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14.3 IEEE 802.22 Physical Layer > 14.3.2 CBP Packet Format - Pg. 396

396 CHAPTER 14 Cognitive radio in TV bands spreading by a factor of 4 is applied to the output of the mapper, resulting in a maximum length of 42 bytes. The FCH is transmitted as part of the DS protocol data unit (PDU) in the DS subframe and uses the basic data rate mode. The length of FCH is 4 bytes and it carries, among other things, the length (in bytes) information for the DS-MAP if it exists or the length of the US-MAP. The FCH is sent in the first two subchannels of the symbol immediately following the preamble symbol. To increase the robustness of the FCH, the encoded and mapped FCH data may be retransmitted, which is indicated through the SCH. The receiver can combine corresponding symbols from the two or three OFDM slots and decode the FCH data to determine the lengths of the following fields in the frame. These MAPs are transmitted using the base data rate mode in the logical sub- channel immediately following the FCH logical subchannels. The length of the DS-MAP PDU, if present, or the US-MAP PDU is indicated in the FCH. If the DS-MAP is present, the length of the US-MAP, DCD, and UCD are specified in the DS-MAP and follow the DS-MAP in that order. The number of subchannels required to transmit these fields is determined by their lengths and could possibly exceed the number of subchannels allocated per symbol. In this case, the transmission of these PDUs will continue in the next symbol, starting with the first logical subchannel. In most situations, no more than two symbols are required to transmit the FCH and MAPs.