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9.3. HomePlug

High-throughput home networking is partially driven by the arrival of DSL lines and cable modems to homes with multiple PCs. HomePlug is the result of a common industry desire to have a single transmission protocol for interconnecting PCs as well as entertainment electronics “wirelessly” over the in-home power lines. The formation of thirteen companies for the HomePlug Powerline Alliance was announced on April 10, 2000. Alphabetically, these founding companies were 3COM, AMD, CISCO, Compaq, Enikia, Intel, Intellon, Motorola, Panasonic, S3 (Diamond Multimedia), Tandy/RadioShack, and TI. The HomePlug group followed a development process for the proposal, the field test, and the selection of a transmission technology similar to the one that has been successfully practiced at HomePNA. Under the sponsorship of RadioShack and Compaq, five residences of different sizes in the Dallas, Texas, area were chosen to field test prototypes of different power line–based transmission systems from Adaptive Network, Cogency, Enikia, Intellon, and Itran. Based on field-test performances of these prototypes, the system from Intellon was chosen as the basis for the HomePlug technology, and the decision was finalized on June 5, 2000. Intellon also immediately made the first draft of the HomePlug specification available to member companies. Technical working groups were formed to address issues such as field validation tests, radio emission limits, and coexistence with other power line–based systems. Extensive field tests of prototypes conforming to the draft specification were conducted at more than 500 homes during the first quarter of 2001 by many participating member companies. Version 1.0 of the HomePlug specification was subsequently released on June 26, 2001.

The HomePlug uses burst mode orthogonal frequency division multiplexing as its basic transmission technique. OFDM is also called Discrete MultiTone (DMT) for the modulation method used by ADSL. OFDM waveforms are generated by the inverse fast Fourier transform (IFFT) converting data bits allocated in many frequency subcarriers to time domain. Each complete cycle of IFFT produces one OFDM symbol. In time domain, each HomePlug OFDM symbol lasts 420 sampling points. With a sampling frequency of 50 MHz, HomePlug's symbol rate is about 119 kHz. HomePlug has an IFFT size of 256 real time points or 128 complex subcarriers in frequency domain. Adjacent subcarriers are therefore 50,000,000/256 = 195312.5 Hz apart. Extended OFDM symbols of 428 points are created by concatenating the last 172 time points in front of the original 256 IFFT time points. Extended symbols are sent over the power line with eight overlapping points between adjacent ones.


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