16HomePlug AV2

16.1 INTRODUCTION

The HomePlug AV 2.0 Specification (AV2) adds new features to HomePlug AV that

provide a significant increase in throughput and coverage performance. The Home-

Plug AV 2.0 Specification provides a significantly higher data rate of 1.5 Gbps

compared to 200Mbps for HomePlug AV. A notable performance point for AV2 is

the coverage performance of �90Mbps User Datagram Protocol (UDP) network

throughput (three equal UDP streams of 30Mbps each) for 99% of networks with

four or more devices assuming immediate repeating is implemented, based on field

test measurements.

New physical layer features include Multiple Input Multiple Output (MIMO),

wider frequency band, efficient notching and short (lower overhead) delimiter. New

MAC Layer features include delayed acknowledgment, immediate repeating, and a

power save mode. The following sections provide detail on each of these features.

16.2 MIMO

Most powerline wiring contains three individual wires: Line (or Phase), Neutral, and

Ground (or Protective Earth). Most powerline modems such as HomePlug AV 1.1

modems use the Line-Neutral wire pair for communication. However, the third wire,

Ground, may be used in combination with either Line or Neutral to support sending a

312

HomePlug AV and IEEE 1901: A Handbook for PLC Designers and Users, First Edition.

Haniph A. Latchman, Srinivas Katar, Larry Yonge, and Sherman Gavette.

� 2013 by The Institute of Electrical and Electronics Engineers, Inc. Published 2013 by John Wiley & Sons, Inc.

second, independent signal to increase capacity. Note that only two unique signals

can be transmitted on three wires due to Kirchoff’s voltage law: the signal third wire

pair is the sum of the signals on the other two wire pairs.

On receive, there are up to four possible receive channels: Line-Neutral, Line-

Ground, Neutral-Ground, and common mode. Note that the three wire pairs are

subject to Kirchoff’s voltage law, but in practice the third wire pair does provide

additional diversity evidently because of non-ideal implementations of the coupling

to the powerline wiring. Common mode is the signal between true earth ground and

the three wire pairs and is generally only available in products that have a relatively

large effective ground plane, such as a flat screen television.

Note that is some cases (e.g., older construction), and in some countries such as

Japan, the Ground wire is not present. MIMO cannot be used in these cases.

2xN MIMO spatial multiplexing with Eigen beamforming is specified in Home-

Plug AV. The “2” in “2xN MIMO” refers to two transmitters and “N” refers to the

number of receivers, which can be 2, 3, or 4 receivers. The typical performance gain

from MIMO compared to HomePlug AV 1.1 (SISO—Single-Input Single-Output) is

100% (two times) on all channels ranging from poor channels to good channels.

Eigen beamforming is required in order to achieve a performance benefit from

MIMO on poor channels.

Figure 16.1 shows a block diagrammatic representation for the physical layer of

the HomePlug AV2 transmitter and receiver based on a 200MHz Sampling Clock.

On the transmitter side, the PHY layer receives its inputs from the Medium Access

Control (MAC) Layer. Three separate processing chains are shown because of the

different encoding for HomePlug 1.0.1 Frame Control (FC) data, HomePlug AV2

Frame Control data, and HomePlug AV2 Payload data. AV2 Frame Control data is

processed by the AV2 Frame Control Encoder, which has a Turbo Convolutional

Encoder and Frame Control Diversity Copier while the HomePlug AV2 payload data

streampasses through a Scrambler, a TurboConvolutional Encoder, and an Interleaver.

The HomePlug 1.0.1 Frame Control data passes through a separate HomePlug 1.0.1

FrameControlEncoder. Theoutputs of the FCEncoders andPayloadEncoder lead into

a commonMIMOOrthogonal Frequency DivisionMultiplexing (OFDM)Modulation

structure, consisting of a MIMO Stream Parser that provides up to two independent

data streams to two transmit paths which includes two Mappers, a phase shifter that

applies a 90� phase shift to one of the two streams, a MIMO precoder to apply

transmitter beamforming operations, two Inverse Fast Fourier Transform (IFFT)

processors, Preamble, and Cyclic prefix insertion, and symbol Window and Overlap

blocks, which eventually feeds the Analog Front End (AFE) module with one or two

transmit ports that couple the signal to the powerline medium.

