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DESIGN AND USRP2 IMPLEMENTATION OF ADAPTIVE SPECTRUM SENSING AND INTERFERENCE SUPPRESSED
SECONDARY TRANSMISSION FOR DSA
1
Predrag Spasojevic
Samson Sequeira, Srinivas Pinagapany, Ashwin Revo, (WINLAB, Rutgers U) Yasunori Futatsugi, Masayuki Ariyoshi (NEC Corporation, Japan)
WINLAB Industrial Advisory Board Meeting December 2, 2010
Opportunistic Spectrum Access 2
potential solution to the spectrum scarcity crisis.
incumbent primary users are protected from opportunistic secondary interference.
secondary users assured some QoS: do not have a dedicated bandwidth and have to overcome the interference from the primary.
adaptive sense-and-carefully-transmit system that
detects the presence of primary users
provides opportunistic access to secondary users in the vacant frequencies.
Experimental Objectives: Avoiding the Primary
3
a) On/Off: ♦ no secondary transmission when a
primary user is detected and
♦ transmit using OFDM when the primary user is not detected.
b) NC-OFDM: ♦ When primary is detected, the
corresponding NC-OFDM sub-carriers are deactivated
c) NC-OFDM w/ IA-PFT suppression: ♦ time windowing and cancellation
carrier schemes which result in maximum side-lobe suppression.
Opportunistic Spectrum Access 4
Transmission data
Quiet period
Transmission data
Quiet period
Time
Freq
uenc
y
Time
Freq
uenc
y
Time
Freq
uenc
y
Primary system operation
Secondary system operation
Link P1
Link P2
Link S1
Ch1 Ch2 Ch3 Ch4
Ch1 Ch2 Ch3 Ch4
Start
Spectrum Sensing
Primary existence?
NC-OFDM transmission
Regular OFDM transmission
YES
NO
Quiet period
System Model 5
Primary Tx: Wireless Mic FM signal
Emulated on VSG
Secondary Tx: USRP2 Spectrum Sensing
NC-OFDM w/ IA-PFT suppression
Secondary Receiver: USRP2 NC-OFDM
Spectrum sensing power spectral density (PSD) detector
detection threshold: frequency dependent noise floor estimation
NC-OFDM transmitter The subcarriers used by primary transmitter are deactivated.
Interference Avoidance by Partitioned Frequency- and Time-domain (IA-PFT) suppresses the spectral leakage within the spectrum inactivity range
Primary Transmitter: Wireless Mic 6
Vector Signal Generator transmits the wireless microphone signal. FM signal: tone frequency and frequency deviation depend on the mode of operation Signal bandwidth 200kHz The transmit power is -30dBm
Noise Power Calculation
Sensing Module: Block Diagram 7
PSD Calculation
Detection Threshold
Decision Spectral Mask
Secondary Transmitter
Automatic noise floor estimation based on rank-order filtering
Motivation for Automatic Noise Floor Estimation
8
Measuring the noise floor when the signal is not present requires taking the system offline.
Calibrating the system in this way is a lengthy and tedious procedure.
Noise floor may change with time and also with the aging of components.
Changes in the signal environment will cause the noise floor to change.
Noise floor may not be flat over the entire bandwidth.
Ready, M.J.; Downey, M.L.; Corbalis, L.J.; , "Automatic noise floor spectrum estimation in the presence of signals," Signals, Systems & Computers, 1997. Conference Record of the Thirty-First Asilomar Conference on , vol.1, no., pp.877-881 vol.1, 2-5 Nov 1997
Noise Floor Estimation 9
1. calculate PSD 2. “open” the PSD vector:
♦ “erode” the vector ♦ “dilate” the vector.
3. change in the noise floor is very small? ♦ Yes: stop ♦ No: Go to 2
♦ why “open”? ♦ eliminates the spectral peaks
from the PSD at each step.
Yes
No
Open
Calculate PSD
Erode
Dilate
Noise-Floor
Increase kernel size K
while change_in_power
> epsilon
Rank-Order Filter 10
A rank-order filter of rank ‘m’ takes a vector of length ‘N’ as the input and outputs the ‘mth’ smallest value in the vector.
