HomePlug AV2 is the solution identified by the HomePlug Alliance to achieve the improved data rate performance required by the new generation of multimedia applications without the need to install extra wires. Developed by industry-leading participants in the HomePlug AV Technical Working Group, the HomePlug AV2 technology provides Gigabit-class connection speeds over the existing AC wires within home. It is designed to meet the market demands for the full set of future in-home networking connectivity. Moreover, HomePlug AV2 guarantees backward interoperability with other HomePlug systems. In this paper, the HomePlug AV2 system architecture is introduced and the technical details of the key features at both the PHY and MAC layers are described. The HomePlug AV2 performance is assessed, through simulations reproducing real home scenarios.
The convergence of voice, video, and data within a variety of multifunction devices, along with the evolution of High Definition (HD) and 3-Dimensional (3D) video, are today driving the demands for home connectivity solutions. Home networks are required to support high throughput connectivity guaranteeing at the same time a high level of reliability and coverage (the percentage of links that are able to sustain a given throughput in two-node or multinode networks). Applications such as HD Television (HDTV), Internet Protocol Television (IPTV), interactive gaming, whole-home audio, security monitoring, and Smart Grid management have to be supported by the new home networks.
During the last decade, in-home power line communication (PLC) has received increasing attention from both the industry and research communities. The reuse of existing wires to deploy wide band services is the main source of attractiveness of the in-home PLC technologies. Another major advantage of PLC is the ubiquity of the power lines which can be used to provide whole-home connectivity solutions. However, the power line medium has not been originally designed for data communication; the frequency selectivity of the channel and different types of noise (background noise, impulsive noise, and narrow band interferers) make the power line a very challenging environment and require state of the art design solutions.
In 2000, the HomePlug Alliance [
In this paper, we highlight the key differentiating HomePlug AV2 features compared to the HomePlug AV technology. At the physical (PHY) layer HomePlug AV2 includes the following. (i)
HomePlug AV2 transmitter and receiver PHY layer.
All the features listed above improve the Quality of Service of AV2 power line modems, by improving coverage and robustness of communication links.
The following sections provide technical details for all the above mentioned features. Moreover, the AV TWG evaluated the performance of the HomePlug AV2 system. The adoption of all the above listed features provides a significant gain of HomePlug AV2 compared to HomePlug AV. The remainder of this paper is organized as follows. Section
HomePlug AV employs PHY and MAC technology that provides a 200-Mbps class power line networking capability. The PHY operates in the frequency range of 2–28 MHz, uses windowed Orthogonal Frequency Division Multiplexing (OFDM) and a powerful Turbo Convolutional Code (TCC) that provides robust performance within 0.5 dB of Shannon’s limit. Windowed OFDM provides more than 30 dB spectrum notching. OFDM symbols with 917 usable carriers (tones) are used in conjunction with a flexible guard interval. Modulation densities from BPSK to 1024 QAM are adaptively applied to each carrier based on the channel characteristic between the transmitter (Tx) and the receiver (Rx).
On the MAC layer, HomePlug AV provides a Quality of Service (QoS) connection oriented, contention-free service on a periodic Time Division Multiple Access (TDMA) allocation, and a connectionless, prioritized contention-based service on a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) allocation. The MAC receives MAC Service Data Units (MSDUs) and encapsulates them with a header, optional Arrival Time Stamp (ATS), and a checksum to create a stream of MAC frames. The stream is then divided into 512-octet segments, encrypted and encapsulated into serialized PHY Blocks (PBs), and packed as MAC Protocol Data Units (MPDUs) to the PHY unit which then generates the final PHY Protocol Data Unit (PPDU) to be transmitted onto the power line [
HomePlug AV2 enhances significantly the capability above to accommodate new generation of multimedia applications with Gigabit performance.
The HomePlug system is specified for in-home communication adapted to the power line channel. The in-home communications are established with Time Division Duplexing (TDD) mechanism to allow for symmetric communication between peers, as opposed to the classical access system in ADSL with two different downstream and upstream throughputs.
The PHY layer employs OFDM modulation scheme for better efficiency and adaptability to the channel impairments (such as being frequency selective and suffering from narrowband interference and impulsive noise). The HomePlug AV2 OFDM parameters correspond to a system with 4096 carriers in 100 MHz, but only carriers from 1.8 to 86.13 MHz are supported for communication (3455 carriers). The subcarrier spacing of 24.414 kHz was chosen in the HomePlug AV system according to the power line coherence bandwidth characteristic and is maintained in HomePlug AV2 for interoperability.
