Abstract
In sparse code multiple access (SCMA)-based grant-free (GF) access, both user’s channel and the selected SCMA codebook (CB) are considered as a transmission signature for multi-user detection (MUD). In conventional SCMA-based grant-free (GF) access where a single codebook (CB) is employed per a user’s random access transmission, however, random selection of CBs and random fading channel reduce the minimum Euclidean distance among overloaded layers (signatures). This significantly degrades the performance. Furthermore, it is also challenging to reduce the collision rate in the course of selecting random access resources while maintaining low preamble overhead. In this paper, we consider a new approach that maps user’s consecutive symbols to codewords from the different SCMA CBs in a round-robin manner. It is shown that a combinatorial nature of multiple CBs and the inherent code diversity therein allow to simultaneously reduce collision probability while providing sufficient separation among transmissions, consequently decreasing users’ activity misdetection rate. It is also shown that the proposed multi-codebook SCMA (MC-SCMA)-based GF access scheme outperforms the conventional single CB-based SCMA version by overloading of more users (almost up to 200%) as it allows a large number of highly non-orthogonal preambles to be employed while keeping the same preamble length (overhead). Finally, we conclude that the proposed joint design of non-orthogonal preambles and data transmission based on multiple CBs provides a significant performance improvement for the grant-free access system. It can also readily be extended to other code-based non-orthogonal multiple access (NOMA) schemes.
Highlights
One of the requirements of the fifth generation (5G) mobile communication is to support a massive number of machinetype communication devices with their intrinsic characteristics such as intermittent channel access, small packet size and in some cases, tight latency requirement
The contribution of this paper can be summarized with these points: 1) A framework for scalable sparse code multiple access (SCMA)-based GF access with multiple codebooks provides code diversity while increasing Euclidean distance among signatures of active transmissions 2) By exploiting the code diversity provided, a preamble transmission based on non-orthogonal Zadoff-Chu sequences is proposed to reduce signaling overhead 3) A joint channel estimation and multi-user detection (MUD) framework is presented which exploits the intrinsic nature of multi-codebook SCMA (MC-SCMA) to assist active user detection (AUD) and multi-path fading channel estimation
NUMERICAL RESULTS To evaluate the performance of the proposed scheme, we have considered a GF access in which a single time-frequency region for contention transmission units (CTUs) is defined with Ns = 128 subcarriers and 8 OFDM symbols
Summary
One of the requirements of the fifth generation (5G) mobile communication is to support a massive number of machinetype communication (mMTC) devices with their intrinsic characteristics such as intermittent channel access, small packet size and in some cases, tight latency requirement. It is shown that by associating multiple preamble sequences with a codebook, the random access resources can be increased while users can still be distinguished from each other even if they have employed the same codebook for symbol mapping as long as their channel is sufficiently different and reliably estimated This approach allows transmission codebooks and users’ channel together to be considered as signatures which identify users and their corresponding data in the MUD. The contribution of this paper can be summarized with these points: 1) A framework for scalable SCMA-based GF access with multiple codebooks provides code diversity while increasing Euclidean distance among signatures of active transmissions 2) By exploiting the code diversity provided, a preamble transmission based on non-orthogonal Zadoff-Chu sequences is proposed to reduce signaling overhead 3) A joint channel estimation and MUD framework is presented which exploits the intrinsic nature of MC-SCMA to assist active user detection (AUD) and multi-path fading channel estimation. Let diag(h) denotes a diagonal matrix with elements of h on its diagonal
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