Gautham Prasad and Lutz Lampe

Introducing In-Band Full Duplexing for Broadband Power Line

Communication

Gautham Prasad and Lutz Lampe

Or can we?

Introducing In-Band Full Duplexing for Broadband Power Line

Communication

Introduction

• Simultaneous bidirectional communication over the same line and in the same band potentially doubles transmission efficiency

• Other (more important?) benefits include:– Alternate PHY solution to the hidden node problem

– Full duplex relaying in multi-hop networks to improve throughput efficiency

– Continuous cognitivity

– Relaxed scheduling constraints

In-Band Full Duplex (IBFD)

• Hindrance: Interference of the large “self-transmitted” signal (echo/SI) with the received signal-of-interest (SOI)

To/From PLC

TX

RX

TXS

SOI

SOI Echo/SI

Echo Cancellation (EC)

• Very old – Analog telephone: transformer based isolation

– DSL/Ethernet/Wireless• Hybrid/Circulator: Elementary isolation in analog domain

• Analog Cancellation: analog circuitry to cancel non-linear components and achieve additional cancellation

• Digital cancellation: digital filters implemented in time or frequency domain or a combination of both

– Low-rate narrowband PLC: line hybrid and digital cancellation [IEC 62488-1:2012, Tripodi, Ferraro, Pighi, Raheli, “Echo cancellation in a power line modem in the presence of abrupt channel variations”, IEEE ISPLC 2014]

Factors for Consideration of IBFD for BB-PLC

• Cancellation gains:– Transmit PSD = -55dBm/Hz, noise PSD (NPSD) > -130dBm/Hz

– Max gain required for BB-PLC applications < 75 dB

• Non-linear effects: – BB-PLC Analog Front Ends restrict non-linear SI components at 75 to

80 dB below max SI power

• Increase in Quantization Noise (QN): – SQNRdB = 6.02ENOB + 4.77 - PAPRdB

– About 60 dB SQNR for a 12-bit ADC

– QN < -55 - 60 = -115 dBm/Hz

use hybrid and a digital cancellation circuit

Proposed IBFD Solution

• Use hybrid and a digital cancellation circuit

TX

FIR EC Filter

Hybrid

RX

To/From PLC

1

2

3

Active hybrid circuit, e.g. [Wenzel, “Low FrequencyCirculator/Isolator Uses No Ferrit or Magnet”, RF Design, 1991.]

Low Frequency Hybrid • Ideally, Z1=ZTX, Z2=ZPLC, Z3=ZRX

for maximum isolation• At Port 1:

ZS is typically small,AFE constraints=> Set Z1≅ ZS

• At Port 2:ZPLC is unknown and varying=> Set e.g. Z2=100W

• At Port 3:ZRX typically large=> Set Z3=ZRX

Hybrid Isolation – Preliminary Simulation Results

- Maximum isolation at ZPLC = 100 + j0 W

where isolation = Vs/VRX

Digital Cancellation

• The received signal contains

• This is used to train the adaptive filter weights using an LMS algorithm and the resulting estimated signal is subtracted fromy(n) y(n)

Digital Cancellation

• Weight update and signal cancellation can be performed in time or frequency or a combination of time and frequency domain

AFEs &AFEs &

LMS Adaptation to LPTV Changes

• Traditional LMS algorithm performs poorly under fast channel changes

• Channel changes in PLC are often Linearly Periodic Time Varying (LPTV)

• We modify the existing LMS algorithm to adapt to LPTV changes by re-using previously updated filter weights for the same channel condition

Our LPTV-LMS Algorithm• LPTV changes are periodic to one AC half cycle (HC)• Detect the cyclic channel change (CCC) positions

during the first/fresh HC• In every following HC, re-use the last updated

weights of every CCC from the previous HC• Detect non-CCC (N-CCC) by monitoring the resulting

MSE levels• Reset weights with an LS estimate and jump back to

the fresh HC when a N-CCC is detected

CC

CN

-CC

C

Some Preliminary Results*• The EC filter, which replicates the echo, is trained using the received signal y(n)

• Notice that the strength of the SOI component decreases with increase in PLC channel attenuation => less overall ‘noise’ for echo estimation, resulting in better echo estimate and higher EC gain (ECG)

*Note: use with caution

20 25 30 35 40 4540

42

44

46

48

50

52

54

56

58

60

PLC channel + hybrid attenuation (dB)

Inte

rfere

nce c

ancela

tion

gain

(dB

)

PSD

, dB

m/H

zTXS

Noise

SOI

SI

Ch

ann

el attenu

ation

When canceled-SI PSD is in -

DRG < 1

1 < DRG < 2

DRG = 2

Hybrid isolation

ECG

Canceled-SI

Data Rate Gain (DRG) – Some Estimations*

*Note: max. const. 1024 QAM, SNR-to-rate translation using results for CC rate ¾ from [Tsugi and Itami, “A study on adaptive modulation of OFDM under impulsive rate”, ISPLC 2008]

Noise PSD = -105dBm/Hz Noise PSD = -115 dBm/Hz

Ch. Att

ECG SNR BHD SCINR BFD DRG SNR BHD SCINR BFD DRG

14 46 36 10 22 6 1.2 46 10 22 6 1.2

20 52 30 8 21 6 1.5 40 10 21 6 1.2

26 56 24 6 20 6 2.0 34 10 20 6 1.2

34 58 16 4 15 4 2.0 26 6 15 4 1.3

Conclusions

• Interesting possibilities with IBFD for BB-PLC

• Suggest the use of a low frequency active hybrid to provide impedance balance and initial isolation

• In tandem with a digital echo cancellation• Need to work out some details on the

hybrid, EC, and the gains achievable