208879492
Grant-Free Uplink Transmission With Multi-Codebook-Based Sparse Code Multiple Access (MC-SCMA)
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.
22221919
A Survey on Non-Orthogonal Multiple Access for 5G Networks: Research Challenges and Future Trends
Non-orthogonal multiple access (NOMA) is an essential enabling technology for the fifth-generation (5G) wireless networks to meet the heterogeneous demands on low latency, high reliability, massive connectivity, improved fairness, and high throughput. The key idea behind NOMA is to serve multiple users in the same resource block, such as a time slot, subcarrier, or spreading code. The NOMA principle is a general framework, and several recently proposed 5G multiple access schemes can be viewed as special cases. This survey provides an overview of the latest NOMA research and innovations as well as their applications. Thereby, the papers published in this special issue are put into the context of the existing literature. Future research challenges regarding NOMA in 5G and beyond are also discussed.
3608951
Compressive Sensing-Based Random Access with Multiple-Sequence Spreading for MTC
In this paper, we present a new structure for compressive sensing(CS)-based random access (RA) scheme for machine-type communications (MTC). In the proposed scheme, we consider a one-shot transmission in which users transmit their packets right away by spreading them with multiple spreading sequences (MS) over a frame without waiting for scheduling grant from the base station. The scheme is designed for delayconstrained & sporadic traffic where a massive number of users have a very low activity factor, small packet sizes and limited power. It has been shown that by employing the proposed scheme the channel access success rate can be increased without incurring additional detection complexity. In different regions of users' activity factor, two sources of random access failure have been identified. In the design of the MS scheme, the tradeoff between these two factors can be considered to optimize the system. For strictly delay-intolerant applications where retransmission is not allowed, it is shown that about 10 times more users can be supported by employing MS in comparison with a type of random access in long term evolution (LTE) network.
10444186
Pattern Division Multiple Access—A Novel Nonorthogonal Multiple Access for Fifth-Generation Radio Networks
In this paper, pattern division multiple access (PDMA), which is a novel nonorthogonal multiple access scheme, is proposed for fifth-generation (5G) radio networks. The PDMA pattern defines the mapping of transmitted data to a resource group that can consist of time, frequency, and spatial resources or any combination of these resources. The pattern is introduced to differentiate signals of users sharing the same resources, and the pattern is designed with disparate diversity order and sparsity so that PDMA can take the advantage of the joint design of transmitter and receiver to improve system performance while maintaining detection complexity to a reasonable level. System level simulation results show that PDMA can support <inline-formula><tex-math notation="LaTeX">$\text{six}$</tex-math> </inline-formula> times simultaneous connections than that of conventional and at least <inline-formula> <tex-math notation="LaTeX">$\text{30}\%$</tex-math></inline-formula> improvement in spectrum efficiency over orthogonal frequency division multiple access.
8141354
Power Allocation for Max-Sum Rate and Max-Min Rate Proportional Fairness in NOMA
In this letter, we consider proportional fairness scheduling (PFS) for downlink non-orthogonal multiple access (NOMA) with two users. Unlike conventional multiuser diversity systems, the power allocation plays a key role in PFS for NOMA as it can support multiple users simultaneously with positive transmission rates. With different criteria, we find optimal power allocation. Among those, it is shown that the PFS scheme that maximizes the minimum normalized rate can provide not only proportional fairness, but also small variation of transmission rates.
21115002
SCMA codebook design based on constellation rotation
Sparse code multiple access (SCMA) is a competitive non-orthogonal multiple access technique for the fifth generation (5G) wireless communications. The SCMA codebook design is a very essential problem. This paper presents a constellation-rotation-based method for designing codebooks for downlink SCMA system. The basic idea is to make the minimum Euclidean distance of the main-constellation as larger as possible, so as to achieve good BER (bit error rate) performance. By constructing proper sub-constellations and Latin matrix, it is able to achieve a good shaping gain. Simulation results show that the proposed SCMA codebooks provides good BER performance in both additive white Gaussian noise (AWGN) and flat fading channels.
53281734
Design and Analysis of SCMA Codebook Based on Star-QAM Signaling Constellations
In this paper, a novel codebook design method for sparse code multiple access (SCMA) is proposed and an analytical framework to evaluate the bit error rate (BER) performance is developed. In particular, to meet the codebook design criteria based on the pairwise error probability, a novel codebook with large minimum Euclidean distance employing the star quadrature amplitude modulation signal constellations is designed. In addition, with the aid of the phase distribution of the presented SCMA constellations, BER of SCMA system over downlink Rayleigh fading channel is obtained in closed-form expression. The simulation and analytical results show that the presented SCMA codebook outperforms the existing codebooks and low-density signature, and the proposed design is more efficient for the SCMA codebook with large size and/or high dimension. Moreover, the derived theoretical BER results match well the simulation results, especially in the high signal-to-noise ratio regimes.
