# Reliable Positioning and mmWave Communication via Multi-Point Connectivity +
## 1 Introduction  
To tackle the exponential increase in data traffic, use of millimeter-wave (mmWave) spectrum (30-300 GHz) is gaining more popularity. In fact, mmWave communication is expected to be one of the key components for future 5G system [1]. Ultra-wide band transmission and massive multiple-input-multiple-output (MIMO) techniques at the mmWave frequencies not only contribute in achieving giga-bit wireless access, but also provide accurate positioning capabilities because of higher temporal and angular resolution [2]. Thus, unlike the legacy communication systems, positioning will be an integral feature of mmWave based radio-access-technology (RAT), for example, mentioned in a recent 3 GPP study [3]. Widely adopted positioning methods rely mostly on conventional techniques, such as on time(-difference)-of-arrival (ToA/TDoA), angle-of-arrival (AoA) and received-signal-strength (RSS). The key idea is based on estimating the positioning parameters, such as distance, angle or both from two or more base stations (BSs) [2, 4]. Related work showed that massive MIMO with full-digital beamforming architecture can provide sub-meter level localization accuracy for both sub-6 GHz frequencies [5] and mmWave frequencies [6]. However, full digital solutions are costly and power inefficient when the number of antennas become large due to high power consumption by ADC/DAC and other RF components. Thus, alternative approaches based on all-analog or hybrid structures areneeded in practice. Commercially available mmWave hardware features phased-array of antennas with limited number of baseband processing chains [7]. Existing work with such hardware constraints is limited. For example, author of [8] considered Wi-Fi access-points (APs) and localized users with three or more APs, using the conventional triangulation techniques. In contrast to most prominent localization methods based on signals from two or more BSs, mmWave RAT can accurately estimate user's position with the single BS by processing orthogonal pilots in different analog beams [9; 10]. The author of [9; 10] provided closed-form expressions for the Cramer-Rao Lower Bound of the achievable localization accuracy, considering point-to-point communication. Author of [11] studied trade-off between positioning quality and achievable throughput with the increasing training overhead for single-user case and extended to multi-user scenario in [12], while the authors of [13; 14] investigated the rate-0 positioning trade-off for different beam training strategies. Similar trade-offs are also drawn in [15] for different bandwidths and power splitting factors between positioning and communication. The mmWave channel is characterized by \"quasi\"-optical propagation properties [1], thus the main contribution to communications is from the line-of-sight (LoS) path. However, dominant paths are also susceptible to blockage due to relatively low diffraction and higher penetration loss [16], which results in loss of connection and gives rise to unreliable communication. To solve these problems, one possible solution consists of coordinated multi-point reception, whereby enabling each user to have concurrent connections with multiple BSs for better communication robustness [17]. Coordinated transmission/reception has been well studied in the literature over the past decade, mostly under the context of Long Term Evolution (LTE) and LTE-Advanced mobile communication systems [18; 19; 20]. Widely adapted techniques, such as coordinated beamforming and multi-point selection are included in 3 GPP standards [21] and are currently deployed in 4G systems to enhance network capacity and cell-edge performance. However, localization in the conventional systems were solely based on the dedicated technologies such as Global Positioning System (GPS) and Global Navigation Satellite Systems (GNSS).