Chapter 2 Introduction of MB-OFDM proposal
2.2 Frame structure
802.15.3a is a packet base transmitting communication system. The structure of this packet is shown in figure 2.2.1
figure 2.2.1 PLCP frame format (source : 802.15.3a proposal)
Three sections are contained in a complete PLCP frame:
1. PLCP preamble 2. PLCP header 3. Data
A PLCP preamble is added prior to the PLCP header to aid receiver algorithms related to synchronization, CFO recovery, and channel estimation. Two portions, the time domain packet and frame synchronization sequence for total 24 or 12 OFDM symbol durations long, and the frequency domain channel estimation sequences compose the PLCP preamble for 6 OFDM symbol durations long.
There are four packet synchronization patterns defined (see appendix). Each time domain preamble is decided by the four patterns and cover sequence pc(n) (of length 24 for standard preamble) ,the table is in the appendix, and the combined packet and frame synchronization portion are generated as the Kronecker product of the two sequences. In additional to standard preamble, a shorten preamble is also defined for use in the streaming mode when a burst of packet is transmitted, separated by a MIFS time. For data rates of 200 Mbps and lower, all the packets in the burst mode use the standard PLCP preamble.
However, for data rates higher than 200 Mbps, the first packet uses the standard preamble, while the remaining packets may use the shorten preamble. This portion of the preamble can be used for packet detection and acquisition, coarse carrier frequency estimation, and coarse symbol timing. As for the channel estimation portion, the training sequence is generated by passing the frequency-domain sequence defined in table 2.2.1 through the IFFT, and appending a zero pad interval consisting of 37 zeros to the resulting time-domain output. This portion of the preamble can be used for estimation of the channel frequency response, fine CFO estimation, and fine symbol timing.
The OFDM training symbols shall be followed by the PHY header, which
contains the RATE of the MAC frame body, the length of the frame payload (which does not include the FCS), the seed identifier for the data scrambler, and information about the next packet-whether it is being sent in streaming mode and whether it employs a shortened preamble or not. The RATE field conveys the information about the type of modulation, the coding rate, and the spreading factor used to transmit the MAC frame body.
The PHY header field shall be composed of 40 bits, as illustrated in figure 2.2.2.
Bits 3-7 encode the RATE, as shown in table 2.2.5. Bits 8–19 encode the LENGTH field, with the least significant bit (LSB) being transmitted first. Bits 22–23 encode the seed value for the initial state of the scrambler, which is used to synchronize the descrambler of the receiver. Bit 26 encodes whether the packet is being transmitted in “burst” or streaming mode. In burst mode, the minimum packet size shall be 1 byte. In non-burst mode, the minimum packet size should be zero byte. Bit 27 encodes the preamble type (set to 0 for long or 1 for short preamble) used in the next packet if in streaming mode. All other bits are reserved for future use and are set to zero.
figure 2.2.2 PHY header bit assignment (source : 802.15.3a proposal)
Rate (Mb/s) R1 – R5
53.3 00000 80 00001
110 00010
160 00011 200 00100 320 00101 400 00110 480 00111 Reserved 01001, 01011–11111
Table 2.2.5 Rate-dependent parameters
The HCS (header check sequence) field right next to the MAC header field is used to protect the PHY and MAC header with CCITT CRC-16 rule. The CCITT CRC-16 HCS is the ones complement of the remainder generated by the modulo-2 division of the protected combined PHY and MAC headers by the polynomial, x16+ x12+ x5+1. The HCS should notedly be one’s complement of the remainder. A schematic of the processing is shown in figure 2.2.3.
figure 2.2.3 CCITT CRC-16 implement (source : 802.15.3a proposal)
Whole 802.15.3a packet except PLCP preamble and PHY header pass a scrambler to randomize the bit stream. The scrambler in 802.15.3a use the polynomial g(D) = 1 + D14 + D15 of the pseudo-random binary sequence (PRBS) generator to produce a sequence, and then exclusive or (modulo-2 addition) with the unscrambled bit stream. Based on this polynomial, the corresponding PRBS, xn, is generated as xn=xn-14⊕xn-15. The scrambled bits, sm, are obtained as sm=dm⊕xm, where dm is the unscrambled bit. The descrambler of the receiver is initialized with the same vector initialization vector used in the transmitter. The initialization vector is determined from the scrambler init field contained in the PLCP header of the received frame. The initialization vector or seed value corresponds to the scramble init or seed identifier as shown in table 2.2.6.
