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When a vehicle moves from one base station (BS) to another, a large number of on-board user equipments (UE) may simultaneously and individually perform a handover (HO) procedure, resulting in increased HO overheads. A mobile relay node (MRN), connected via a wireless backhaul to a donor base station (DBS), is deployed on the rooftop of a bus to improve the link quality and reduce the associated HO overhead via group mobility. However, at moderate to high speeds, the on-board UEs can still suffer from frequent HOs due to the MRN failing to HO to a new DBS using the legacy downlink measurement-based HO (DL-HO) method. As a consequence, the connection towards all associated mobile users will be lost which poses tight reliability requirements on the backhaul link to avoid becoming a single point of failure (SPoF). In order to improve the reliability during group handover, in this work, we propose an uplink reference signal (UL RS) based HO procedure (coined as UL-HO) for the MRN which relies on the existing sounding reference signal in long term evolution (LTE) /new radio (NR). In the proposed scheme, and unlike the legacy DL-HO procedure in LTE/NR, the measurement report (MeasReport) transmission is not required between MRN and the DBS, therefore the HO delay can be reduced, decreasing the SPoF chances and thus, uninterrupted services can be provided to on-board UEs. We investigate the gain in terms of HO rate, HO failure rate, ping-pong rate and power consumption (both at the UE and the BS). Performance evaluations demonstrate that the proposed UL-HO scheme outperforms the legacy DL-HO scheme in current cellular networks.

Coping with the limited amount of available spectrum, time division duplexing (TDD) system is considered as an attractive duplexing method due to exploiting channel reciprocity as well as flexible resource management. The conventional scheduling scheme is based on the channel quality indicator (CQI) reported from the user equipment (UE) to estimate instantaneous data rates for the scheduling metric calculation. However, CQI is insufficient to reflect the state of the channel variation in terms of frequency and time. Based on the channel reciprocity of TDD systems, we utilize uplink sounding reference signal (SRS) to estimate downlink channel status. However the received SRS power is a result of uplink power control where power control effect should be compensated to estimate channel status in downlink scheduling. In order to solve this problem, we propose the SRS path loss estimation method based on the power headroom report. By using this scheme, the base station (BS) can obtain the compensated signal-to-interference-plus-noise-ratio (SINR) and determine the scheduling metric based on its calculated SINR instead of reported CQI from UE. Simulation results show that the proposed scheduling algorithm outperforms the conventional scheme in total throughput, whereas the fairness index experienced by the proposed algorithm is lesser than of the conventional scheme based on CQI.

The SRS are physical signals transmitted in uplink to enable the eNB to estimate the CSI over a range of frequencies in LTE system. The estimation of the CSI assists the eNB scheduler to properly allocated radio resource to the UE. The SRS transmission can be also be used to support downlink beamforming. The sub-frame in which SRS is transmitted by any UE within the cell is signaled via cell specific broadcast signaling, there are 15 possible sets of subframe in which SRS may be transmitted within each radio frame. The SRS is based on the extended zadoff-chu sequence and transmitted in the last SC-FDMA symbol of an uplink subframe, the SRS transmitted by the UEs are multiplexed in the time and freq. domain through configuring SRS periodicity SRS, frequency comb pattern, and SRS bandwidth. Different sets of UE-specific sounding signals are independently allocated for SRS transmission, including transmissi on bandwidth, frequency comb pattern, cyclic shift. In this paper, transmission subframe, bandwidth, freq. comb pattern, for it suggests ways to reduce the measurement performance and HW complexity of the UE between the Sounding RS separated by a Cyclic shift value and Timing offset measurement method.

In 3GPP long term evolution (LTE) specification for uplink, single antenna for transmission is adopted due to its simplicity. However, when it evolves to LTE-A, more transmit antennas are needed for higher spectrum efficiency, which means more reference signals. In the physical uplink share channel (PUSCH) of LTE-A, the estimated channel based on sounding reference signal (SRS) is used for precoder and modulation and coding scheme (MCS) selection. In this paper, an improved SRS design is proposed, which achieves minimum interference between different reference signals. More importantly, it is perfectly backwards compatible with the SRS in LTE uplink. In addition, we propose an improved SRS channel estimation for multiple transmit antennas. Simulation results show that the proposed SRS design leads to less channel estimation error, and the proposed SRS channel estimation can reach similar accuracy to exponential power delay profile (PDP) channel estimation, while with much lower complexity.

Coordinated Multi-Point (CoMP) transmission is a key technology for LTE-Advanced (LTE-A) to increase the downlink cell-edge throughput. Generally speaking, multi-cell channel state information (CSI) is needed at multiple coordinated eNBs to achieve the performance gain promised by CoMP in downlink. Sounding reference signal (SRS) could be an efficient tool to enable multi-cell CSI acquisition for time division duplex (TDD) LTE-A (TD-LTE-A) systems because of the inherent channel reciprocity in TDD systems. In this study, we investigate multi-cell SRS coordination schemes for TD-LTE-A downlink CoMP. We introduce a CoMP-specific sounding zone that facilitates multi-cell SRS coordination and then propose and compare three coordinated SRS resource allocation schemes within the CoMP-specific sounding zone, i.e., cyclic shift (CS) coordination, comb coordination, and orthogonal cover code (OCC) for the SRS. Computer simulations show the effectiveness of multi-cell SRS coordination and its necessity is revealed by the sum-rate analysis of TD-LTE-A downlink CoMP.