206854727
Multi-Dimensional SCMA Codebook Design Based on Constellation Rotation and Interleaving
Sparse code multiple access (SCMA) is a new non- orthogonal multiple access scheme, which effectively exploits the shaping gain of multi-dimensional codebook. In this paper, a multi-dimensional SCMA (MD-SCMA) codebook design based on constellation rotation and interleaving method is proposed for downlink SCMA systems. In particular, the first dimension of mother constellation is constructed by subset of lattice Z2. Then the other dimensions are obtained by rotating the first dimension. Further, the interleaving is used for even dimensions to improve the performance in fading channels. In this way, we can design different codebooks for the aim of spectral efficiency or power efficiency. And the simulation results show that the bit error rate (BER) performance of MD-SCMA codebooks outperforms that of the existing SCMA codebooks and low density signature (LDS) in downlink Rayleigh fading channels.
89617635
Uplink Grant-free Multi-codebook SCMA Based on High-overload Codebook Grouping
Fifth generation (5G) wireless networks are expected to support massive connectivity with a great number of users. Current LTE uplink (UL) system is not able to efficiently support massive connectivity, because of the extra overhead and latency with granting. So UL grant-free transmission is proposed for 5G. Sparse code multiple access(SCMA) is a typical non-orthogonal multiple access technology in 5G. With SCMA, multi-user data is spread to same time-frequency resources, such as OFDMA tones, through codebook mapping. In this paper, the general design of high-overload SCMA codebooks is proposed. In the UL grant-free system, Active Users Detection (AUD) in receiver detects the active pilots to determine the active users and their codebooks, and the number of pilots will affect the AUD performance. Every user may use different number of codebooks to encode data to multiple codewords, because of different service, and that is called Multi-codebook SCMA (McSCMA). In this paper, based on grant-free McSCMA, codebook grouping(CG) is proposed for decreasing the number of combinations of codebook(CoC) in order to reduce the number of pilots to improve the AUD performance. The simulation results demonstrate that the design of high-overload codebook is feasible, and CG can improve the AUD performance based on grant-free McSCMA.
5455257
Blind detection of SCMA for uplink grant-free multiple-access
Sparse code multiple access (SCMA) is a new frequency domain non-orthogonal multiple-access technique which can enable massive connectivity and grant-free transmission in wireless radio access. With SCMA, different incoming data streams are directly mapped to codewords of different multi-dimensional cookbooks, where each codeword represents a spread transmission layer. Multiple SCMA layers share the same time-frequency resources of OFDMA. The sparsity of codewords allows low complexity of multi-layer detection for excessive codeword overloading which is the key feature to enable massive connectivity. In this paper, a blind detection solution is introduced and analyzed to support massive connectivity in a SCMA-based UL grant-free multiple access. The proposed solution is based on two major components: i) blind detection of active pilots/users with reasonable complexity, and ii) blind decoding of active users' data with no knowledge of active codebook set. Different activity detection algorithms and schemes are proposed, described, and analyzed. Simulation results are provided to evaluate the performance of the proposed schemes in various scenarios. Our analysis and performance evaluation confirm the proposed SCMA-based blind reception solution is a promising technology to enable massive connectivity for grant-free multiple-access transmission mode in future wireless networks.
12940624
A Fixed Low Complexity Message Pass Algorithm Detector for Up-Link SCMA System
Sparse code multiple access (SCMA) is a new modulation and non-orthogonal multiple access scheme for 5G systems, in which the coded bits are mapped to multi-dimensional sparse codewords, and the message passing algorithm (MPA) is employed to detect multi-user signals. However, the relative high computational complexity of MPA detection is still a big concern for practical implementation, even the sparse structure has already been employed. In this letter, a novel fixed low complexity detector for uplink SCMA system is proposed, based on partial marginalization (PM). Our analysis and simulation results show that, compared with the existing MPA detector, the implementation complexity of the new PM-based detector is reduced substantially, while the bit error ratio (BER) performance degrades unnoticeably (less than 0.3 dB).
