# Interference Avoidance Using TDMA-Beamforming in Location Aware Small Cell Systems
## 1 Introduction  
The future communication systems that operate in higher frequency bands promise to exploit wider bandwidths to cater higher data rates for the end users [1]. For instance, the highest bandwidth range that is obtainable in millimetre wave (mmWave) systems is approximately 10 Gigahertz (GHz) over a frequency range of 30-300 GHz which achieves about 100 bits/sec/Hz spectral efficiency, and obtains terabyte data rate at the physical layer [2; 3]. This is triggered by the limitation of the current wireless communication systems in terms of spectrum limitation as well as fairness issues. In wireless systems, different channel conditions and/or improper resource allocation and scheduling are experienced by signals. Therefore, fairness is one of the issues worth to look at in such systems. In [4], the downlink throughput of OFDMA small cells is maximised subject to short-term and long-term fairness indices. This is achieved using a fixed transmission power. The proposed scheme utilises cumulative fairness constraint to replace the long-term fairness and further improve the system's fairness. Similarly, the authors of [5] used non-orthogonal multiple access (NOMA) to maximise the fairness of drone small cells. More works on fairness can be found in [6; 7; 8]. Similarly, extensive researches focused on investigating bandwidths of up to 10 Terahertz (THz) because it offers extremely broader spectrum leading to huge capacity enhancements. Nevertheless, THz signals are very fragile in longer distances due to the high frequency. In other words, THz communication performs best in indoor capacities due to the proximity between transmitters and receivers [9; 10]. On the other hand, multiple input multiple output (MIMO) technique is exploited toachieve enhanced beamsteering gains in which multiple antennas are tightly bundled together over a small spatial area. Subsequently, the technical and hardware complexities are few of many constraints that researchers had to realise in approaching the technique. The authors of [11] investigated the performance and the complications of signals phase-uncertainty when deploying low-complexity indoor wireless units that utilise hybrid beamforming techniques. The study concluded that the number and the size of antennas are proportional to the performance degradation. The authors of [12] proposed a hybrid beamforming that is able to map a beamsteering codebook based on the channel state information (CSI). It consists of a digital beamformer that eliminates interference by dealing with regulated channel inversion (RCI) to reduce the overall complexity. It is meaningful to mention that the proposed algorithm is designed to eliminate inter-band-interference that is caused by orthogonal frequency division multiplexing (OFDM) carrier offsets (CFO) in [13]. The authors of [14] presented an experimental analysis of the in-band full-duplex communication (IBFD) end-fire arrangement array transceiver and then developed a MIMO testbed that communicates at 12.9 GHz carrier frequency. However, the scheme suppressed the self generated interference by using the OFDM techniques and did not show evaluations in terms of data rates and bit error rates. In view of that, the consideration of transmission and reception, proper channel modelling, and multipath and fading effects lead to the smart antennas techniques. In [15], smart antennas were categorised into adaptive arrays and switched beam antennas. While the latter describes the antennas that have multiple beams and able to switch to any beam at any given time, the former estimates the arrival direction and in-lines the beams with that direction, thereby resulting less interference. Subsequently, the switched beam antennas do design the multi-user constellation depending on how interference is perceived. Interference is mostly eliminated or suppressed through minimum mean square error (MMSE) [16], zero-forcing (ZF) [17; 18], or time division multiple access (TDMA) techniques [19].