m Dependent) sub-layer. Features Mandatory/optional BPSK, QPSK, 16QAM, 64QAM Modulation Mandatory BCC error correction code Mandatory LDPC error correction code Optional Short guard interval (400 ns) Optional MIMO (up to four streams) Optional Beamforming Optional Optional Table 4.5. Characteristics of PMD sub-layer 4.3.1. Transmission chain Transmission in the HT_MF and HT_GF modes is generated from the following function blocks: a) Scrambler scrambles the data to reduce the probability of long sequences of bits to ZERO or to ONE. b) Encoder parser, if BCC encoding is to be used, demultiplexes the scrambled bits among NES (number of BCC encoders for the Data field) BCC encoders, in a round robin manner. Wi-Fi Integration to the 4G Mobile Network c) FEC encoders encode the data to enable error correction. An FEC encoder may include a binary convolutional encoder followed by a puncturing device, or it may include an LDPC encoder. d) Stream parser divides the outputs of the encoders into blocks that are sent to different interleaver and mapping devices. The sequence of the bits sent to an interleaver is called a spatial stream. e) Interleaver interleaves the bits of each spatial stream (changes order of bits) to prevent long sequences of adjacent noisy bits from entering the BCC decoder. Interleaving is applied only when BCC encoding is used. f) Constellation mapper maps the sequence of bits in each spatial stream to constellation points (complex numbers). g) STBC encoder spreads constellation points from Nss spatial streams into NSTS space-time streams using a space-time block code. STBC is used only when NSS<NSTS. h) Spatial mapper maps space-time streams to transmit chains. This may include one of the following: - direct mapping: constellation points from each space-time stream are mapped directly onto the transmit chains (one-to-one mapping); - spatial expansion: vectors of constellation points from all the space- time streams are expanded via matrix multiplication to produce the input to all the transmit chain