At the receiver, an AFE with NRX¼ one, two, three, or four receive ports operates

with individual Automatic Gain Control (AGC) modules and one or more time-

synchronization modules to feed separate Frame Control and Payload data recovery

circuits. Receivers plugged to power outlets which are connected to the three wires

Line, Neutral, and Protective Earth might utilize up to three differential mode Rx

ports and one common mode Rx port. The common mode signal is the voltage

between the sum of signals on the three wires and the ground. The Frame Control

MIMO 313

data is recovered by processing the received signals through a 1024-point FFT (for

HomePlug 1.0.1 delimiters) and multiple 8192-point FFTs, and through separate

Frame Control Decoders for the HomePlug AV2/AV1.1 and HomePlug 1.0.1 Modes.

The payload portion of the sampled time domain waveform, which contains only

HomePlug AV2 formatted symbols, is processed through the multiple 8192-point

FFT (one for each receive port), a MIMO Equalizer that receives NRX signals,

performs receive beamforming and recovers the two transmit streams, two Demod-

ulators, a Demultiplexer to combine the two MIMO streams and a Channel De-

interleaver followed by a Turbo Convolutional Decoder and a De-scrambler to

recover the AV2 Payload data.

MIMO has an added benefit when communicating with legacy SISO devices or

AV2 SISO devices. The two transmitters can use beamforming to improve the signal

arriving at a SISO device. The two or more receivers can use Maximal Ratio

Combining to improve performance when receiving from a SISO device.

16.3 EXTENDED FREQUENCY BAND

The HomePlug AV 1.1 Specification utilizes the frequency band from 1.8 to 30MHz,

and the IEEE 1901 Standard increase that to 1.8–50MHz. HomePlug AV2 extends

the frequency band even further to 1.8–86.13MHz.

One of the challenges with the frequency band above 30MHz is the regulations

require a 25–30 dB reduction in transmit power spectral density (PSD) above

30MHz. The powerline wiring radiation characteristics obviously do not change

suddenly at 30MHz. Rather, this is an artifact due to the way the regulations are

specified for different frequency bands. In the United States, at least three factors

in the FCC Part 15 regulations contribute to this step in PSD at 30MHz: a

reduction in the measurement distance from 30 to 3m, an increase in the field

strength from 30 to 100mV/m and an increase in the measurement bandwidth from

9 to 120 kHz.

The performance gain provided by the 30–86.13MHz band is generally quite

high on medium to good channels due to the relatively wide channel bandwidth.

However, this additional band does not provide much benefit on the poorest

channels, for example, the worst 5% of connections, because of the low PSD level

allowed at the transmitter. However, this band does contribute to the coverage

performance in two ways. First, most powerline channels fall in the good to

medium category, and the higher data rate provided on these channels enables

them to reduce the time-on-wire for their traffic thus enabling more time-on-wire

to be available for traffic on poorer channels. Also, when the higher frequency

band is combined with immediate repeating, described in Section 16.6, the

performance on the poorest paths can typically be improved dramatically by

taking advantage of higher data connections through a repeater. This combination

is a significant factor in achieving the 99% coverage performance mentioned in

Section 16.1.

EXTENDED FREQUENCY BAND 315

16.3.1 Power BackOff

Power backoff is a feature that enables higher performance on relatively good

powerline channels. Practical implementations of the transmitter and receiver have

limited dynamic range and thus the OFDM carriers above 30MHz suffer distortion at

the reduced PSD level. This is largely due to quantization noise of the digital-to-analog

and analog-to-digital converters and the limited linearity of the transmit line driver. For

example, a system may support 40 dB dynamic range for the band below 30MHz, but

only 10 dB is available for the band above 30MHz with a 30 dB step in PSD.

To address this problem on good channels where the channel SNR can be quite

high, the transmit PSD in the band below 30MHz can be reduced so that the

distortion to the OFDM carriers above 30MHz can be reduced. For example, a 10 dB

backoff in PSD below 30MHz can increase the dynamic range of the carriers above

30MHz by 10 dB (i.e., allows 20 dB of dynamic range).