More significantly, HomePlug AV2 incorporates MIMO capability (see details in the next sections) to improve throughput and coverage.
A block diagram of the PHY layer is shown in Figure
Besides, two OFDM paths are shown in order to implement the MIMO capabilities with 2 transmission ports.
Apart from the Hybrid or AV-only Frame Control symbols, the payload data can be sent using adaptive bit loading per carrier or robust modes (ROBO) with fixed Quadrature Phase Shift Keying (QPSK) constellation and several copies of data interleaved in both time and frequency.
Looking at the data path details in the block diagram (Figure
The outputs of the FC Encoders and Payload Encoder lead into a common MIMO OFDM modulation structure, consisting of a MIMO Stream Parser that provides up to two independent data streams to two transmit paths which include two Mappers, a phase shifter that applies a 90-degree phase shift to one of the two streams (to reduce the coherent addition of the two signals), 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 feed the AFE module with one or two transmit ports that couple the signal to the power line medium.
At the receiver, an AFE with one, two, three, or four (
The Frame Control 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/AV 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
The HomePlug AV2 specification incorporates Multiple-Input Multiple-Output (MIMO) capabilities with Beamforming, which offers the benefit of improved coverage throughout the home, particularly for hard to reach outlets. MIMO technology enables HomePlug AV2 devices to transmit and receive on any two-wire pairs within a three-wire configuration. Figure
MIMO-PLC channel: different feeding and receiving options.
The numbers of used transmit ports
Some regions and maybe homes with older electrical installations do not have the third wire installed in the private buildings. In this scenario, HomePlug AV2 automatically switches to Single-Input Single-Output (SISO) operating mode. HomePlug AV2 incorporates also selection diversity in SISO mode. The ports used for feeding and receiving might be different from the traditional L-N feeding. If, for example, the path from L-PE to L-N offers better channel characteristics than L-N to L-N, the transmitter can choose to use the L-PE port for feeding.
Figure
Magnitude of the transfer functions (
More information about the MIMO-PLC channel characteristics and channel modeling may be found in [
The channels presented in Figure
The MIMO-PLC channel is described by a
Figure
Schematic MIMO channel and decomposition into parallel and independent
Figure
Attenuation of the decomposed PLC-MIMO channel of Figure
The decomposition into parallel and independent
Depending on how many
Precoded Spatial Multiplexing or Beamforming was chosen as the MIMO scheme as it offers the best performance by adapting the transmission in an optimum way to the underlying Eigen modes of the MIMO-PLC channel. The best performance is achieved for various channel conditions. On the one hand, the full spatial diversity gain is achieved in highly attenuated and correlated channels when each symbol is transmitted via each available MIMO path. On the other hand, a maximum bit rate gain is achieved for channels with low attenuation when all available spatial streams are utilized. Beamforming also offers flexibility with respect to the receiver configuration. Only one spatial stream may be activated by the transmitter when only one receive port is available, that is, if the outlet is not equipped with the 3rd wire or if a simplified receiver implementation is used that supports only one spatial stream. Since Beamforming aims to maximize one MIMO stream, the performance loss of not utilizing the second stream is relatively small compared to the Spatial Multiplexing schemes without precoding. This is especially true for highly attenuated and correlated channels, where the second MIMO stream carries only a small amount of information. These channels are most critical for PLC and adequate MIMO schemes are important. A comparison and analysis of different MIMO schemes may be found in [
Beamforming requires knowledge about the channel state information at the transmitter to apply the optimum precoding. Usually, only the receiver has channel state information, for example, by channel estimation. Thus, the information about the precoding has to be fed back from the receiver to the transmitter. The HomePlug AV2 specification supports adaptive modulation [
The optimum linear precoding matrix
Figure
Precoding at the transmitter.
