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Uplink Non-Orthogonal Multiple Access for 5G Wireless Networks
Mohammed Al-Imari, Pei Xiao, Muhammad Ali Imran, Rahim Tafazolli
TL;DR
Uplink OMA cannot reach the system capacity limit under exclusive allocation, while existing NOMA reduces spectral efficiency through coding or spreading redundancy. The paper proposes bounded-load uplink NOMA with shared subcarriers, joint detection, and sum-rate-oriented allocation. It reports link-level BER close to the single-user case and higher spectral efficiency and fairness than OFDMA.
Problem
Uplink OMA cannot achieve the system upper bound, particularly under fairness constraints, while existing NOMA uses redundancy that reduces spectral efficiency.
Method
The paper proposes uplink OFDM-NOMA with non-exclusive subcarrier sharing, an upper limit on users per subcarrier, joint multi-user detection, and subcarrier-power allocation.
Results
The proposed NOMA achieves link-level BER close to the single-user case and improves spectral efficiency and fairness compared with OFDMA.
Takeaways & Limitations
At K = 50, the proposed NOMA achieves about 95% of the system upper bound, while OFDMA achieves 81%.
Abstract
from arXiv · showhide
Orthogonal Frequency Division Multiple Access (OFDMA) as well as other orthogonal multiple access techniques fail to achieve the system capacity limit in the uplink due to the exclusivity in resource allocation. This issue is more prominent when fairness among the users is considered in the system. Current Non-Orthogonal Multiple Access (NOMA) techniques introduce redundancy by coding/spreading to facilitate the users' signals separation at the receiver, which degrade the system spectral efficiency. Hence, in order to achieve higher capacity, more efficient NOMA schemes need to be developed. In this paper, we propose a NOMA scheme for uplink that removes the resource allocation exclusivity and allows more than one user to share the same subcarrier without any coding/spreading redundancy. Joint processing is implemented at the receiver to detect the users' signals. However, to control the receiver complexity, an upper limit on the number of users per subcarrier needs to be imposed. In addition, a novel subcarrier and power allocation algorithm is proposed for the new NOMA scheme that maximizes the users' sum-rate. The link-level performance evaluation has shown that the proposed scheme achieves bit error rate close to the single-user case. Numerical results show that the proposed NOMA scheme can significantly improve the system performance in terms of spectral efficiency and fairness comparing to OFDMA.
I. INTRODUCTION
Uplink orthogonal access cannot reach the system capacity limit, especially under fairness constraints, while existing NOMA sacrifices spectral efficiency through coding or spreading redundancy. The paper proposes an uplink NOMA scheme that permits shared subcarriers without such redundancy and limits per-subcarrier users to control receiver complexity.
- Existing NOMA schemes facilitate signal separation through coding or spreading, but this redundancy reduces spectral efficiency.
- Uplink OMA is spectrally inefficient and cannot achieve the system upper bound, with the problem becoming more prominent when user fairness matters.
- The proposed uplink OFDM-NOMA scheme removes subcarrier allocation exclusivity and uses multi-user detection without coding or spreading redundancy.
- The scheme limits the number of users per subcarrier to control receiver complexity and includes a subcarrier and power allocation algorithm.
II. BACKGROUND AND SYSTEM MODEL
The system models uplink transmission as a multi-user channel in which users may share subcarriers, creating interference that requires multi-user detection. The proposed scheme bounds the number of users per subcarrier, enabling joint detection without spreading or coding redundancy while trading spectral efficiency against receiver complexity.
- In the uplink model, multiple users transmit to one base station over frequency-selective channels and divide power across subcarriers under individual power constraints.
- OMA restricts each subcarrier to at most one active user, eliminating inter-user interference but reducing spectral efficiency and fairness through exclusive allocation.
- NOMA permits multiple users per subcarrier, but shared resources create inter-user interference and conventional schemes add spreading or coding redundancy for lower-complexity detection.
