# Multi-Backup Beams for Instantaneous Link Recovery in mmWave Communications
## 1 Introduction  
Millimeter wave (mmWave) frequencies represents a major component of standalone (SA) 5G networks for high data rates support in enhanced mobile broadband (eMBB). One key advantage here is the contiguous available spectrum at these bands. However, the aggregated path losses impose the use of beamforming techniques to achieve higher link gains. The use of directional transmission and reception here (absence of omni-directional modes), requires the base station (BS) and mobile station (MS) to scan over all spatial directions to determine the best beamforming and combining vectors that yield the highest received signal level [1]. Consequently, this creates high computational complexity and prolonged access times, i.e., long control-plane latencies. This, in turn, contradicts with the International Mobile Telecommunications (IMT) framework requirements that define 10 milliseconds (ms) latency levels for eMBB in 5G systems [2]. Therefore, initial beam access schemes need to attain reduced times to achieve short control plane latencies. Consequently, beam access and adaptation arise as challenging problems in mmWave systems. Once initial access procedures are performed, the BS and MS need to maintain robust link adaptation when signal levels drop due to mobility and blockage effects. In particular, mmWave links are highly vulnerable to obstacles in the propagation paths between the MS and BS, which degrades signal levels and triggers link blockage [3]. This deficiency is more likely to occur when transmitting at narrow beams, i.e., short coherence times and low-channel ranks. Therefore, efficient beam recovery schemes are required to overcome link blockages, maintain communication sessions without drops, and reduce requirements for repeated beam access procedures.  
In light of the above, this paper presents a novel beam recovery scheme to overcome link blockage effects and provides instantaneous beam recovery times, without the requirements for beam scanning. The scheme develops a bundle of simultaneously radiated beams that compensate for the blocked beam using an analog beamforming architecture. Namely, when blockage effects are introduced, the link is transiting from a line-of-sight (LoS) to a non-line-of-sight (NLoS) operation, which decaysthe signal quality. Conventional schemes require the beam to find alternative spatial directions by performing beam scanning. Meanwhile, the proposed scheme radiates the beam-bundle after blockage occurs, i.e., acting as backup beams. This, in turn, eliminates the need for beam scanning since the bundle radiates in different directions, hence resulting in signal aggregation at the receiver.  
This paper is organized as follows: Firstly, Section 2 presents a survey on recent studies on beam recovery methods. Then, a proposed scheme is presented by first proposing the novel beamforming model in Section 3, along with the channel and signal models. This is followed by the bundle-beam recovery scheme in Section 4. Then, the performance evaluation is presented in Section 5, and finally, conclusions and future directions are discussed in Section 6.