Random Access Identifier-Linked Receiver Beamforming with Transmitter Filtering in TDD-Based Random Access

Abstract

This paper proposes a novel random access identifier (RAID)-linked receiver beamforming method for time division duplex (TDD)-based random access. When the number of receiver antennas at the base station is large in a massive multiple-input multiple-output (MIMO) scenario, the channel estimation accuracy per receiver antenna at the base station receiver is degraded due to the limited received signal power per antenna from the user terminal. This results in degradation in the receiver beamforming (BF) or antenna diversity combining and active RAID detection. The purpose of the proposed method is to achieve accurate active RAID detection and channel estimation with a reasonable level of computational complexity at the base station receiver. In the proposed method, a unique receiver BF vector applied at the base station is linked to each of the M RAIDs prepared by the system. The user terminal selects an appropriate pair comprising a receiver BF vector and a RAID in advance based on the channel estimation results in the downlink assuming channel reciprocity in a TDD system. Therefore, per-receiver antenna channel estimation for receiver BF is not necessary in the proposed method. Furthermore, in order to utilize fully the knowledge of the channel at the user transmitter, we propose applying transmitter filtering (TF) to the proposed method for effective channel shortening in order to increase the orthogonal preambles for active RAID detection and channel estimation prepared for each RAID. Computer simulation results show that the proposed method greatly improves the accuracy of active RAID detection and channel estimation. This results in lower error rates than that for the conventional method performing channel estimation at each antenna in a massive MIMO environment.

HARQ Using Hierarchical Tree-Structured Random Access Identifiers in NOMA-Based Random Access

“…However, since the base station cannot recognize the relationship between the RAIDs in the initial transmission and retransmission, the base station cannot perform packet combining. Furthermore, when channeldependent RAID selection is assumed such as in the RAIDlinked receiver BF method [26,27], reselection of the RAID, which does not match a given user channel at retransmission, reduces the received signal power after receiver BF, which represents the combined received signal power from among all receiver antennas. This degrades the transmission performance.…”

“…This degrades the transmission performance. References [26] and [27] include some quantitative evaluations on the impact of the unmatched receiver BF applied to the user channel on the achievable packet error rate (PER).…”

“…Table 2 gives the major simulation parameters. Random access with a 4.6-MHz transmission bandwidth and 0.5-ms time slot, which is equal to the packet length, using the RAID-linked receiver BF [26,27] is assumed as the baseline random access method. A combination of the turbo and repetition codes is used for channel coding.…”

“…On the other hand, if the RAID of the retransmitted packet is randomly reselected by the user, the RAID collision can be resolved at retransmission, but packet combining becomes impossible. Furthermore, when channel-dependent RAID selection is assumed such as in the RAID-linked receiver beamforming (BF) method [26,27], reselection of a RAID that does not match a given user channel at retransmission degrades the transmission performance due to the reduced received signal power after BF.…”

“…For example, when two users incur a RAID collision at the t−1-th transmission, the probability that the RAID collision is resolved in the next t-th retransmission is 1−(1/R (t) ). When the proposed method is applied to the RAID-linked receiver BF method in [26,27], the received BF vector associated with RAID i in the initial transmission is also applied to all the RAIDs in all the retransmission RAID groups that are linked to the initial RAID i.…”