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Parameterized Cancellation of Partial-Band Partial-Block-Duration Interference for Underwater Acoustic OFDM Zhaohui Wang , Shengli Zhou , Josko Catipovic , and Peter Willett Department of Electrical and Computer Engineering

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SLIDE 1

Parameterized Cancellation of Partial-Band Partial-Block-Duration Interference for Underwater Acoustic OFDM

Zhaohui Wang†, Shengli Zhou†, Josko Catipovic‡, and Peter Willett†

†Department of Electrical and Computer Engineering

University of Connecticut

‡Naval Undersea Warfare Center

WUWNet’11 Seattle December 1-2, 2011

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 1 / 22

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SLIDE 2

Outline

1

Motivation with a story

2

Interference parameterization

3

The proposed OFDM receiver for interference cancellation

4

Simulation results

5

Experimental results

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 2 / 22

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SLIDE 3

An Interesting Story Behind This Work

  • Dr. Catipovic to Dr. Zhou: I’ve collected some OFDM data in the AUTEC. Can

you decode it?

  • Dr. Zhou to Zhaohui: Can you have a try?

Zhaohui to Dr. Zhou: Sure. Zhaohui to Dr. Zhou: Most files can be decoded, but several others cannot.

  • Dr. Zhou to Zhaohui: What’s the reason?

Zhaohui to Dr. Zhou: Here it is, the unknown waveform of 45 ms, 2 kHz

  • the time domain waveform

2 4 6 8 10 −0.03 −0.02 −0.01 0.01 0.02 0.03 Time [s] Passband signal HFM Preamble Interference Interference ZP−OFDM Blocks

  • the time-frequency spectrum
  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 3 / 22

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SLIDE 4

An Interesting Story Behind This Work (Cont.)

  • Dr. Catipovic: These could be the interference from sonar users.
  • Dr. Zhou to Zhaohui: Can you do an investigation on the existing interference

cancellation methods? Zhaohui to Dr. Zhou: Sure.

◮ Limited research on interference cancellation in UWA communications ◮ External interference cancellation in radio communications ⋆ Narrowband interference with large time-duration ⋆ Impulsive interference with large bandwidth

Zhaohui to Dr. Zhou: These methods do not apply to this kind of partial-band (e.g., 2 kHz vs 5 kHz) partial-block-duration (e.g., 45 ms vs 150 ms) interference

  • Dr. Zhou to Zhaohui: Hmm..., let’s develop our own method.

After a couple of weeks’ thinking and discussions, on a sunny morning, ⇒ A parameterized interference cancellation approach! ¨ ⌣ ¨ ⌣ ¨ ⌣

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 4 / 22

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SLIDE 5

Generality of the Proposed Method

During the development of this work, we realized that... The method applies as long as the interference can be parameterized. External interference, e.g.,

◮ Unintentional interference, e.g. interference from sonar operations ◮ Interference from marine mammals ◮ Dumb malicious jamming, as you’ll see later

Internal interference, e.g.,

◮ Interblock interference ◮ Multi-access interference

⇒ How does it work?

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 5 / 22

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SLIDE 6

Interference Parameterization

Assume the prior of the interference time-duration TI and frequency band BI = [fIc − BI/2, fIc + BI/2], and define NI = ⌈BITI⌉ The baseband representation of the interference by NI unknowns: {cl} I(t) =

∞

  • l=−∞

clej2π l

TI t ≈

NI/2−1

  • l=−NI/2

clej2π l

TI t,

t ∈ [0, TI] , The passband representation of the interference, ¯ fl := fIc + l/TI ˜ I(t) = 2Re  

NI/2−1

  • l=−NI/2

clej2π¯

flt

  , t ∈ [0, TI]

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 6 / 22

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SLIDE 7

Impact of Interference on OFDM

Illustration of interference in OFDM in time domain

T

I I

T t ) ( ~ t y

g

T T

τI: relative delay of the interference to the start of the OFDM block Illustration of interference in OFDM in frequency domain

BI (MI subcarriers)

2 /

I

N

f

1 2 /

I

N

f

l

f

m

f NI samples

2 / K

f

1 2 / K

f

c

f

Ic

f

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 7 / 22

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SLIDE 8

Modeling Interference for Block-by-Block OFDM Receiver

For interference ˜ I(t − τI), frequency component at the mth subcarrier ν[m] = 1 T T+Tg ˜ I (t − τI) e−j2πfmtdt = e−j2πfmτI

