5-17 MINI-LINK BAS
EN/LZB 111 0542 P2B Technical Description
5.4.2 Frame Alignment
Frame alignment for the radio frame is performed by using the first
group polling permit plus the CRC-P as a FAW.
Frame recovery is achieved in the NU by detecting the FAW. This
unique pattern marks the beginning of each radio frame. Two
consecutive FAWs will be far apart as many bits as specified by the
frame format.
When FAW is detected for three consecutive times, the receiver in NU
can be assumed as aligned with the transmitting RN. If FAW is not
detected in the expected position for four consecutive times, then the
receiver in NU can be considered to have lost the frame alignment to
the transmitting NCU and consequently a loss of sync is issued.
5.4.3 Scrambling
Data stream received from the MAC layer is scrambled in order to
guarantee the right shape in output spectrum, to provide enough
transition for clock recovery purposes, to support security of data.
Scrambler is synchronous with the TDMA/TDM frames; at the
beginning of a TDMA/TDM frame the scrambler register is preset to a
known value. This value can be either all “1”s fixed or derived from
the RN identity, 20 ASCII characters.
In the RN the slots received from the MAC unit are scrambled, FEC
encoded and input to the modulator. The data stream is then converted
to analogue signal, frequency up-converted and sent towards the
outdoor RAU.
The signals coming from the outdoor RAU are frequency down-
converted and in the demodulation process translated to a digital data
stream, then FEC is decoded, descrambled and sent to the MAC unit.
FAW, guard and preamble are not scrambled.
5.4.4 FEC
Error performance is a major issue for wireless ATM systems. Two
main error sources are expected:
• Errors due to propagation anomalies, mainly rain outage
• Errors due to presence of unwanted signals
• Errors due to interference from neighbouring cells
FEC capability is foreseen in the physical layer to improve robustness
to transmission impairments.
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