Seed identifier (S1, S0) Seed value (x14 … x0) 0,0 0011 1111 1111 111 0,1 0111 1111 1111 111 1,0 1011 1111 1111 111 1,1 1111 1111 1111 111
Table 2.2.6 Scramble seed selection
The scrambled data is then pass to a convolutional encoder of coding rate 1/3 with constraint length 7(figure 2.2.4), and puncture those coded bits corresponding to the required coding rate (figure 2.2.5 through 2.2.8).
To protect the data from burst error, the coded bits of convolutional code are interleaved. The 802.15.3a interleaving operation is performed in three stages.
First of all, interleaving operated across OFDM symbols. The bits equivalent to six OFDM symbols ( 6*NCBPS/TSF bits) are grouped together. Coded bits of each group are then permuted using a block interleaver of size (6/TSF)*NCBPS. The input-output relationship is given by:
( )
⎭⎬
⎫
⎩⎨
⎧ ⎟⎟+
⎠
⎜⎜ ⎞
⎝
= ⎛ CBPS
CBPS
N N i
U i i
S() Floor 6Mod , where S(i) = output, U(i) = input, i=0,
1,2,…, (6/TSF)*NCBPS-1
In the second stage, named tone interleaver, the bits output from symbol interleaver are grouped together into blocks of NCBPS and then permuted using a block interleaver of size NTINT*10m where NTINT=NCBPS/10. The input-output relationship is given by:
( )
⎭⎬
⎫
⎩⎨
⎧ ⎟⎟⎠+
⎜⎜ ⎞
⎝
= ⎛ Tint
T
N N i
S i i
T() Floor 10Mod ,
int
where T(i) = output, S(i) = input, i=0,1,2…,
NCBPS-1
The output of the tone interleaver is then passed through the third stage which cyclic shift the tones of NCBPS bits of bth block. The relationship is as follow,
)) ),
( mod(
, ( ) ,
(b i T b i A b NCBPS
V = + where V(b,i) = input, T(b,i) = output, i==0,1,2…, NCBPS-1
A(b) = b*33, b=0,1,2, for conjugate symmetric modes and NCBPS=100.
A(b) = b*66, b=0,1,2, for non-conjugate symmetric modes with TSF=2.
A(b) = b*33, b=0,1,2,…,5, for non-conjugate symmetric modes with TSF=1.
figure 2.2.4 Convolutional encoder: rate R = 1/3, constraint length K = 7 (source : 802.15.3a proposal)
figure 2.2.5 11/32 coding rate bit-stealing and bit-insertion procedure (source : 802.15.3a proposal)
figure 2.2.6 1/2 coding rate bit-stealing and bit-insertion procedure (source : 802.15.3a proposal)
figure 2.2.7 5/8 coding rate bit-stealing and bit-insertion procedure (source : 802.15.3a proposal)
figure 2.2.8 3/4 coding rate bit-stealing and bit-insertion procedure (source : 802.15.3a proposal)
The only modulation mapping defined in 802.15.3a is QPSK. The constellation map and bit coding is shown in figure 2.2.9. After mapping, a normalization factor, 1/√2, should be multiply the mapping value to normalize the transmitting power.
figure 2.2.9 QPSK constellation bit encoding (source : 802.15.3a proposal)
After interleaving, modulation mapping, and zero-padding prefix adding, a complete packet is formed. The digital signals is then pass through up-sampler, digital filter, DA converter, up-converter, and pass to the antenna. A packet transmitting is complete. The 802.15.3a packet forming procedure is as shown figure 2.2.10
figure 2.2.10 802.15.3a packet forming procedure