16.4 EFFICIENT NOTCHING

In HomePlug AV 1.1, OFDM with windowing was specified to support 30 dB deep

notches without the need for FIR or IIR filters. This feature provides for program-

mable notches as may be needed for different regulatory regions or to accommodate

changes in regulations and to permanently notch the Amateur bands. In HomePlug

AV 1.1, the default tone mask specifies 917 active carriers out of 1155 total carriers.

A total of 238 OFDM carriers are permanently turned off out to create notches for the

10 Amateur bands in the 1.8–30MHz frequency band.

However, the approach to deep notches used in HomePlug AV1.1 requires �12%

in overhead for additional guard interval needed to support the time domain window,

which is 4.96ms in duration, and 8% in overhead to support a guard band of 195 kHz

(eight OFDM carriers) for each notch required to achieve the 30 dB notch depth.

The requirement to notch using windowing was removed from the HomePlug AV

2.0 Specification to allow for implementations to allow alternate implementation of

the transmitter to reduce this overhead, such as adding fixed and/or programmable

FIR or IIR filters, or a combination of windowing and filters. To support this, smaller

guard intervals were added and protocol changes were made to support additional

OFDM carriers when supported by a transmitter (e.g., when a small guard band can

be used, some carriers in the guard band used in AV1.1 can be enabled). This feature

will also help products that need to meet CENELEC EN50561-1 requirements (refer

to Section 1.3.3.2) to minimize the performance loss due to the additional notch

requirements for the aeronautical and broadcast bands.

16.5 SHORT DELIMITER AND DELAYED ACKNOWLEDGMENT

The Short Delimiter and Delayed Acknowledgment features were added to Home-

Plug AV2 to improve efficiency by reducing the overhead associated with trans-

mitting payloads over the powerline channel. In HomePlug AV 1.1, this overhead

316 HomePlug AV2

results in relatively poor efficiency for TCP payloads. One goal that was achieved

with these new features was TCP efficiency was improved to be relatively close to

that of UDP.

In order to send a packet carrying payload data over a noisy channel, signaling is

required for a receiver to detect the beginning of the packet and to estimate the

channel so that the payload can be decoded, and additional signaling is needed to

acknowledge the payload was received successfully. Inter-frame spaces are also

required between the payload transmission and the acknowledgment for the proc-

essing time at the receiver to decode and check the payload for accurate reception

and to encode the acknowledgment. This overhead is even more significant for TCP

payload since the TCP acknowledgment payload must be transmitted in the reverse

direction.

16.5.1 Short Delimiter

The delimiter specified in AV 1.1 contains the preamble and frame control symbols

and is use for the beginning of data PPDUs as well as for immediate acknowledg-

ments. The length of the AV 1.1 delimiter is 110.5ms and can represent a significantamount of overhead for each channel access. A new single OFDM symbol delimiter

is specified in AV2 to reduce the overhead associated with delimiters by reducing the

length to 55.5ms. Figure 16.2 shows that every fourth carrier in the first OFDM

symbol is assigned as a preamble carrier, and the remaining carriers encode the

Frame Control. The following OFDM symbols encode data the same as in AV1.1.

One of the limitations of the Short Delimiter is that it cannot be easily detected

asynchronously, which is necessary for CSMA channel access. Thus, reception of the

short delimiter requires that the receiver know the position in time where the Short

Delimiter was transmitted. Thus, the use of the Short Delimiter is limited to Selective

Acknowledgment of CSMA and TDMA Long MPDUs, Reverse Start-of-Frame, and

TDMA Start-of-Frame.

Figure 16.3 shows a PPDU with the Short Delimiter. The sample points indicated

are based on a 200MHz clock. The guard interval for the Short Delimiter is 9.56ms

Preamble Carrier Frame Control Carrier Data Carrier

OFDM Symbol 1 OFDM Symbol 2 OFDM Symbol N

FIGURE 16.2 Short delimiter.

SHORT DELIMITER AND DELAYEDACKNOWLEDGMENT 317

to provide for sampling error by the receiver. The receiver can detect the sampling

error from the Preamble Carriers, and since every fourth carrier is a Preamble carrier

the receiver can detect and error up to one fourth of the OFDM symbol time, or

�5.12ms, and will use the correction for the sampling location of the data symbols as

well as to correct for the sampling location for receiving future Short Delimiters from

a particular transmitter.