The precoding matrix
There are two possible modes of operation. If only one spatial stream is utilized, single-stream Beamforming (or Spot-Beamforming) is applied. In this case, the precoding is described by the first column vector of
Since the information about the precoding matrix has to be fed back from the receiver to the transmitter, an adequate quantization is required. To achieve this goal, the special properties of
The unitary property of
These properties allow to represent the complex
According to the phase-invariance property, the first entry of each column (
In both modes, Spot-Beamforming and Eigen-Beamforming, the Beamforming vector or Beamforming matrix, respectively, is described by both angles
If the MIMO Equalizer is based on zero-forcing (ZF) detection, the detection matrix
The SNR of the MIMO streams after detection is calculated as
Figure
Influence of precoding on the SNR: SNR of the first (a) and second (b) streams with and without precoding, transmit power to noise power of
The two markers X in Figure
Figures
The Beamforming matrix is quantized efficiently to reduce the amount of feedback required to signal
MIMO power allocation is applied to two-stream MIMO transmissions. The power allocation adjusts the power of a carrier on one stream relative to the other stream. For SISO transmissions and MIMO Spot-Beamforming transmissions, MIMO power allocation can be bypassed since there is only one transmit stream. In this case, the only available option is to allocate all the power to the single stream. The power allocation module is located between the Mapper and the precoding block (see Figure
HomePlug AV2 devices support an expanded frequency spectrum (up to 86.13 MHz, see Section
During the specification development, the AV TWG realized a measurement campaign, where power line channel and noise measurements were performed in 30 homes located in different countries, from Europe to USA. Such variability was a key ingredient to obtain insight into the power line at frequencies above those used in HomePlug AV (1.8–30 MHz). In particular, in every home, all the possible links among at least 5 different nodes were measured.
Examples of recorded channel attenuation and noise PSD are reported in Figures
HomePlug AV2 measurement campaign. Example of channel measurement.
HomePlug AV2 measurement campaign. Example of noise measurement.
Based upon the results of the measurement campaign, the present EMC regulations, coverage, and complexity targets, the final outcome of the analysis was the selection of the 30–86 MHz band for the following reasons. The FM band region 87.5–108 MHz shall be avoided. In fact this frequency region presents higher attenuation and higher noise compared to the 30–86 MHz band (see also Figures The 30–86 MHz frequency band appears to offer a throughput increase especially at the low to mid coverage percentages; the reason is that while the attenuation is greater compared to the 1.8–30 MHz band, noise is lower. A concern derives from the fact that EMC requirements for the 30–86 MHz frequency band are generally tighter than in the 1.8–30 MHz band. However, this observation could be mitigated in view of the fact that, for instance, the upcoming standard FprEN 50561-1 appears to be more restrictive in the 1.8–30 MHz frequency band. Within the HomePlug AV2 specification it has been possible to provide flexibility in choosing the stop frequency in the 30–86 MHz interval. In particular, an AV2 device implementing a frequency band 1.8- The 30–86 MHz frequency band extension allows devices to be fully interoperable with the IEEE 1901 devices that use the 1.8–50 MHz frequency band. In fact, HomePlug AV2 devices that implement a frequency band extension shall support at least the IEEE 1901 bandwidth.
HomePlug AV2 increases throughput by allowing devices to minimize the overhead incurred due to EMC notching requirements. While in HomePlug AV the mechanism (“windowed OFDM”) for creating the PSD notches is fixed and relatively conservative, HomePlug AV2 devices may gain up to 20% in efficiency if they implement additional techniques to accommodate sharper PSD notches. The 20% includes the gain of guard carriers which were excluded by HomePlug AV modems and the reduced Transition Interval in time domain. Such devices gain additional carriers at the band edges and may utilize shorter cyclic extensions, which reduces the duration of the OFDM symbols.
The FFT process uses a rectangular window to cut data out of a continuous stream to convert them from time to frequency domain. The FFT of a rectangular function in the time domain is a sin
OFDM side lobes of a single carrier. Comparison of various window functions.
The process of multiplying a window with an OFDM symbol (see Figure
OFDM symbol with GI and window.
The process of how a window is applied is shown in Figure
The two descending slopes in the time domain could overlap, to save communication resources at consecutive OFDM symbols, as shown in Figure
Consecutive OFDM symbols, guard interval (GI), roll-off interval (RI), and windowing.
With HomePlug AV2, the Transition Interval TI is introduced. The shape of the windowing is transmitter implementation dependent; it does not affect interoperability. The pulse shaping window and Guard interval might be reduced even to zero to minimize overhead in time domain and to notch efficiently. In order to guarantee backward compatibility with previous HomePlug versions the definition and timings of the parameter RI as shown in Figure
There is a balance between this intervals and obtaining a better side lobe attenuation.
