SIGNAL PROCESSING CHALLENGES IN THE DESIGN OF THE HOMEPLUG AV POWERLINE STANDARD TO ENSURE CO-EXISTENCE WITH HOMEPLUG 1.0.1

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1 SIGNAL PROCESSING CHALLENGES IN THE DESIGN OF THE HOMEPLUG POWERLINE STANDARD TO ENSURE CO-EXISTENCE WITH HOMEPLUG Brent Mashburn 1, Haniph Latchman 2, Tim VanderMey 3, Larry Yonge 1 and Kartikeya Tripathi 2 1 Intellon Corporation, 5100 W. Silver Springs Blvd., Ocala, FL, ECE Department, University of Florida, Gainesville, FL Proton Engineering and Design, Inc., 1352 Charlotte St., Altamonte Springs, FL ABSTRACT The HomePlug standard was released in 2000 by the HomePlug Powerline Alliance [1] and is now in use worldwide in Powerline Communication products for inhome LANs [2] and for Broadband Powerline (BPL) Internet Access [3], providing typical aggregate TCP throughput of 5-7 Mbps. The HomePlug standard, expected to be released in mid 2005, will provide a tenfold improvement in average throughput over While is optimized for multimedia Audio-Visual () communication, it is important that devices coexist with those of To achieve this, the preamble which begins all packets should be robustly detectable by both and nodes. devices must also be capable of transmitting compatible priority and control waveforms, which conform to each region s spectral mask. This paper describes the hurdles that needed to be scaled to generate the compliant waveforms for coexistence, using the structures. 1. INTRODUCTION One significant challenge encountered in the development of the HomePlug standard was finding a way for both and nodes to coexist, without requiring that nodes contain large subsystems of the hardware in order to create compatible time domain waveforms. In addition to setting certain carriers to zero amplitude, the technology required digital filters in order to lower remaining spectral roll-off energy in certain bands so as not to interfere with other licensed devices. As the technology was designed for a fixed spectral mask, the exact number of filters, taps, and bits of resolution could be computed and simplified by designers ahead of time. In contrast, a requirement of the technology was that it must have a flexible spectral mask so that it may be configured according to the variable regulations of different countries. Thus even if an system did incorporate all required hardware to create time domain waveforms, at a sizeable increase in cost and complexity, there would still be an unresolved problem with respect to making the spectral mask flexible enough to operate in any country. Designers would have to incorporate multiple programmable banks of digital filters and even then the technology could not be guaranteed to meet requirements for all cases. The solution was to use s flexible spectrum signal processing engine to create waveforms that are not exact replicas of those generated by true nodes, but similar enough so as to be robustly detectable by existing nodes. The remainder of the paper elucidates that procedure. Section 2 presents the requirements needed for nodes to coexist. Section 3 describes the orthogonal frequency division multiplexing (OFDM) symbol shaping and the effect on the spectral mask. Sections 4, 5, and 6 detail the signal processing steps required to produce the coexistent Priority Resolution Slot () symbols, preamble, and frame control respectively. Section 7 presents some conclusions and comments on the required signal fidelity of said compatible waveforms. 2. REQUIREMENTS OF CO-EXISTENCE In order to understand co-existence we must first review the channel access mechanism. The protocol uses carrier sense multiple access with collision avoidance (CSMA/CA) [4]. In a nutshell, nodes transmit special symbols in predetermined priority resolution slots in order to signify to other nodes the priority of their pending traffic. All nodes that have traffic of the highest signaled priority wait a random period of time before attempting to gain access to the channel. While contending nodes are waiting to gain access, each listens to the medium until their given transmit time in order to detect if another node has won the luck of the draw and already begun transmitting (in which case each node backs off for another random period of time). Therefore, for nodes to co-exist with nodes, they must have a means to generate recognizable symbols and the beginning part of frames (the preamble) must appear as a preamble to nodes.