NI/2−1

  • l=−NI/2

ulρm,l with ul = TI/Tcl, ρm,l=sin(π(fm−¯ fl)TI) π(fm−¯ fl)TI e−jπ(fm−¯

fl)TI

A compact representation: ν = ν[−K/2], · · · , ν[K/2 − 1]T ν = Λ(τI)ΓIu where, Λ(τI): a K × K diagonal matrix, ΓI: a matrix of size K × NI, and u: a column vector of length NI, with [Λ(τI)]m,m=e−j2πfmτI, [ΓI]m,l=ρm,l, u=

  • u−NI/2, · · · , uNI/2−1

T

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 8 / 22

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SLIDE 9

OFDM System Model for Interference Cancellation

The input-output relationship in the frequency domain z = Hs + Λ(τI)ΓIu + w Unknowns to estimate:

◮ Channel matrix: H ◮ Information symbols in s ◮ Interference vector: u ◮ Interference relative delay: τI in Λ(τI)

What we have:

◮ Frequency measurements: z ◮ Pilot symbols in s

Receiver structure

◮ Joint estimation of all the unknowns using the Bayesian method:

high computational complexity

◮ Iterative receiver structure: to estimate the unknowns iteratively

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 9 / 22

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SLIDE 10

An OFDM Receiver for Interference Cancellation

No Interference Subtraction Nonbinary LDPC Decoding i = i + 1 Decisions Yes Preprocessing, i = 0 Success or i = imax? Channel Estimation Noise Variance Update Noise Variance Update Yes No Interference Present? GLRT Interference Detection ICI Equalization Initialization

z = Hs + Λ(τI)ΓIu + w Initialization: with interference prewhitening, to get an initial estimate of H and s GLRT detector: to estimate the interference parameter and detect the presence of the interference Conventional OFDM receiver: channel estimation, ICI equalization and nonbinary LDPC decoding Stopping criteria: once the parity check condition of the nonbinary LDPC decoder is satisfied, or i reaches imax, the iteration stops.

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 10 / 22

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SLIDE 11

Interference Detection and Estimation

With ˆ H and ˆ s from the initialization or the last iteration Define Θ as a selector matrix of size MI × K, with MI = ⌈BI/∆f⌉ The interference frequency components within the frequency band ¯ z = Θ(z − ˆ Hˆ s) = B(τI)u + ¯ w, where B(τI) = ΘΛ(τI)Γ, ¯ w = Θw + Θ(ˆ Hˆ s − Hs) Assume ¯ w ∼ CN(0, σ2

BIIMI)

H0: absence of interference H1 : presence of interference Generalized log-likelihood ratio test (GLRT) statistic L(¯ z) = max

{τI,u} ln f(¯

z|τI, u, H1) f(¯ z|H0) = max

{τI,u}

1 σ2

BI

¯ zHB(τI)u + uHBH(τI)¯ z − uHBH(τI)B(τI)u

  • ≷Γth
  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 11 / 22

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SLIDE 12

Interference Detection and Estimation

Define an objective function J = ¯ zHB(τI)u + uHBH(τI)¯ z − uHBH(τI)B(τI)u Setting the derivative ∇Ju to zero yields ˆ u =

  • BH(τI)B(τI)

−1 BH(τI)¯ z, The estimate of τI: obtained using 1-D grid search ˆ τI = arg max

τI

¯ zHB(τI)

  • BH(τI)B(τI)

−1 BH(τI)¯ z, GLRT statistic L(¯ z) = 1 σ2

BI

ˆ uHBH(ˆ τI)B(ˆ τI)ˆ u≷Γth Γth: determined based on the probability of detection or the probability of false alarm

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 12 / 22

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SLIDE 13

Conventional OFDM Receiver Processing

If the presence of the interference is declared, the desired OFDM components ˇ z = z − Λ(ˆ τI)ΓIˆ u = Hs + ˇ w, Sparse channel estimator: to estimate H LMMSE equalizer: to estimate information symbols in s Nonbinary LDPC decoder: to recover information bits

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 13 / 22

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SLIDE 14

Simulation Setup

OFDM Parameters: Center frequency: fc = 13 kHz Bandwidth: B = 9.77 kHz Number of subcarriers: K = 1024, including 672 data subcarriers, 256 pilot subcarriers, and 96 null subcarriers Symbol duration: T = 104.86 ms Guard interval: Tg = 24.6 ms A rate-1/2 nonbinary LDPC code and a 16-QAM constellation are adopted. Channel profile: The sparse channel: 10 discrete paths Inter-arrival time of paths: exponentially distributed with the mean 1 ms Amplitudes of paths: decay exponentially, with the difference at the beginning and end of the guard interval 20 dB The Doppler rate of each path: a zero mean uniform distribution with the std σvfc/c, with σv: std of platform speed; c: sound speed in water 1500 m/s.