The typical decoding process for receiving an AV1.1 PPDU is as follows:

1. Detect the presence of the Preamble signal.

2. Estimate the position of the Preamble signal.

3. Generate an estimate of the channel from the Preamble signal.

4. Sample the Frame Control symbol based on the position estimate from the

Preamble, perform an FFTand decode the Frame Control using the estimate of

the channel from the Preamble signal.

5. Estimate the channel from the Frame Control by re-encoding the Frame

Control to determine the reference.

6. Perform an FFT and decode each payload symbol using the estimate of the

channel from the Frame Control and the tone map based on the index indicated

in the Frame Control data.

Decoding of the payload of an AV2 PPDU with the Short Delimiter is similar:

1. Perform an FFT of the Short Delimiter symbol based on the position estimate

2. Generate an estimate of channel from the Preamble carriers

3. Decode the Frame Control using the estimate of the channel from the Preamble

carriers

4. Re-encoded the Frame Control and generate and estimate of the channel from

the Preamble and Frame Control carriers

5. Perform an FFT and decode each payload symbol using the estimate of the

channel from the Frame Control and the tone map based on the index indicated

in the Frame Control data

Figure 16.4 demonstrates the efficiency improvement when the AV2.0 Short

Delimiter is used for the acknowledgment of a CSMA Long MPDU compared to the

AV1.1 delimiter. Not only is the length of the delimiter reduced from 110.5 to

55.5ms, the Response Inter-Frame Space (RIFS) and Contention Inter-Frame Space

AV2 SD

992

GISD

81921912

D1GI1512

81921512

D2GI1512

81921512

D3GI

8192X

FIGURE 16.3 PPDU format with Short Delimiter.

318 HomePlug AV2

(CIFS_AV) can be reduced to 5 and 10ms, respectively. Reduction of RIFS requires

delayed acknowledgment, which is described in Section 16.5.2. Backwards com-

patibility when contending with legacy AV 1.1 devices is maintained by indicating

the same length field for virtual carrier sense in both cases so that the position of the

PRS contention remains the same. A field in the Frame Control of the Long MPDU

indicates the Short Delimiter format to an AV2.0 device so that it can correctly

determine the length of the payload.

16.5.2 Delayed Acknowledgment

The processing time to decode the last OFDM symbol and encode the acknowledg-

ment can be quite high, thus requiring a rather large the Response Inter-Frame Space

(RIFS). In AV1.1, since the Preamble is a fixed signal, the preamble portions of the

acknowledgment can be transmitted while the receiver is still decoding the last

OFDM symbol and encoding the payload for the acknowledgment. With the Short

Delimiter, the preamble is encoded in the same OFDM symbol as the payload for the

acknowledgment, so the RIFS would need to be larger than for AV1.1, eliminating

much of the gain the Short Delimiter provides.

Delayed acknowledgment solves this problem by acknowledging the segments

ending in the last OFDM symbol in the acknowledgment transmission of the next

PPDU, as shown in Figure 16.5. This permits practical implementations with a very

small RIFS, reducing the RIFS overhead close to zero. AV2 also allows the option of

delaying acknowledgment for segments ending in the second to last OFDM symbol

to provide flexibility for implementers.

16.5.3 TCP and UDP Efficiency Improvements

The combination of Short Delimiter and Delay Acknowledgment can provide a

significant improvement in efficiency. Figures 16.6 and 16.7 show the improvement

in throughput for the AV2 PPDU format with the Short Delimiter and Delay

Acknowledgment compared to the AV 1.1 PPDU format for CSMA channel access

and TDMA channel access, respectively. For example, the TCP throughput on a

channel that supports 70Mbps throughput with the AV 1.1 PPDU format will support

92Mbps with the AV2 PPDU format in CSMA. This assumes the PHY rate for the

payload portion of the PPDU is constant. A payload size of approximately 20,000

bytes was used for this analysis.

Preamble+FC+data Data (second) Data (last) SACK

RIFS

Preamble+FC+data Data (second) Data (last) SACK

RIFS

Acknowledge segments that endin the last data symbol in a future

(or next) SACK transmission

Acknowledge segments in all but thelast data symbol in the immediately

following SACK transmission

FIGURE 16.5 Delayed acknowledgment.

