Spectrally Efficient Frequency Reuse With Higher Order Sectorisation and Directional Relays in a Two-Hop Relay Network

Two-hop relay networks for critical industries require high reliability within a limited spectrum. The lower hop between densely populated user equipment (UE) and the relay nodes (RNs) is assumed to have low spectral efficiency due to the inexpensive low-powered UEs equipped with omnidirectional antennas in non-line-of-sight (NLOS) conditions. Thus, spectrally efficient frequency reuse (FR) in the upper hop between RNs and base stations (BSs) is pivotal in enhancing the two-hop relay network's uplink coverage and capacity. To achieve this objective, we propose the deployment of higher order sectorisation (HOS) at BS and directional antennas (DAs) at RNs in the upper hop and propose rotational (RFR), double (DFR), and triple (TFR) frequency reuse schemes. These schemes allow FR by exploiting the orientation of the DAs at RNs and are compared with conventional distance-based FR. Furthermore, an optimisation method is formulated to partition the limited bandwidth between the two hops to increase the average end-to-end uplink capacity while using the proposed FR schemes on the upper hop. When evaluating system performance with HOS at BSs (upper hop) and RNs (lower hop) in irregular cellular layout, the statistical evaluation of uplink interference becomes computationally expensive. To tackle this issue, we developed a semi-analytical model to efficiently compute the statistical distribution of uplink interference with reduced time and complexity. It is shown that this model is an accurate tool for performance assessments, which also validate the end-to-end two-hop capacity improvements up to 73% and 110% using the proposed DFR and TFR schemes, respectively.