Uplink Non-Orthogonal Multiple Access Implementation

Introduction : Non-orthogonal multiple access (NOMA), which can significantly improve the spectrum efficiency (SE) is a promising candidate for 5G wireless communications, in which the superposed coded signal can be successfully decoded and de-modulated at the receiver by applying successive interference cancellation (SIC). Unlike orthogonal multiple access (OMA), the multiple users in NOMA can be assigned to the same frequency, time, and code, but in different power domain to improve the SE of the system.
In this project, we need to model an over-the-air implementation of a uplink NOMA system, while providing the solutions to the most important practical problems.
Difficulties : Challenges faced in uplink NOMA compared to downlink NOMA :
1. Being different to the downlink NOMA, time synchronisation is challenging for the uplink NOMA due to the fact that the mobile communications channel is usually dynamic in nature, and users are spatially distributed.
2. Carrier frequency offset (CFO) can be considered as one of the most common impairments found in a practical communication system. CFO occurs when the frequency is not stable in the transmitter and the receiver oscillators, and also due to the Doppler effect as the transmitter or the receiver is moving. This CFO leads to a large amount of multiple access interference in the uplink NOMA system, since CFO misaligns the sub-carriers in the frequency domain.
3. For the downlink NOMA, channel estimation is a trivial task, since user equipments (UE) only has to estimate the channel between the UE and the base station (BS). However, by exploiting the channel reciprocity, BS can determine the uplink channel when UE estimates the downlink channel and transmit it back to the BS through a feedback channel. This method is mainly applicable for time division duplex (TDD) systems that use the same frequency for the uplink and the downlink. Also, using channel reciprocity for channel estimation leads to many practical difficulties, due to the non-symmetric characteristics of the RF front-end circuitry at the receiver and the transmitter.

Model : We need to model the basic uplink NOMA as addressed in the paper [URef] in our lab, with fully equipped setups. The basic idea is sketched below :
The uplink users estimate the CFO by using a reference signal transmitted by the BS and compensate it at the radio frequency (RF) front-end. The reference signal adopts the primary synchronisation signal (PSS) of LTE. Thereafter design a preamble for each uplink NOMA user, and then, and it is appended to every frame before it is transmitted through the air. This preamble will be used for time synchronisation, and channel estimation at the BS. Also, to implement a lowcomplexity, iterative linear minimum mean squared error (LMMSE) detector for multi-user decoding. The proposed architecture is suitable for a frequency division duplex (FDD) uplink NOMA system, where the uplink frequency is different from the downlink, and hence channel reciprocity does not apply.
Setup : Wherein all the real-time signal processing tasks, i.e., CFO synchronisation, time synchronisation, channel estimation, modulation, soft-demodulation, are performed on laptops using the GNU Radio framework, an open-source software development tool-kit which facilitates us to implement signal processing modules for SDRs. USRP hardware driver (UHD) is the interface between GNU Radio and SDR.
Future work : To extend the work in [URef], we need to implement the same for real-time operation:
The current uplink NOMA system [Ref] has not been sufficiently integrated, BS can not operate with all the necessary functions in real time for the iterative decoding process. Therefore, offline experiments has been carried out for evaluating the over-the-air performance of uplink NOMA, where the soft-demodulated data and the channel estimates are dumped to a file by the BS, and then, which are used to recover the user transmit data using a simulation program
[URef] S. Abeywickrama, L. Liu, Y. Chi and C. Yuen, "Over-the-Air Implementation of Uplink NOMA," in Proc. IEEE Global Communications Conference (GLOBECOM), pp. 1-6, Dec. 2017.

Supervisor name: 
Dr. Mathini Sellathurai
Supervisor and Deputy email addresses: 
M.Sellathurai@hw.ac.uk

MSc programme:

Project Type:

Project location: 
EM1.30
Deputy name: 
Dr. C-X Wang