Abstract
Semi-grant-free (SGF) transmission has recently received significant attention due to its capability to accommodate massive connectivity and reduce access delay by admitting grant-free users to channels which would otherwise be solely occupied by grant-based users. In this paper, a new SGF transmission scheme that exploits the flexibility in choosing the decoding order in non-orthogonal multiple access (NOMA) is proposed. Compared to existing SGF schemes, this new scheme can ensure that admitting the grant-free users is completely transparent to the grant-based users, i.e., the grant-based users’ quality-of-service experience is guaranteed to be the same as for orthogonal multiple access. In addition, compared to existing SGF schemes, the proposed SGF scheme can significantly improve the robustness of the grant-free users’ transmissions and effectively avoid outage probability error floors. To facilitate the performance evaluation of the proposed SGF transmission scheme, an exact expression for the outage probability is obtained and an asymptotic analysis is conducted to show that the achievable multi-user diversity gain is proportional to the number of participating grant-free users. Computer simulation results demonstrate the performance of the proposed SGF transmission scheme and verify the accuracy of the developed analytical results.
Highlights
The generation Internet of Things (NGIoT) is envisioned to be an important use case for beyond 5G mobile networks [1]
The first group applies random access protocols originally developed for computer networks [3], the second group relies on the excess spatial degrees of freedom offered by multiple-input multiple-output (MIMO) techniques [4], [5], and the third group employs non-orthogonal multiple access (NOMA) which encourages spectrum sharing among the devices [6]–[10]
We have proposed a new NOMA assisted SGF transmission scheme
Summary
The generation Internet of Things (NGIoT) is envisioned to be an important use case for beyond 5G mobile networks [1]. In SGF transmission, a crucial task is how to guarantee a grant-based user’s quality of service (QoS) experience when admitting grant-free users to the same resource block. Recall that SGF Scheme II directly decodes the grant-free users’ signals by treating the grant-based user’s signal as interference, which means that interference always exists for the grant-free users. The new SGF scheme can realize interference-free transmission for the grant-free users, and can offer significantly improved achievable data rates for grant-free transmission. For both existing SGF schemes, their outage probabilities exhibit error floors, when there is no transmit power control, e.g., the grant-free and grant-based users increase their transmit powers without coordination. The asymptotic expressions demonstrate that the proposed SGF transmission scheme avoids an outage probability error floor and realizes a multi-user diversity gain of M.
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