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

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  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