320 HomePlug AV2

16.6 IMMEDIATE REPEATING

AV2 supports repeating and routing of traffic to not only handle hidden nodes but

also to improve coverage (i.e., performance on the worst channels). The basic

repeating and routing function used by AV2 is the same as in IEEE 1901 and Green

PHY (refer to Section 14.2.6).

45

30 40 50 60 70 80 90 100 110 120

AV Data Rate (Mbps)

Impr

ovem

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(Mbp

s) 40

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30

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20

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5

0

UDPTCP

FIGURE 16.6 Throughput improvement for CSMA.

80

20 40 60 80 100 120 140 160

AV Data Rate (Mbps)

Impr

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V2

Dat

a R

ate

(Mbp

s)

70

60

50

40

30

20

10

0

UDPTCP

FIGURE 16.7 Throughput improvement for TDMA.

IMMEDIATE REPEATING 321

With HomePlug AV2, hidden nodes are extremely rare. However, some links may

not support the data rate required for some applications such as a 3D HD video

stream. In a network where there are multiple AV2 devices, the connection through a

repeater typically provides a higher data rate than the direct path for the poorest 5%

of channels.

Immediate repeating is a new feature in AV2 that enables high efficient repeating.

Immediate repeating provides a mechanism to use a repeater with a single

channel access, and the acknowledgment does not involve the repeater. As shown

in Figure 16.8, Station A transmits to the repeater R. In the same channel access,

repeater R transmits all payload received from station A to station B. B sends an

acknowledgment directly to A. With this approach, latency is actually reduced with

repeating, assuming the resulting data rate is higher, the obvious criteria for using

repeating in the first place. Also, resources required by the repeater are minimized

since the repeater uses and immediately frees memory it would require for receiving

payload destined for it. Also, the receiver has no retransmission responsibility for

failed segments.

Figure 16.9 shows the result of a coverage analysis based on data measured in 25

homes for the 2–86.13MHz frequency band where the channel characteristics for all

combinations of paths between five locations were measured in each home. The

analysis shows the coverage for networks of 2, 3, 4, and 5 AV2 SISO nodes where all

nodes are capable of repeating. For each connection in the network, the best

throughput of the direct path or one of all available two hop repeating paths is

selected. Note the 2 node case has the same results as the direct path for 3, 4, and 5

nodes network where repeating is not supported. Note that this analysis shows the

coverage throughput for the 98%, 99%, and 100% coverage points improve by

approximately 57%, 122%, and 409%, respectively, in the case of a five nodes

network with repeating.

16.7 POWER SAVE

The power save protocol in the HomePlug AV 2.0 Specification is the same as in the

HomePlug Green PHY Specification. Refer to Section 15.3.1 for more detail.

SOF Payload (A -> R) SOF Payload (R -> B) SACK

SOF IndicatesMPDU Burst

B acknowledgesdirectly to A

Segments from A are retransmitted by R to BBad segments are replaced with dummy segments

Normal ChannelContention

PRS CRS

FIGURE 16.8 Immediate repeating channel access for CSMA.

322 HomePlug AV2

16.8 SUMMARY

This chapter provided an overview of the HomePlug AV 2.0 Specification which

builds on the strengths of HomePlug AV but uses key extensions, such as MIMO,

augmented bandwidth, increased modulation density and repeating, to achieve

1.5Gbps throughput.

As illustrated especially in the last several chapters, HomePlug AV has become

the foundation of several other derivative works such as the IEEE 1901 Standard as

well as the HomePlug GreenPHY and AV2 Specifications.

It is the hope of the authors that this book, HomePlug AV and IEEE 1901: A

Handbook for PLC Designers and Users will prove useful as a PLC reference and in

giving an accessible exposition of the core technologies that have revolutionized

high speed Powerline Communications over the past decades.

0 10 20 30 40 50 60 70 80 90 10090

91

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96

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99

100

PHY Data Rate (Mbps)

Cov

erag

e (%

)

Direct3 Nodes4 Nodes5 Nodes

FIGURE 16.9 AV2.0 SISO PHY rate with repeating.

SUMMARY 323