ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación descargar pdf (Pagina 111)

Each frequency sub-band conveys OFDM frames comprising two main parts:

•

The CP (Cyclic Prefix) is used for the temporal delimitation of the part convey-

ing the data.

•

The data frame consists of OFDM symbols, each of which consists of 428

samples.

The OFDM blocks of the HomePlug frame consist of 20 or 24 symbols. Those

of the ROBO frame only comprise 40 symbols.

Figure 5.6 gives details on an OFDM symbol and the respective times for its var-

ious parts: 8.4 μs for HomePlug 1.0 and 40.96 μs for HomePlug AV.

The long data frame is itself composed of 20 to 120 OFDM blocks forming the

data of the data link layer and the service blocks.

The OFDM symbols are modulated in each frequency sub-band with phase

modulation according to the quality of the link between PLC devices.

OFDM Transmission Schemes

Unlike single-carrier transmission schemes, OFDM transmission schemes are used to share

the complexity of power equalization for the signal transmitted between the sender and

the receiver. This ensures simple and cost-effective implementation of PLC receivers.

The other advantages of OFDM transmission schemes are the following:

•

Efficient use of the frequency band unlike conventional frequency-division

multiplexing techniques. The various channels overlap in spectral terms while

remaining fully orthogonal.

•

Digital equalization and simple and optimum decoding thanks to the use of guard

spaces even if accompanied by a lower data rate. Used in conjunction with

convolutional codes, Viterbi codes, and block codes (Reed-Solomon codes), this

technique proves to be highly efficient.

•

Robustness to burst noise thanks to a multicarrier technique. Each carrier is affected

by a noise independent from the other carriers. In the single-carrier technique, the

92 Frames

Next OFDM

symbol

Figure 5.6 OFDM symbol details

1 2 ... 106 107 108 109 110 111 112 113 114 115 116 ... 348 349

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ZyXEL Communications PLA-470 V2 - V3.0.5 Guía de instalación Pagina 111