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 14 / 22

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SLIDE 15

Simulation Setup

Interference parameters:

◮ Interference generation: pass white Gaussian noise of a given

duration through a bandpass filter

◮ Time duration: TI = 26.2 ms ◮ Center frequency: fIc = 15 kHz ◮ Bandwidth: BI = 2.4 kHz ◮ Delay relative to OFDM block: τI ∼ U[0, T + Tg − TI]

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 15 / 22

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SLIDE 16

BLER Performance: Test Case 1

With a fixed SIR: 0 dB σv = 0 m/s

8 9 10 11 12 13 14 15 10

−3

10

−2

10

−1

10 SNR per Symbol [dB] BLER Conventional Proposed Without interference With interference

σv = 0.20 m/s

7 8 9 10 11 12 13 14 10

−3

10

−2

10

−1

10 SNR per Symbol [dB] BLER Conventional Proposed Without interference With interference

With interference, the proposed receiver → the conventional receiver without interference Without interference, slight performance degradation with the proposed receiver

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 16 / 22

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SLIDE 17

BLER Performance: Test Case 2

With different SIRs: σv = 0 m/s

8 9 10 11 12 13 14 15 10

−3

10

−2

10

−1

10 SNR per Symbol [dB] BLER SIR = −12dB SIR = −9dB SIR = −6dB SIR = −3dB SIR = 0dB SIR = 6dB

As the SIR ↑, the proposed receiver → conventional receiver in the scenario without interference

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 17 / 22

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SLIDE 18

AUTEC10: Experiment Setup

Location: the AUTEC around the Andros Island, Bahamas, 2010 Stationary receiver: the receiving hydrophones and the transmitter were at least 4 km apart, and depths of them varied from 1.5 km to 2 km OFDM parameters:

◮ Center frequency: fc = 11 kHz ◮ Bandwidth: B = 5 kHz ◮ Number of subcarriers: K = 860, with 336 data subcarriers ◮ Symbol duration: T = 170.7 ms ◮ Guard time: Tg = 250 ms ◮ A rate-1/2 nonbinary LDPC code and a QPSK constellation are

used. Interference parameters:

◮ Center frequency: fIc = 13 kHz ◮ Bandwidth: BI = 2 kHz ◮ Time duration: TI = 45 ms

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 18 / 22

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SLIDE 19

BLER Performance: Test Case 1

Semi-experimental data set: created by adding Gaussian noise to the interference-contaminated received signal BLER performance at different SNRs: SIR varies from 0 dB to 9 dB

7 8 9 10 11 12 0.001 0.01 0.1 1 SNR [dB] BLER it 0 it 1 it 3 it 5

without interference cancellation with interference cancellation

The proposed receiver outperforms the conventional receiver considerably

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 19 / 22

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SLIDE 20

BLER Performance: Test Case 2

Semi-experimental data set: created by adding two received waveforms, SNR about 7.9 dB The relative delay: τI ∼ U[0, T + Tch − TI], with channel length Tch = 10 ms

1 2 3 4 5 6 7 8 9 −0.02 −0.01 0.01 0.02 Time [s] Passband signal OFDM Blocks HFM Preamble 1 2 3 4 5 6 7 8 9 −0.02 −0.01 0.01 0.02 Time [s] Passband signal

BLER performance at different SIRs

−4 −2 2 0.001 0.01 0.1 1 SIR [dB]

BLER

it 0 it 1 it 3 it 5 without interference cancellation with interference cancellation

The proposed receiver outperforms the conventional receiver considerably Iterative interference cancellation helps

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 20 / 22

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SLIDE 21

Conclusions

Proposed a general parameterized representation of the partial-band partial-block-duration interference in UWA communications; Developed an iterative interference cancellation receiver for UWA OFDM; Simulations and experimental results demonstrated that the proposed receiver outperforms the conventional receiver considerably; Further investigation is required to address the interference with fast-varying time-frequency characteristics.

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 21 / 22

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Thanks ! Questions ?

  • Z. Wang, S. Zhou, J. Catipovic, and P

. Willett () Interference Cancellation for OFDM WUWNet’11 22 / 22