SRS Interference Management in TDD CJT for 5G

One of the work items in fifth generation (5G) radio layer 1 (RAN1) in the Release-18 third-generation partnership project (3GPP) standardization activity concerns sounding reference signal (SRS) enhancements to deal with interference management in time-division duplex (TDD) coherent joint transmission (CJT) for multiple transmission reception points (multiple TRPs or mTRP). The work item proposed to study two approaches: 1) interference randomization techniques (IRT) and 2) capacity enhancements (CE) of SRS. This paper discusses this work item in detail along with the authors’ contributions to it. This paper can be broadly divided into three parts. In the first part, we provide an overview of the various techniques discussed by different companies in the standards. In the second part, we present our contribution of a novel SRS receiver with enhanced SRS capacity. We show how our improved SRS receiver allows the transmission of six SRSs over 12 subcarriers, which results in better performance (greater than 5 dB gain) than a conventional SRS receiver that supports only four SRSs over 12 subcarriers, as per the existing 3GPP standard. Capacity improvement is achieved by enabling closer placement of the SRS in the cyclic shift (CS) domain, which depends on leakage. Conventional SRS receivers are based on discrete Fourier transform (DFT), which have more leakages than the proposed Slepian-based SRS receiver, which has fewer leakages. We analysed in detail the effects of both DFT and Slepian on leakages in the CS domain. This capacity and/or performance improvement allows more user equipments (UEs) to simultaneously transmit SRS, allowing lower uplink transmission power to attain the same performance as conventional SRS receivers, thereby improving coverage. We investigate the changes that need to be implemented in existing standards to support such receivers, which can achieve an enhanced SRS capacity. In the third part, we present three novel enhancement techniques, that we proposed, namely, per-port cyclic shift (PP-CS) allocation scheme, CS hopping in a subset and muting of SRS transmission. CS hopping in a subset was accepted in 5G RAN1 Release-18 3GPP standardization activity as a means of reducing interference in TDD CJT for mTRP. In the section on simulation results in this paper, we show that there is a distinct improvement in performance (compared to existing legacy systems) when CS hopping is employed. This is because, in the CS domain, as the CS associated with a user hops across orthogonal frequency-division multiplexing (OFDM) symbols, it has different neighbouring users in different symbols and leakages due to these neighbours get reduced as the channel estimates are averaged over different OFDM symbols. We demonstrate that it is possible to mitigate almost all interference and reach the zero-interference lower bound of a single TRP (sTRP) case with a combination of PP-CS, CS hopping in a subset, and muting of SRS transmission. The key idea is to recognize that the extra propagation distance (EPD) of UEs from other TRPs results in higher frequency selectivity in the channel. To address this, we designed efficient SRS CS resource-allocation strategies.