Figure
OFDM spectra with different notch widths and depths achieved with different roll-off factors.
To create notches in the OFDM spectrum, a model is implemented using QAM modulation and notches of omitting a various number of carriers: 1, 2, 3, 4, 5, and 10. A max-hold function is implemented in order to create these figures. The 10 carrier notch shows the spectral benefit of windowing. The influence of windowing is hardly visible up to the point of the 5-carrier notch. At the 5-carrier notch, the difference between no window and the highest simulated
The additional overhead in the time domain is extremely large compared to the improved notch depth. Windowing with a small roll-off factor is sufficient in order to suppress the side lobes outside the used spectrum, but it is not recommended to increase the depth of a single carrier notch.
In the case of the HomePlug AV specification using a
Of course an ideal implementation as described above is not possible, but digital band stop filters increase the sharpness of the notches as well the implementation efforts in hardware. Shrinking the semiconductor manufacturing process to smaller structures, allowing to integrate additional functions on the same die size, shifts the balance towards hardware implementation efforts.
HomePlug AV2 specification gives maximum freedom to the chip implementer. Filter algorithm, order, and structure are implementation dependent. An example is documented in [
The HomePlug AV2 standard introduces two novel techniques that can be used to optimize the use of transmit power. The first one, named “transmit power back-off”, is a technique that reduces the transmitted power spectral density for a selected set of carriers when this can be done without adversely affecting performance. Conversely, the second technique, called “EMC Friendly Power Boost” is a technique that allows the transmitter to increase the power on some carriers with the knowledge that this can be done without exceeding regulatory limits.
In power line communications the transmit power limit is typically defined as a power spectral density (PSD) mask applicable over the range of frequencies used in the standard. And since power line modems are directly connected to the electrical wiring, they are traditionally designed to transmit with the maximum allowed transmit PSD on each frequency (i.e., they do not need to be sensitive to a limited battery supply). In many cases maximizing transmit power leads to the best performance; however, certain definitions of PSD masks combined with certain channel conditions can produce cases where modems can benefit from transmitting at less than the maximum allowed power level.
We illustrate the benefits of transmit power control using the North American regulatory limits as an example. FCC regulations that are applicable to power line devices in North America are commonly interpreted to allow a transmit PSD of −50 dBm/Hz from 1.8 to 30 MHz, and −80 dBm/Hz from 30 MHz up to 86 MHz. This 30 dB drop in the PSD (at 30 MHz) causes the signal from the higher frequency carriers (above 30 MHz) to have much smaller amplitude than the signal for the low band carriers (up to 30 MHz). Consequently, when the overall signal is represented in a quantized digital domain, the high band signal has lower resolution than the signal in the lower band and will therefore also have a limited SNR. This will be evident at the transmitter where the 30 dB drop will result in a reduction of 5 bits of resolution for the high band signal. If the transmit power is backed off in the low band, so that the PSD drop is reduced, then the high band signal will be represented with an increased number of bits of resolution and enjoy increased SNRs out of the transmitter.
Another limiting factor is the limited dynamic range of the analog to digital converter (ADC). We illustrate its impact using once again the example of the North American PSD limits. For the sake of simplicity, we assume a flat power line channel, and flat noise spectrum in Figure
Benefits of power back-off. (a) No power back-off, (b) 10 dB power back-off.
In this example, the power back-off technique results in a 5 dB SNR reduction for the carriers in the lower frequency band, and a 5 dB SNR increase for the carriers in the upper frequency band. Given the larger bandwidth of the upper frequency band there will be an overall throughput gain due to transmit power back-off.
Transmit power back-off is also an effective interference mitigation technique. For instance, in Europe, the ability of a PLC transmitter to reduce the transmit power depending on the attenuation link is a possible requirement considered in [
The EMC Friendly Power Boost is a mechanism introduced in the HomePlug AV2 specification to optimize the transmit power by monitoring the input port reflection coefficient at the transmitting modem. This coefficient is known as the
In order to compensate for the frequency selective impedance mismatch at the interface between the device port and the power line network, HomePlug AV2 modems adapt their transmission mask, upon measurement of the
A statistical analysis was conducted on the practical values of the
The
The measurement set used in this analysis consists of 478 frequency sweeps, that can be categorized as follows: 6 different locations in Germany and 3 different locations in France. 264 measurements outdoor at 10 m, 43 measurements outdoor at 3 m, and 171 measurements indoors.
