Physical Layer (PHY)

The goal of this layer is to provide excellent, yet simple, performance. The PHY layer must be able to adapt to different conditions and also needs to be flexible for jumping from channel to channel without errors in transmission or losing clients (CPEs). This flexibility is also required for being able to dynamically adjust the bandwidth, modulation and coding schemes. Orthogonal Frequency Division Multiple Access (OFDMA) will be the modulation scheme for transmission in up and downlinks. With OFDMA it will be possible to achieve this fast adaptation needed for the BS's and CPEs. OFDMA is a multi-user version of the Orthogonal frequency-division multiplexing (OFDM) digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of sub-carrier frequencies to individual CPEs as shown in the illustration below. This allows simultaneous low data rate transmission from several users.

By using just one TV channel the approximate maximum bit rate is 19 Mbit/s at a 30 km distance. The speed and distance achieved is not enough to fulfill the requirements of the standard. The feature Channel Bonding deals with this problem. Channel Bonding enables to use more than one channels for transmission and reception. This allows the system to have higher bandwidth which will be reflected in a better system performance. Failure of one or more of the channels does not lead to data loss, only a longer transmission time. Failure of one or more channels leads to the automatic rerouting of missed packets, without data loss.