2 A third requirement is that the nodes be able to transmit information about packet length. This information, contained in the payload of four OFDM symbols following the preamble and referred to as the frame control (FC), must be able to be reliably decoded by nodes in order to know when the next opportunity for contention begins (there is no physical carrier sense). By meeting these three requirements, nodes can control when nodes may contend for access to the medium. Therefore, at certain times, nodes are free to use another protocol that is more quality-of-service (QoS) friendly for multimedia delivery (e.g. time division multiple access or TDMA) without fear of collision with traffic. From the technology point of view, all of these compatible waveforms 1) must be easily adaptable to conform to various spectral masks, 2) should take advantage of the increased bandwidth of (1.8-30MHz compared with s MHz), and 3) should use the existing signal processing engine as much as possible to reduce size and complexity. 3. HOMEPLUG SYMBOL SHAPING One significant difference in the physical layer (PHY) payload parameters [5] of the technology in comparison to is that uses a much longer OFDM symbol length. The IFFT interval is eight times the length of that used in In standard operation, uses a guard interval of either 5.56 µs or 7.56 µs, compared to s guard interval of 3.28 µs. One benefit of the longer IFFT interval is that even though the guard interval has increased, overhead due to the guard interval is substantially less that that of the technology. Another benefit of the increased symbol length is that longer portions of each OFDM symbol can be shaped and overlapped as shown in Figure 1. In the figure, RI refers to the samples comprising the shaped roll off interval, GI refers to guard interval samples, and the IFFT Interval refers to the OFDM symbol samples read directly out of the IFFT (before addition of the cyclic prefix). roll-off of means that at either side of a desired spectral notch only a handful of carriers must be turned off to significantly reduce transmit power in the licensed band. Figure 2 demonstrates how spectral energy drops off versus bandwidth from the last on carrier, assuming a semi-infinite number of carriers that are also on before this final carrier. As each of the 1155 carriers spanning 1.8 MHz to 30 MHz are spaced approximately khz apart, it can be seen that if only 4 to 5 additional carriers on either side of the desired notch are set to zero amplitude, regardless of which of the two standard guard interval lengths is used, the energy inside the licensed band can be guaranteed to be at least 30 db lower than the normal transmit power. This amount of power reduction is adequate to meet most spectral masking requirements. The challenge with applying the same technique to the compatible symbols is that these symbols are much shorter than those of and therefore cannot be easily modified to have smoother transitions from symbol to symbol without introducing considerable distortion and/or altering the symbol pitch. Figure 2. Spectral Roll-off of Semi-Infinite Number of Carriers 4. CO-EXISTENT PRIORITY RESOLUTION SLOT SYMBOLS Figure 1. OFDM Symbol Timing This slow tapering, which occurs on the first and last 4.96 µs of each OFDM symbol, translates into faster spectral roll-off. Whereas the technology required digital filters to push down energy in licensed bands (in order to avoid having to turn off large numbers of carriers on either side of each licensed band) the faster spectral The creation of compatible symbols was the simplest hurdle to overcome. In the technology, a node transmits six consecutive symbols in a given slot to signify priority. Each of the six symbols is 5.12 µs in length while the total slot time lasts µs. In the frequency domain, one symbol consists of a known phase for each carrier that is turned on (all used carriers have the same magnitude). When cycled through the inverse fast Fourier transform, this results in a cyclic frequency sweep or chirp beginning at 20.7 MHz and ending at 4.5 MHz.

3 For, the same phases are used for each of the carriers, though new phases are used for carriers in the extended bandwidth (i.e. the chirps span MHz). As these new carriers are orthogonal, they have no impact when processed by nodes. While details regarding the waveform are still being finalized, in all likelihood the waveform will be that shown below in Figure 3 (or some minor variation thereof). An additional half of a symbol is affixed to either side of the six standard symbols used in and the first and last RI samples are pulse-shaped. Transmission of the waveform begins 2.56 µs before the start of the actual slot time, so that the attenuation of the signal while in the first part of the ramp-up phase does not affect detection reliability. The result is a new waveform which has essentially the same reliability for nodes as those generated by nodes, has a flexible spectral mask, and allows nodes to take advantage of the extended bandwidth to increase robustness preamble, the IFFT is first loaded with the FFT of the nominal un-masked preamble waveform as shown below in Figure µs 2.56 µs (Last Half) 5.12 µs 5.12 µs Figure 4. Nominal Un-masked Preamble Waveform (First Half) All carriers that are to be masked in accordance with the µs packet body symbols are then set to zero amplitude. Next the IFFT is performed, resulting in a nominal masked preamble. Portions of the waveform are then windowed and overlapped to effectively insert an extra in order to lengthen the preamble as shown in Figure 5. RI 4.96 µs RI 2.56 µs 2.56 µs µs (Slot Time) Figure 3. Waveform 5. COEXISTENT PREAMBLE Similar to the symbols, the preamble also consists of a repeating, known, pattern of chirps. One significant difference with the preamble is that the chirps are not all identical. Following 6 consecutive symbols, there are also 1.5 symbols, which are simply symbols with a 180 phase shift. For synchronization, this / transition may be detected via autocorrelation in order for a receiving node to set up symbol timing for processing the following packet body. At first glance, it may appear that the same approach could be applied to the preamble as was used on the symbols, namely shaping the first and last symbols. However, this will not result in the correct spectral roll-off due to the immediate transition from to in the middle of the preamble. This poses a problem since the preamble cannot be significantly modified (e.g. to have a smooth, well shaped transition from to ) without affecting either synchronization reliability or symbol timing. The solution to the problem lay in treating the collection of 5.12 µs and preamble symbols initially as a single large OFDM symbol µs in length. To generate a spectrally correct version of the Figure 5. Extending the Masked Preamble Finally both the beginning and ending portions of this extended preamble are shaped resulting in the extended shaped preamble shown in Figure 6. Figure 6. Extended Shaped Preamble It should be noted that due to the masking, the resulting multiple and symbols in the extended preamble are not identical to each other, unlike those generated by a true node. However, they are similar enough such that there is a negligible effect on (and ) synchronization performance. Through