1368383
Low Complexity Message Passing Algorithm for SCMA System
Sparse code multiple access (SCMA) is a novel non-orthogonal air-interface technology in which several users share a same frequency resource simultaneously. The SCMA decoder is based on a message passing algorithm (MPA) for the sparsity of codeword. In the original MPA, all the users update a message until the maximum number of iterations is reached, resulting in high computational complexity. This letter puts forward a threshold-based MPA, where a belief threshold is applied to control the algorithm process. Simulation results show that the proposed scheme obtains a low computational complexity with only a slight performance degradation when the threshold is set appropriately.
51877809
A Contention Transmission Unit Allocation Scheme for Uplink Grant-Free SCMA Systems
In order to satisfy the requirement of supporting massive connectivity accompanied by the cost of transmission latency and signaling overhead, uplink (UL) grant-free Sparse Code Multiple Access (SCMA) design has been proposed. Contention transmission unit (CTU) is the basic resource for contention transmission for UL SCMA. Most CTU allocation schemes allocate CTU just by the UE_ID, which may bring about high packet loss rate due to the collision of CTUs when the activeness of UEs is not similar. In this paper, we first study the equation of the average packet loss rate, where the optimization of average packet loss rate is proved to be a NP-hard problem as a function of the allocation scheme. In this paper, we proposed a novel scheme based on activeness via dynamic programming transforming the NP-hard problem to a solvable and low complexity problem, which can obtain the Pareto optimal solution ultimately. Simulation results demonstrate that the proposed CTU allocation scheme can greatly reduce the packet loss rate with extremely low complexity.
12347084
Message Passing Receiver Design for Uplink Grant-Free SCMA
In this paper, we consider the uplink grant-free SCMA detection where the base station blindly decodes the users'data without knowing the user activity and their channel state information. In order to shorten the pilots overhead, a message passing receiver that jointly performs the channel estimation,user detection and data decoding is proposed. We formulate the belief propagation (BP) framework based on the factor graph of SCMA. However, the direct use of BP for multi-variable problem is prohibitively complex. Motivated by the idea of approximate inference, expectation propagation is used to project the intractable distributions into Gaussian families such that a linear complexity decoder is obtained. Simulation results show that the proposed algorithm has a better performance compared with the existing methods.
434411
Uplink contention based SCMA for 5G radio access
Fifth generation (5G) wireless networks are expected to support very diverse applications and terminals. Massive connectivity with a large number of devices is an important requirement for 5G networks. Current LTE system is not able to efficiently support massive connectivity, especially on the uplink (UL). Among the issues that arise due to massive connectivity is the cost of signaling overhead and latency. In this paper, an uplink contention-based sparse code multiple access (SCMA) design is proposed as a solution. First, the system design aspects of the proposed multiple-access scheme are described. The SCMA parameters can be adjusted to provide different levels of overloading, thus suitable to meet the diverse traffic connectivity requirements. In addition, the system-level evaluations of a small packet application scenario are provided for contention-based UL SCMA. SCMA is compared to OFDMA in terms of connectivity and drop rate under a tight latency requirement. The simulation results demonstrate that contention-based SCMA can provide around 2.8 times gain over contention-based OFDMA in terms of supported active users. The uplink contention-based SCMA scheme can be a promising technology for 5G wireless networks for data transmission with low signaling overhead, low delay, and support of massive connectivity.
70311173
A Novel Analytical Framework for Massive Grant-Free NOMA
In this paper, we consider a massive grant-free non-orthogonal multiple access (GF-NOMA) scheme, where devices have strict latency requirements and no retransmission opportunities are available. Each device chooses a pilot sequence from a predetermined set as its signature and transmits its selected pilot and data simultaneously. A collision occurs when two or more devices choose the same pilot sequence. Existing GF-NOMA schemes assume that a collision of at least one pair of users entails a collision for all simultaneously transmitting users, which is sub-optimal in terms of individual outage and system throughput. For that, we propose a novel framework, where collisions are treated as interference to the remaining received signals. With the aid of Poisson point processes and ordered statistics, we derive simplified expressions that can well approximate the outage probability and throughput of the system for both successive joint decoding (SJD) and successive interference cancellation (SIC). Numerical results verify the accuracy of our analytical expressions. For low data rate transmissions, results show that the performance of SIC is close to that of SJD in terms of outage probability, for packet arrival rates up to 10 packets per slot. However, SJD can achieve almost double the throughput of SIC and is, thus, far more superior.