Erode: Rank-order filter the bins corresponding to the kernel in the PSD vector with rank 1, i.e. R(K, 1).
Dilate: Rank-order filter the bins corresponding to the kernel in the PSD vector with rank K, i.e. R(K, K).
Sort in ascending
order
Output mth value
N – length vector mth – smallest value
Noise Floor Estimation: Results 11
Secondary Transmitter: Non-Contiguous OFDM
12
Non-Contiguous OFDM (NC-OFDM) transceiver: a modified OFDM transceiver in which few of the subcarriers are
deactivated as dictated by the carrier mask
Center frequency Secondary
Primary
f 0 0 0 0 1 1 0 0 0 1 0 0 0 1 1 1 0 1 1 1 0 0 1 1 1
Carrier mask
NC-OFDM Transceiver Architecture 13
RF up convert
Adaptive Spectrum Sensor
Carrier Mask
RF down convert
Channel
Sub-carrier mapper
OFDM Modulator
Serial-to-Parallel
OFDM Demodulator
Sub-carrier mapper
Parallel-to-Serial
Digital-to-Analog
Analog-to-Digital
NEC’s IA-PFT System Model
Cancellation Carriers
NC-OFDM Modulator
Time Windowing
NC-OFDM Modulator
+Modulator Sub-carrier
Mapper IA-PFT Transmission
IA-PFT System Description
Time Windowing (TW) : The time windowing block shapes the CP to reduce the spectral
leakage in the notch Time windowing is more efficient in reducing leakage at the
center of the notch
Cancellation Carriers (CC): Lobes on either side of the notch are suppressed by CC tones 2 CC tones are added on either side of the notch CC is more efficient in suppressing leakage at the
edges of the notch
Cancellation Carriers
CC:
2 CC sub-carriers added on either side of notch
CC tones
CC suppressed sub-carriers
Sub-carrier index
IA-PFT: TW+CC
CC Zero Padding: CC symbol are zero padded to match the length of the time windowing
OFDM symbols
IA-PFT: CC and Time Windowing streams are combined
Time
OFDM symbol (i+1) CP OFDM symbol (i) CP Freq
uenc
y
ZP ZP OFDM symbol (i) CC tones symbol (i)
OFDM symbol (i+1) CC tones symbol (i+1)
Spectrum Diagram 18
3.84 MHz
2.16 MHz
RB0 RB1 RB2 RB3 RB5 RB7 RB8 RB9 RB10 RB11 RB6 RB4
BIN1 BIN2 BIN3 BIN4 BIN5 BIN6 BIN7 BIN8 BIN9 BIN10 BIN11 BIN0
180 kHz
15 kHz
FFT size = 256 Resource Block (RB) bandwidth = 180 kHz No. of frequency bins in each RB = 12 Total number of significant frequency bins = 144
Interference avoidance 19
During sensing time
Sensing result 0 0 0 0 0
NC-OFDM mask 0 0 0 0 0
During transmission time
Transmitted RBs
LTE –RB 180 kHz
f
f
Primary OFF
During sensing time
Sensing result 0 0 1 0 0
NC-OFDM mask 0 1 1 1 0
During transmission time
Transmitted RBs
WM 200 kHz
f
Primary ON
When a RB is occupied, the particular RB and its 2 adjacent RBs are nulled out from the NC-OFDM transmission
In addition to this IA-PFT technique is used to reduce the out of band emissions in the notch.
Simulation 20
Experimental setup
USRP2
Wireless Microphone (Vector Signal Generator Antenna)
16.5 m
Sensing Unit & Secondary Transmitter
Monitor
19.1 m
10.4 m
21
Results: MATLAB Simulated 22
Spectrum of MATLAB simulated IA-PFT based NC-OFDM transmitter.
Results: GNU Radio Output 23
~12 dB
Results: Spectrograms 24
Primary transmitter (Wireless Mic: FM BW = 200 kHz)
Secondary transmitter (OFDM: BW = 500 kHz)
Primary transmitter (Wireless Mic: FM 200 kHz)
Secondary transmitter (NC-OFDM: BW = ~ 2.16 MHz)
video: http://www.orbit-lab.org/~srinivas/videos/ADSA_FINAL.mpeg
Advanced DSA
Normal DSA