- The proposed scheme caps subcarrier occupancy at L users, preventing overloaded subcarriers while retaining non-exclusive resource sharing.
- With L ≪ K, optimum multi-user detection has complexity O(|X|^L), compared with up to O(|X|^K) for an unconstrained generic MAC.
- At the receiver, symbols from L users are superimposed after OFDM demodulation and processed by multi-user detection before channel decoding.
- For link-level evaluation, the proposed NOMA uses two users per subcarrier and ML-MUD, with BER versus Eb/N0 compared against OFDMA using BPSK.
III. SUBCARRIER AND POWER ALLOCATION
The paper formulates subcarrier and power allocation as a sum-rate maximization problem and proposes a suboptimal allocation algorithm for the new NOMA scheme.
- Subcarrier and power allocation is formulated first as sum-rate maximization, followed by a suboptimal algorithm for the proposed NOMA scheme.
A. Problem Formulation
The allocation problem uses CSI-based joint processing with binary subcarrier assignments, power allocation, and a per-subcarrier user limit. Its interference-containing objective is non-convex, and relaxing binary assignments would violate the proposed scheme’s loading constraint.
- The base station uses CSI to assign subcarriers and power, with successive decoding for joint processing.
- The allocation formulation optimizes subcarrier and power assignments while constraining the number of users sharing each subcarrier.
- The optimization is non-convex because subcarrier assignments are binary and interference appears in the objective.
- Relaxing binary assignments is unsuitable because it could allow every user to interfere on every subcarrier instead of limiting sharing to L users.
B. Suboptimal Algorithm
A low-complexity iterative algorithm alternates power allocation, subcarrier selection, and allocation until capacity or rate-improvement limits are reached. It compares local-rate and global-objective allocation criteria.
- The proposed algorithm is suboptimal because the optimal subcarrier and power allocation problem is intractable and impractical.
- Each iteration performs single-user water-filling with interference considered, then calculates users’ rates on available subcarriers.
- For each user, the algorithm selects the unallocated subcarrier providing the maximum rate.
- Allocation criteria: Local Rate Maximization allocates a selected subcarrier to the user with the highest rate on it.
- Allocation criteria: Global Objective Maximization allocates a selected subcarrier to the user producing the greatest increase in the objective function.
- The iterations continue until subcarriers reach their limit L or users’ rates can no longer increase.
IV. NUMERICAL RESULTS
System-level simulations compare the proposed NOMA with OFDMA across spectral efficiency and fairness. NOMA remains closer to the generic-MAC upper bound and is fairer, while greater subcarrier loading increases fairness but also receiver complexity.
- Spectral efficiency: At K = 50, proposed NOMA achieves about 95% of the system upper bound, while OFDMA achieves 81%.
- Spectral efficiency: Proposed NOMA significantly outperforms OFDMA in spectral efficiency and remains closer to the generic-MAC system upper bound as user count increases.
- Spectral efficiency: LRM and GOM achieve almost the same spectral efficiency.
- Fairness: Proposed NOMA is fairer than OFDMA, with the gap becoming more significant at high user counts.
- Fairness: GOM is considerably fairer than LRM, although the two criteria have the same spectral efficiency.
- Subcarrier loading: Allowing more users to share subcarriers improves fairness but increases receiver complexity, while producing only a marginal spectral-efficiency increase.
V. CONCLUSION
The paper proposes uplink NOMA that shares subcarriers without coding or spreading redundancy, uses optimum multi-user detection, and limits per-subcarrier loading to control complexity. Simulations report improvements in spectral efficiency and fairness over orthogonal access.
- The proposed uplink NOMA lets users share subcarriers without coding or spreading redundancy.
- Optimum multi-user detection separates users’ signals, while a per-subcarrier user limit controls receiver complexity.
- The proposed scheme achieves link-level performance very close to the single-user case despite inter-user interference.
- System-level simulations show significantly improved spectral efficiency and fairness compared with orthogonal multiple access.