Statistical analysis of this experimental data allowed designing the practical implementation of the EMC Friendly Power Boost technique. In the following, we define the Impedance Mismatch Compensation (IMC) factor as (in dB)
Figures
CDF of
CDF of IMC parameter.
On Figure
More interestingly, Figure For 40% of the records, the Tx power could be increased by more than 2 dB to compensate for the impedance mismatch. For 10% of the records, the Tx power could be increased by more than 4 dB to compensate for the impedance mismatch.
We then focused on the effect of the EMC Friendly Power Boost technique on the radiated EMI statistics. The recorded values allowed the computation of two statistics: the statistical CDF of the recorded EMI in terms of E-field for all frequencies and feeding possibilities without applying any power boost, the statistical CDF of the recorded EMI in terms of E-field for all frequencies and feeding possibilities when applying the EMC Friendly Power Boost.
Figure
Difference in dB between the CDFs of the radiated EMI before and after applying the EMC Friendly Power Boost.
Different observations can be made from this figure. First, the application of the EMC Friendly Power Boost leads to an increase of the radiated field CDF comprised between 0 and 6 dB. Note that the extreme value of 6 dB arises for one of the lowest values of radiated field, and, hence, is not relevant. Secondly, in general, the application of the EMC Friendly Power Boost increases the radiated power CDF by about 2 dB. More importantly, the increase of the radiated power CDF is lower than 2 dB for the 25% most radiating cases. This practically means that in the worst case scenarios where the modems produce the largest EMI, the application of the IMC factor does not increase the EMI by more than 2 dB. This value can be compared with the CDF of the IMC factor given in Figure
Based on this study, we conclude that the application of the EMC Friendly Power Boost technique provides a significant gain in terms of transmit power increase for a large number of configurations, where the impedance mismatch causes the dissipation of the signal at the transmitter. In addition, the statistical analysis shows that this technique will not lead to an increase of the undesirable radiated interference, in particular in the worst EMI scenarios, as long as a margin
In addition to the MIMO technology, the frequency band extension, the Efficient Notching, and power optimization techniques such as the power back-off and the EMC friendly power boost, other elements of the PHY layer were modified as presented in the paragraphs below.
In the HomePlug AV2 specification, a number of time domain parameters were refined. As the sampling frequency has increased from 75 MHz to 200 MHz, the number of time samples for a given symbol duration is increased by a factor 8/3; the IFFT interval is 8192 samples in length, and the number of samples in the HomePlug AV Guard Interval has increased accordingly. In addition, new features have been added: The Transition Interval defines the part of the Roll-off Interval dedicated to the transition window, allowing more flexibility in the choice of the window (see Section A new Guard Interval has been defined for the HomePlug AV2 Short Delimiter (see Section The payload symbol Guard Interval has been made variable and can be as short as 0
In HomePlug AV, the maximum constellation size is 1024-QAM, corresponding to 10 coded bits per carrier. HomePlug AV2 also provides support for 4096-QAM, which corresponds to 12 bits per carrier. The higher constellation size increases the peak PHY rates by 20%. Practically, the increased throughput will be available mostly on average to very good channels, but even some of the poorer channels sometimes have frequency bands in which high SNRs can be achieved, and the increased constellation size can be used.
HomePlug AV2 uses the same duobinary Turbo Code as HomePlug AV. In addition to the code rates of 1/2 and 16/21, HomePlug AV2 also provides support for a 16/18 code rate. This allows more granularity in the compromise between robustness and throughput degradation. For this new code rate, a new puncturing structure is defined, as well as a new channel interleaver. In addition, a new Physical Block size of 32 octets is defined, which includes specification of a new termination matrix for the FEC as well as a new interleaver seed table. The 32-byte octet PBs are used in the PHY level acknowledgements and allow for the acknowledgement of much larger packet sizes that are supported with the increased PHY rates possible in HomePlug AV2.