4 the masking and shaping, the spectrum of this preamble can now easily be changed to be similar to the packet body symbols. Two challenges remain: how to construct the following frame control symbols and make them spectrally correct and flexible, and how to combine them with the preamble in such a way as to have the same pitch with respect to the / transition as a true preamble. 6. COEXISTENT FRAME CONTROL Similar to the method used in the creation of the preamble, the compatible frame control begins with loading a 3072-IFFT with a frequency image of a nominal waveform, known as the macro symbol and shown in Figure 7. Also similar to the preamble, this single symbol contains multiple symbols. bit s contribution to the amplitude and phase of the fundamental and surrounding carriers is summed (excluding all masked carriers). Likewise a sum is also performed for the contribution of the elements at the beginning of the waveform. The IFFT is then performed, a cyclic postfix is added (for overlapping with the following OFDM symbol) and the beginning and end portions of the macro symbol are shaped. When the first RI samples are overlapped with the last RI samples of the preamble, the resulting portions overlap constructively (so as not to compromise synchronization performance), and the frame control symbols have the same pitch with respect to the / transition as Similar to the preamble s and symbols, the frame control symbols are not exactly the same as those generated by a true node. However, they are similar enough to have no measurable effect on detection reliability. 7. CONCLUSION Figure 7. HomePlug Macro Symbol Though the macro symbol contains a pre-formed frame control, the four FC symbols have no modulation. If one were to examine the time domain representation of the carrier output for a single frame control symbol one would see a waveform similar to that shown in Figure µs Figure 8. Single Carrier-Symbol Waveform In the frequency domain, this waveform has a sin(x)/x amplitude profile, meaning that most of the signal power resides in frequencies closest to the fundamental. The phase is dependent on carrier frequency and position in time (i.e. in which of the four symbols this waveform appears). Based on this amplitude and phase information, a waveform corresponding to one carrier s BPSK modulated bit can be recreated using the un-modulated 3072-IFFT macro symbol frequency image and the magnitude and phase of the fundamental and of a few surrounding carriers (empirically it was determined that using 7 carriers on either side of the fundamental provides a waveform with acceptable levels of distortion). Each In conclusion, a methodology has been presented whereby nodes utilizing the state-of-the-art HomePlug technology can communicate enough information so as to be able to co-exist with the widely deployed nodes, while still providing QoS for streaming multimedia. While the -created symbols transmit exactly the same magnitude and phase on carriers as true nodes, the preamble and frame control are not exact copies. However, as both the preamble synchronization and frame control were designed to be the most robust elements of the PHY, they operate at significantly reduced SNR levels. Therefore the imperfections introduced in creating their counterparts still result in signals with a noise-free fidelity signal-to-noise ratio (SNR) which is tens of decibels above their failure points. For this reason, the imperfections do not have a noticeable effect on performance. In addition, by taking advantage of the increased bandwidth and using additional carriers, orthogonal and therefore transparent to nodes, the robustness of the priority resolution and synchronization have been improved for nodes. Just as the generated compatible symbols appear as true node traffic to nodes, true traffic is robustly recognizable to nodes, allowing both to contend for the channel during CSMA periods. The ability to gain -only access to the channel for certain portions of time means that nodes can periodically use the more QoSfriendly TDMA-based protocol for latency sensitive streaming multimedia. Finally, as the engine processes groups of the shorter OFDM symbols as a single long OFDM symbol, pulse shaping may be used. The slower waveform transitions due to the shaping translate into

5 faster spectral decay with respect to each carrier s center frequency. Therefore, all waveforms produced by the engine can be easily adapted to meet various countries spectral masks, or used in frequency division multiplexing (FDM) access scenarios, by simply setting the amplitudes of certain carriers to zero to effect deep spectral notches. 8. REFERENCES [1] HomePlug PowerLine Alliance, (April 2005). [2] H. Latchman, L. Yonge, Power Line Local Area Networking, IEEE Communications Magazine, vol. 41, no. 4, April [3] S. Galli, A. Scaglione, K. Dostert, Broadband is Power: Internet Access Through the Power Line Network, IEEE Communications Magazine, vol. 41, no. 5, May [4] S. Baig and N.D. Gohar, A Discrete Multitone Transceiver at the Heart of the PHY Layer of an In-Home Power Line Communication Local Area Network, IEEE Communications Magazine, vol. 41, no. 4, pp , April [5] Van Nee, R.D.J. and R. Prasad, OFDM for Wireless Multimedia Communications, Artech House Publishers, Boston, Massachusetts, January 2000.