M ay 2 01 6 Context Information Based Initial Cell Search for Millimeter Wave 5 G Cellular Networks

Millimeter wave (mmWave) communication is envisioned as a cornerstone to fulfill the data rate requirementsfor fifth generation (5G) cellular networks. In mmWave communication, beamforming is considered as a key technology to comba t the high path-loss, and unlike in conventional microwave co mmunication, beamforming may be necessary even during initi al access/cell search. Among the proposed beamforming scheme s for initial cell search, analog beamforming is a power effici ent approach but suffers from its inherent search delay during i nitial access. In this work, we argue that analog beamforming can st ill be a viable choice when context information about mmWave bas e stations (BS) is available at the mobile station (MS). We the n study how the performance of analog beamforming degrades in case of angular errors in the available context informati on. Finally, we present an analog beamforming receiver archite cture that uses multiple arrays of Phase Shifters and a single RF ch ain to combat the effect of angular errors, showing that it can achieve the same performance as hybrid beamforming. I. I NTRODUCTION AND RELATED WORK In the past few years, there has been an increased interest in millimeter wave (mmWave) technology to fulfill the data rate requirements foreseen for the fifth generation cellula r communication (5G) [1]. However, frequencies in mmWave bands experience high path-loss, which in comparison to microwave bands may result in a significant coverage reduction when considering omnidirectional communication. To overcome these coverage issues, beamforming at mmWave is an effective solution. Due to the small wavelengths at mmWave frequencies, a large number of antennas can be packed in a small space, and this allows to generate high gains and highl y directional beams. Beamforming solutions proposed for LTE are quite different from what is required for mmWave communication. In LTE, initial access is performed using omnidirectional communi cation, and beamforming is used only when directional informa tion is available after initial physical layer access. Howe ver, in mmWave communication beamforming may be required even during the initial access/cell search process to overc me the coverage problems of omnidirectional communication wi th mmWaves. Recently, two different approaches have been considered fo directional initial access/cell discovery. Firstly, in [2 ], considering a HetNet scenario, context information (CI) regardin g mobile station (MS) positioning is provided to the mmWave base station (BS) by the microwave BS. Based on this, the mmWave BS points its beam (using analog beamforming) The work of Michele Zorzi was partially supported by NYU Wire less. in the desired direction. The authors also address the issue of erroneous CI, proposing that the BS, in addition to searching in the CI based direction, also searches the rest o f the angular space by forming beams in different directions and also with different beamwidth (to increase the coverage ). Results showed that the enhanced cell discovery, where in case of positioning error the BS searches the adjacent angul r directions, outperforms the greedy search approach where t BS searches the angular space sequentially. In addition, at the MS, omnidirectional reception is considered, which in comparison to a directional reception results in a reduced gain. Recently, the authors of [2] extended their work by considering a more complex channel model with multiple rays and obstacles [3]. Secondly, a non context information based approach is used in [4]–[6]. In [4], omnidirectional and random directional transmissions are considered from the mmWave BS, and the MS can perform either analog, digital or hybrid beamforming . It is observed that using an omnidirectional transmission a t the BS and digital beamforming at the MS outperforms the other schemes. However, the use of omnidirectional transmission at the BS during initial synchronization/access will reduc e coverage, while random directional transmission will resu lt in large delays. In [5], exhaustive and hierarchical searches are compared while considering analog, digital and hybrid beamforming a t the BS and the MS. In exhaustive search, the whole angular space is covered by sequentially transmitting beams in a time division multiplexing fashion, and initial beamformi ng is done by selecting the best combination of Tx-Rx beams. The hierarchical search, instead, is a multiple step proces s. In the first step, a MS initially utilizes fewer antennas to form a relatively small number of wide beams. The received signal is combined with all the beams and the best combiner beam is selected as a reference for the next step, where several narrower beamwidth combiners are formed, within the initia lly selected beam. Considering scenarios with limited mobilit y, the process finishes when the combiner beam is within the range of5 to 10. However, selecting an incorrect combiner in the initial stage can result in an initial access error in t he following stage. Recently, in [6], iterative and exhaustive search schemes using analog beamforming have been studied and compared, and the authors showed that the optimal scheme depends on the target SNR regime.