The HomePlug AV2 specification describes also the device operation in scenarios where there is no alternating current (AC) line cycle (e.g., a direct current (DC) power line) or when the AC line cycle is different from 50 Hz or 60 Hz. In this case, the Central Coordinator is preconfigured to select a Beacon Period matching either 50 Hz (i.e., Beacon Period is 40 msec) or 60 Hz (i.e., Beacon Period is 33.33 msec). One key use case where this feature is useful is the transfer of data towards a multimedia equipped electrical vehicle during the electrical charging phase (using DC power).
HomePlug AV2 stations improve their energy efficiency in standby mode through the adoption of the specific Power Save Mode already defined in the HomePlug Green PHY [
We introduce some basic terms useful to describe the Power Save Mode. Awake Window: period of time during which the station is capable of transmitting and receiving frames. The Awake Window has a range from a few milliseconds to several Beacon Periods (a Beacon Period is two times the AC line cycle: 40 ms for a 50 Hz AC line and 33.3 ms for a 60 Hz AC). Sleep Window: period of time during which the station is not capable of transmitting or receiving frames. Power Save Period (PSP): interval from the beginning of one Awake Window to the beginning of the next Awake Window. Power Save Period is restricted to 2 Power Save Schedule (PSS): the combination of the values of the PSP and of the Awake Widow duration. To communicate with a station in Power Save mode, other stations in the logical network (AVLN) need to know its PSS.
Potentially, the specification allows aggressive PSSs constituted by an Awake Window duration of 1.5 ms and a PSP of 1024 Beacon Periods, that will cause over 99% energy saving compared to HomePlug AV. In practice, some in-home applications will require lower latency and response time, and a balance will take place reducing the mentioned gain. This is particular appealing for applications that foresee a PLC utilization that is variable during the day (for instance large utilization in daylight time and small utilization during night period). It is worth highlighting that the HomePlug AV2 specification is flexible in allowing each station in a network to have a different PSS. Given these remarks, in order to enable efficient Power Save operation without causing difficulties to regular communication, all the stations in a network need to know the PSSs of the other stations. The network Central Coordinator (CCo) has a key role since it grants the requests of the different stations to enter and exit from Power Save mode operation. Moreover, it distributes the different PSSs to all the stations in the network. When needed a CCo can optionally disable Power Save mode for all stations of the AVLN, optionally wake up a station in Power Save mode.
The shared knowledge of the PSS allows stations communicating during the common Awake Windows (the HomePlug AV2 and HomePlug Green PHY specifications have structured the protocol insuring that at least one superposition of all the Awake Windows occurs). This overlap interval can also be used for transmission of information that needs to be received by all stations within the AVLN.
Figure
Example of Power Save operation in HomePlug AV2 and HomePlug Green PHY.
The Short Delimiter and Delayed Acknowledgement features were added to HomePlug AV2 to improve efficiency by reducing the overhead associated with transmitting payloads over the power line channel. In HomePlug AV, this overhead results in relatively poor efficiency for transmission control protocol (TCP) payloads. One goal that was achieved with these new features was TCP efficiency that 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. Interframe spaces are also required between the payload transmission and the acknowledgement for the processing time at the receiver to decode and check the payload for accurate reception and to encode the acknowledgement. This overhead is even more significant for TCP payload since the TCP acknowledgement payload must be transmitted in the reverse direction.
The delimiter specified in HomePlug AV contains the Preamble and Frame Control symbols and is used for the beginning of data PPDUs as well as for immediate acknowledgements. The length of the HomePlug AV delimiter is 110.5
Short Delimiter.
Figure
Short delimiter efficiency improvement.
The processing time to decode the last OFDM symbol and encode the acknowledgement can be quite high, thus requiring a rather large Response Inter-Frame Space (RIFS). In HomePlug AV, since the preamble is a fixed signal, the preamble portions of the acknowledgement can be transmitted while the receiver is still decoding the last OFDM symbol and encoding the payload for the acknowledgement. With the Short Delimiter, the preamble is encoded in the same OFDM symbol as the Frame Control for the acknowledgement, so the RIFS would need to be larger than for HomePlug AV, eliminating much of the gain the Short Delimiter provides.
Delayed Acknowledgement solves this problem by acknowledging the segments ending in the last OFDM symbol in the acknowledgement transmission of the next PPDU, as shown in Figure
Delayed Acknowledgement.
HomePlug 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).
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 HomePlug 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 HomePlug AV2 that enables high efficient repeating. Immediate Repeating provides a mechanism to use a repeater with a single channel access, and the acknowledgement does not involve the repeater. As shown in Figure
Immediate repeating channel access for CSMA.
