20 minimising fh-nbi through interference cancellation · · 2014-12-01width of the filter....
TRANSCRIPT
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