2010

School of Electrical Engineering &Telecom

munications

UNSW ENGINEERING @ UNSW

14

Author – William Abraham (3290076) Supervisor – Dr. A. von Brasch Co-supervisor – Thomas Dejanovic Assessor – A/Prof W. Zhang

Minimising FH-NBI Through Interference Cancellation

Introduction

Problem statement To mitigate, suppress and/or remove real-world sources of interference (as seen in Fig.1), particularly frequency hopping narrowband interference (FH-NBI) for a network operator in the ISM band - Taggle Systems. Motivation Increase the reliability and performance of a communications network as interference causes signal quality to be reduced/lost. By making changes to the receiver hardware at the baseband, we can improve system performance in hostile interference environments. Proposed solutions The systems considered include:

1 – Adaptive Notch filtering: 2 – Destructive interference cancellation

Adaptive Filtering using IIR notch Transfer function coefficients determined by: a)  Interference centre frequency. b) Bandwidth of the interferer - to determine the 3 dB rejection

width of the filter. Artifact Removal Process (ARP)

To solve this problem we employ the solution below (a) to (d):

Fig 1. Interfering devices (L to R): Ubiquity N900, Trimble SNB900, DNT900 dev kit.

(Above) Filter magnitude and phase response.

(Right) Data capture at baseband of a 923 MHz tone.

Fig 2. When applying the filter above we, notice deterioration of the baseband signal

(a)

(c) (d)

(b)

In (a) we interpolate. Following this, the baseband signal is mixed- up resulting in (b). In (c) our filter is applied, and although the artifact still occurs - it will be removed upon mixing-down the signal in (d). After decimation, comparing figure 2 and 3 shows the successful removal of the artifact that previously occurred at -1 MHz.

Fig 3. After decimation, successful filtering and artifact removal

Our hardware interference cancellation system successfully removes a single real-world, large power NBI. In MATLAB we are able to significantly attenuate the presence of multiple NBIs. And can handle the removal of interference up to 1.5 MHz wide at the baseband.

Hardware Realisation of IIR filter

This is a largely unproven method in high frequency radio applications. This form of AIC is complex to implement and requires significant computing resources. Demodulation of the interfering signal and the need to maintain extremely precise frequency and phase tracking make this method infeasible outside of the optical domain.

Destructive Interference Canceller

Fig 4. Interference suppression declines significantly with miniscule phase variation

Using our MATLAB model, we proceed to a hardware realisation using Icarus - a Verilog simulation tool - prior to implementing our design on a Spartan 6 XC6SLX150 FPGA.

The Verilog designs performance was evaluated in GTKWave, and matched our model. Viewing the “i_filtered_data” waveform in figure 5 shows successful removal of a high powered sinusoidal tone, meaning the hardware implementation, after synthesis, should perform as desired.

Outputting both “i_filtered_data” and “q_filtered_data” into a .csv file we are able to import this data into Octave and view a PSD plot, shown in figure 6 - there remains an artifact, circled in red.

Fig 5. (Top) Sinusoidal interferer “i_data_in” (Bottom) “i_filtered_data” in GTKwave Filter applied to 1 MHz

interferer at baseband causes artifact

Results

Fig 6. PSD plot from Verilog simulation

IC system on multiple NBIs

IC system on a wide interferer