Intersystem Protocol (ISP) allows coexistence between noninteroperable devices sharing the same power line medium. Using the current ISP protocol, noninteroperable devices are able to coexist. The HomePlug AV2 will operate in a 1901-FFT mode [
The ISP protocol allows a TDM scheme to be implemented between coexisting in-home systems and between coexisting in-home and access systems. Each of the PLC system categories is allocated a particular ISP window in a round-robin fashion. The allocation is determined by
The TDM synchronization period for the in-home and access systems is defined with the parameter
Time Division Multiplexing on ISP.
Figure
Coexistence signaling is carried out by the use of periodically repeating ISP signals within the ISP windows. The signals are used to convey information on coexisting system presence, resource requirements, and resynchronization request. Each PLC system category is allocated a particular ISP window in a round-robin fashion, as illustrated in Figure
The ISP signal is transmitted using a range of designated phases that convey a range of information to be used by the system. This set of instantaneous information is termed the Network Status that defines the allocation of resources to each coexisting system.
The ISP signal is merely detected. By monitoring the ISP signal transmitted within the ISP windows allocated to other systems, a coexisting system is able to determine the number and type of coexisting systems present on the line and their resource requirements. Similarly, by monitoring the signal within its own ISP window, a coexisting system is able to detect a resynchronization request from one of the other coexisting systems.
The ISP signal consists of 16 consecutive OFDM symbols. Each OFDM symbol is formed by a set of “all-one” binary phase shift keying (BPSK), modulated onto the carrier waveforms using IFFT, and multiplied by a window function to reduce out-of-band energy complying with the transmit spectrum requirement. Since all devices send the signal simultaneously, the ISP signal must be sent with 8 dB less power than the normal transmission.
Timing parameter that is used for generating an ISP signal is described in Figure
ISP Signal Timing Parameter.
The TDM synchronization scheme mentioned above is used such that each PLC system shares the medium without interfering with one another. However, it is possible that two or more systems are synchronized to two or more different, mutually visible ISP sequences [
In other words, whenever a HomePlug AV2 device starts up or restarts, it needs to be aware of the presence of any other systems with which it is able to coexist. Accordingly, a startup and resynchronization procedure is defined within the ISP protocol that allows the starting system to synchronize with an existing system.
The AV TWG has evaluated the performance of the HomePlug AV2 specification; this activity has been fundamental in order to see if the produced specification meets the requirements of all the stakeholders. The following tables show the performance improvement as compared to HomePlug AV in terms of coverage. These preliminary results are based on a 6-home field test in Florida, home sizes 1900–3300 sq. ft.
Table
Improvement of HomePlug AV2 in a 2-node network.
Coverage based on UDP throughput | Percentage of throughput improvement of HomePlug AV2 compared to HomePlug AV |
---|---|
95% | >136% |
5% | >220% |
Improvement of HomePlug AV2 in a 4-node network.
Coverage based on UDP throughput | Percentage of throughput improvement of HomePlug AV2 compared to HomePlug AV |
---|---|
99% | >131% |
5% | >173% |
Note that the benefits of the HomePlug AV2 technology are expected to be greater (which explains the “>” symbol) than the ones shown in Tables
Another interesting figure is the theoretical maximum PHY throughput for the system, for different options of the standard (Table
Maximum PHY rate computation.
System configuration |
Max PHY rate |
---|---|
HomePlug AV (1.8–30 MHz) |
197 Mbps |
| |
IEEE 1901 (1.8–50 MHz) |
556 Mbps |
| |
HomePlug AV2 SISO (1.8–86.13 MHz) |
1012 Mbps |
| |
HomePlug AV2 MIMO (1.8–86.13 MHz) |
2024 Mbps |
In this paper, an overview of HomePlug AV2 has been presented. The overall system architecture and the key technical HomePlug AV2 improvements introduced at PHY and MAC layers have been described. It has been also shown the related performance improvements were achieved by HomePlug AV2 while ensuring both backward compatibility versus HomePlug AV and the coexistence with other power line technologies.
The HomePlug AV2 performance presented in this work has been assessed by AV TWG through simulations based on field measurements.
The results show the significant benefits introduced by the new set of HomePlug AV2 features, both in terms of achievable data rate and coverage.