[Paper Review] Novel Time Asynchronous NOMA schemes for Downlink Transmissions

Mehdi Ganji, Hamid Jafarkhani|arXiv (Cornell University)|Aug 27, 2018

Advanced Wireless Communication Technologies37 references3 citations

TL;DR

This paper proposes two novel non-orthogonal multiple access (NOMA) schemes—Asynchronous-Power Domain NOMA (AP-NOMA) and Time-domain NOMA (T-NOMA)—that exploit intentional time asynchrony to enhance downlink spectral efficiency. By introducing controlled symbol offsets and precoding, T-NOMA achieves a larger achievable rate region than conventional P-NOMA and AP-NOMA, with superior sum-rate and fairness, especially when users have asymmetric channel conditions.

ABSTRACT

In this work, we investigate the effect of time asynchrony in non-orthogonal multiple access (NOMA) schemes for downlink transmissions. First, we analyze the benefit of adding intentional timing offsets to the conventional power domain-NOMA (P-NOMA). This method which is called Asynchronous-Power Domain-NOMA (AP-NOMA) introduces artificial symbol-offsets between packets destined for different users. It reduces the mutual interference which results in enlarging the achievable rate-region of the conventional P-NOMA. Then, we propose a precoding scheme which fully exploits the degrees of freedom provided by the time asynchrony. We call this multiple access scheme T-NOMA which provides higher degrees of freedom for users compared to the conventional P-NOMA or even the modified AP-NOMA. T-NOMA adopts a precoding at the base station and a linear preprocessing scheme at the receiving user which decomposes the broadcast channel into parallel channels circumventing the need for Successive Interference Cancellation (SIC). The numerical results show that T-NOMA outperforms AP-NOMA and both outperform the conventional P-NOMA. We also compare the maximum sum-rate and fairness provided by these methods. Moreover, the impact of pulse shape and symbol offset on the performance of AP-NOMA and T-NOMA schemes are investigated.

Motivation & Objective

• To address the inefficiency and fairness limitations of conventional orthogonal multiple access (OMA) in downlink transmissions.
• To investigate the potential benefits of time asynchrony in NOMA, traditionally viewed as an impairment.
• To design new NOMA schemes that exploit timing offsets as a resource to improve spectral efficiency and user fairness.
• To develop a precoding and preprocessing strategy that transforms the broadcast channel into parallel streams, eliminating the need for successive interference cancellation (SIC).
• To compare the sum-rate and fairness performance of P-NOMA, AP-NOMA, and T-NOMA under various channel conditions and pulse shapes.

Proposed method

• Proposes AP-NOMA, which introduces intentional timing offsets between user packets to reduce mutual interference in power-domain NOMA.
• Introduces T-NOMA, a precoding-based scheme at the base station and linear preprocessing at users to decompose the broadcast channel into parallel links.
• Employs a precoding matrix at the transmitter and a receiver filter at each user to achieve interference-free parallel transmission without SIC.
• Derives the achievable sum-rate for each scheme using information-theoretic analysis, with closed-form expressions for optimal power allocation.
• Analyzes the impact of pulse shape and symbol offset on performance, showing that optimal shaping can further enhance spectral efficiency.
• Uses mathematical optimization to derive the optimal power allocation for each scheme, comparing the resulting sum-rates.

Experimental results

Research questions

• RQ1 Can intentional time asynchrony improve the achievable rate region in downlink NOMA systems?
• RQ2 How does introducing artificial symbol offsets in AP-NOMA reduce inter-user interference compared to conventional P-NOMA?
• RQ3 What is the maximum sum-rate gain achievable by T-NOMA over P-NOMA and AP-NOMA under different channel conditions?
• RQ4 How does the choice of pulse shape affect the performance of AP-NOMA and T-NOMA in the presence of timing offsets?
• RQ5 Under what conditions does T-NOMA achieve strictly higher sum-rate than both P-NOMA and AP-NOMA?

Key findings

• T-NOMA achieves a strictly larger achievable rate region than both P-NOMA and AP-NOMA, particularly when users have asymmetric channel quality.
• For channels with equal noise power (σ₁ = σ₂), AP-NOMA achieves a higher sum-rate than P-NOMA because it assigns non-zero power to both users, unlike P-NOMA which assigns all power to the stronger user.
• When the channel signal-to-noise ratio difference exceeds (1−g)P, AP-NOMA assigns non-zero power to both users and achieves a higher sum-rate than P-NOMA.
• T-NOMA’s optimal power allocation is given by P₁* = (P + σ₂ − σ₁)/2, enabling balanced power use when channels are asymmetric.
• T-NOMA achieves the highest sum-rate among all schemes because its precoding enables parallel channel decomposition, avoiding SIC and reducing interference.
• The sum-rate of T-NOMA is always greater than or equal to that of AP-NOMA, with strict inequality when both users are allocated non-zero power.