10690969
Automatic configuration of random access channel parameters in LTE systems
In 3G long term evolution (LTE) systems, the random access channel (RACH) is used for initial access, resource request, and handover. Since the random access delay is determined by the arrival rate of the random access preambles and the number of RACH subframes, we should configure the number of RACH subframes given the arrival rate in order to guarantee the delay performance. In this work, by carefully taking account of a tradeoff between the number of RACH subframes and the random access delay, we present an optimization formulation that minimizes the number of RACH subframes for a given delay requirement. Furthermore, since the arrival rate of the random access preambles is time varying in reality, we further propose an estimation scheme for the arrival rate by reflecting the periodicity and the correlation between recent and future arrival rates. Our simulation results show that the proposed scheme for tuning the RACH subframes gives very promising network performance under time-varying environments.
206825315
Zadoff-Chu Sequence Based Signature Identification for OFDM
A signature identification algorithm is a method to obtain the cell identification information for wireless cellular systems or determine the intended user for wireless local area network. In this paper, we propose a simple and efficient signature identification algorithm on the basis of Zadoff-Chu sequence in orthogonal frequency division multiplexing systems. In addition, we prove that the proposed algorithm achieves a maximum likelihood solution if the receiver knows the channel length. Also, the exact probabilities of signature identification failures of the proposed algorithm are provided for different power delay profiles. To demonstrate efficacy of the proposed algorithm in fading channels, we derive the failure probability at high signal-to-noise ratio (SNR). Through a high SNR expression, it is shown that the proposed algorithm fully exploits frequency selective fadings. Especially, we reveal that the slope of the failure probability curves at high SNR is determined by the channel length regardless of power delay profiles. Simulation results show that the proposed algorithm outperforms conventional signature algorithms in frequency selective fading channels. Also, we confirm that our analysis matches well with the empirical results of the proposed signature identification algorithm.
16211728
Zadoff–Chu Sequence Design for Random Access Initial Uplink Synchronization in LTE-Like Systems
A Zadoff–Chu (ZC) sequence is uncorrelated with a non-zero cyclically shifted version of itself. However, this alone is insufficient to mitigate inter-code interference in LTE initial uplink synchronization. The performance of the state-of-the-art algorithms vary widely depending on the specific ZC sequences employed. We develop a systematic procedure to choose the ZC sequences that yield the optimum performance. It turns out that the procedure for ZC code selection in LTE standard is suboptimal when the carrier frequency offset is not small.
2626333
Low Complexity Iterative Receiver Design for Sparse Code Multiple Access
Sparse code multiple access (SCMA) is one of the most promising methods among all the non-orthogonal multiple access techniques in the future 5G communication. Compared with some other non-orthogonal multiple access techniques, such as low density signature, SCMA can achieve better performance due to the shaping gain of the SCMA code words. However, despite the sparsity of the code words, the decoding complexity of the current message passing algorithm utilized by SCMA is still prohibitively high. In this paper, by exploring the lattice structure of SCMA code words, we propose a low-complexity decoding algorithm based on list sphere decoding (LSD). The LSD avoids the exhaustive search for all possible hypotheses and only considers signal within a hypersphere. As LSD can be viewed a depth-first tree search algorithm, we further propose several methods to prune the redundancy-visited nodes in order to reduce the size of the search tree. Simulation results show that the proposed algorithm can reduce the decoding complexity substantially while the performance loss compared with the existing algorithm is negligible.
6629641
Average Performance Analysis for Thresholding
In this letter, we show that with high probability, the thresholding algorithm can recover signals that are sparse in a redundant dictionary as long as the 2-Babel function is growing slowly. This implies that it can succeed for sparsity levels up to the order of the ambient dimension. The theoretical bounds are illustrated with numerical simulations. As an application of the theory, sensing dictionaries for optimal average performance are characterized, and their performance is tested numerically.
3639813
Comprehensive grant-free random access for massive & low latency communication
In this paper, we introduce a comprehensive grant-free random access scheme for machine-type communication which is characterized by massive connectivity and low latency. The scheme presented in here is comprehensive in a sense that, synchronization, channel estimation, and users identification & data detection (multi-user detection) are performed all in a single shot. The scheme employs compressive sensing by exploiting two sparse phenomena: sparsity in users activity and sparsity in multi-path channel. Furthermore, the scheme is designed in such a way that channel estimation and multi-user detection have a bi-directional mutual relationship, enabling one to reinforce the other for accurate detection and estimation. Moreover, the iterative order recursive least square (IORLS) estimation algorithm is modified & employed in such a way that it exploits the joint structure in multi-path channel and multi-user signal sparsity.