one user transmitting on a given time-frequency resource in each cell. Moreover, we focus on non-faded Gaussian channels and assume homogeneous conditions for base station relay station terminal (m-1)-th cell m-th cell (m+1)-th cell … … Fig. 1. A linear two-hop mesh network.
the channel power gains so that the intra-cell MS-to-RS (first hop) and RS-to-BS (second hop) power gains are β 2 and γ 2 , respectively, for all cells, and, similarly, the inter-cell power gains between adjacent cells are α 2 ≤ β 2 and η 2 ≤ γ 2 for first and second hop, respectively. Notice that as in [7] each cell receives signals only from adjacent cells. Moreover, here there exist no direct paths between MTs and BSs and no relevant inter-channels between RSs in adjacent cells. Because of the latter assumptions, we can deal with either full duplex or half duplex transmission at the relays with minor modifications, as explained below. Considering, for simplicity of exposition, full-duplex transmission (by means of perfect echo-cancellation), the signal received at each time by the mth RS (first hop) can be written as
where β and α are the (real) channel gains, and we assume the symbols transmitted by the MTs, X m , to be drawn from a circularly symmetric complex Gaussian distribution with power
Similarly, the signal received by the mth BS is
where the symbols transmitted by the RSs satisfy
By symmetry, w
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