OFDM PHY Specification

3.2.1 Time and Frequency Description of OFDM

The WirelessMAN-OFDM PHY is based on OFDM modulation and designed for NLOS operation in the frequency bands below 11 GHz.

The transmitter energy increases with the length of the guard interval while the

receiver energy remains the same (the cyclic extension is discarded).Thus there is a

On initialization,an SS should search all possible values of CP until it finds the CP being used by the BS.The SS shall use the same CP on the uplink. Once a specific CP duration has been selected by the BS for operation on the downlink, it should not be changed.Changing the CP would force all the SSs to resynchronize to the BS.

Figure 3.2-1 illustrates the time structure of OFDM symbol ,while Figure 3.2-2 gives a frequency description of OFDM signals.

Figure 3.2.1-1 OFDM symbol time structure

Figure 3.2.1-2 OFDM frequency description Data sub-carriers：For data transmission.

Pilot- subcarriers：For various estimation purposes.

Null- subcarriers： No transmission at all, for guard bands, non-active subcarriers and the DC subcarrier.

3.2.2 Parameter Setting

(a) Primitive parameter definitions：

– BW ： This is the nominal channel bandwidth.

– N_used： Number of used subcarriers.

– n：Sampling factor. This parameter, in conjunction with BW and N_used

determines the subcarrier spacing, and the useful symbol time.

– G： This is the ratio of CP time to “useful” time.

(b) Derived parameter definitions ：

– N_FFT：Smallest power of two greater than N_used. – Sampling Frequency：^{Fs = floor}

(

)

– Subcarrier spacing ：Δ =f F N_{s FFT} – Useful symbol time： T_b = Δ 1 f – CP Time： T_g = ⋅G T_b

– OFDM Symbol Time：T_s =T_b+T_g – Sampling time：T N_{b FFT}

(c) Table 3.2.2-1 gives the OFDM Symbol parameters

Parameter Value

u sed F F T

N N _200/256

G 1 4 , 1 8 , 1 16 , 1 32 Frequency offset indices of pilot

carriers –88,–63,–38,–13,13,38,63,88 Frequency offset indices of guard

subcarriers

–128,–127...,–101 (numbers：28) +101,+102,...,127 (numbers：27)

n

For channel bandwidths that are a multiple of

otherwise specified then n = 8/7}

Table 3.2.2-1 OFDM symbol parameters

3.2.3

A Pseudo Random Binary Sequence(PRBS) generator is shown in Figure 3.2.3-1 Data randomization is performed on each burst of data on the downlink and uplink.The randomization is performed on each allocation (downlink or

uplink),which means that for each allocation of a data block (subchannels on the frequency domain and OFDM symbols on the time domain) the randomizer shall be used independently.If the amount of data to transmit does not fit exactly the amount of data allocated,padding of 0xFF (“1” only) shall be added to the end of the transmission block for the unused integer bytes. For RS-CC and CC encoded data,,padding will be added to the end of the transmission block, up to the amount of data allocated minus one byte,which shall be reserved for the introduction of a 0x00 tail byte by the FEC.

Figure 3.2.3-1 PRBS generator for data randomization

On the downlink, the randomizer shall be re-initialized at the start of each frame with the sequence: 1 0 0 1 0 1 0 1 0 0 0 0 0 0 0. The randomizer shall not be reset at the start of burst #1. At the start of subsequent bursts,the randomizer shall be

initialized with the vector shown in Figure 3.2.3-2.

Figure 3.2.3-2 OFDM randomizer downlink initialization vector for burst #2...N

On the uplink,the randomizer is initialized with the vector shown in Figure

3.2.3-3.The frame number used for initialization is that of the frame in which the UL map that specifies the uplink burst was transmitted.

Figure 3.2.3-3 OFDM randomizer uplink initialization vector

3.2.4

A Forward Error-Correction code(FEC),consisting of the concatenation of a

Reed–Solomon outer code and a rate-compatible convolutional inner code, shall be supported on both uplink and downlink. Support of Block Turbo Code(BTC) and Convolutional Turbo Code (CTC) is optional.

The most robust burst profile shall always be used as the coding mode when

requesting access to the network and in the FCH burst.The encoding is performed by first passing the data in block format through the RS encoder and then passing it through a zero-terminating convolutional encoder.

The Reed–Solomon encoding shall be derived from a systematic RS (N = 255, K = 239, T = 8) code.Puncturing patterns and serialization order that shall be used to realize different code rates are defined in Table 3.2.4-2. In the table,“1” means a transmitted bit and “0” denotes a removed bit, whereas X and Y are in reference to Figure 3.2.4-1.

Figure 3.2.4-1 Convolutional encoder of rate 1/2

Table 3.2.4-2 The inner convolutional code with puncturing configuration

Table 3.2-4-3 gives the block sizes and the code rates used for the different modulations and code rates. As 64-QAM is optional for license-exempt bands, the codes for this modulation shall only be implemented if the modulation is

implemented.

Table 3.2.4-3 Mandatory channel coding per modulation

When subchannelization is applied, the FEC shall bypass the RS encoder and use the Overall Coding Rate as indicated in Table 3.2.4-3 as CC Code Rate.The Uncoded Block Size and Coded Block size may be computed by multiplying the values listed in Table 3.2.4-3 by the number of allocated subchannels divided by 16.

In the case of BPSK modulation,the RS encoder should be bypassed.

3.2-5

All encoded data bits shall be interleaved by a block interleaver with a block size corresponding to the number of coded bits per the allocated subchannels per OFDM symbol,Ncbps.Table 3.2-5-1 gives the block size of the Bit Interleaver.

Table 3.2.5-1 Block sizes of the Bit Interleaver

3.2.6 Modulation

After bit interleaving,the data bits are entered serially to the constellation mapper.BPSK,Gray-mapped QPSK,16-QAM,and 64-QAM as shown in Figure 3.2-6-1 shall be supported,whereas the support of 64-QAM is optional for license-exempt bands.The constellations (as shown in Figure 3.2-6-1) shall be normalized by multiplying the constellation point with the indicated factor c to achieve equal average power.For each modulation, b0 denotes the least significant bit (LSB).

Figure 3.2.6-1 BPSK, QPSK,16-QAM,and 64-QAM constellations

The constellation-mapped data shall be subsequently modulated onto all allocated data subcarriers in order of increasing frequency offset index.The first symbol out of the data constellation mapping shall be modulated onto the allocated subcarrier with the lowest frequency offset index.

3.2.7

Pilot subcarriers shall be inserted into each data burst in order to constitute the symbol and they shall be modulated according to their carrier location within the OFDM symbol. The PRBS generator depicted hereafter shall be used to produce a sequence, w .The polynomial for the PRBS generator shall be _k X¹¹+X⁹+ . 1

The value of the pilot modulation for OFDM symbol k is derived from wk. On the downlink ,the index k represents the symbol index relative to the beginning of the downlink subframe. For bursts contained in the STC zone when the FCH-STC is present,index k represents the symbol index relative to the beginning of the STC zone.

In the DL Subchannelization Zone,the index k represents the symbol index relative to the beginning of the burst.On the uplink ,the index k represents the symbol index relative to the beginning of the burst.On both uplink and downlink,the first symbol of the preamble is denoted by k=0.The initialization sequences that shall be used on the downlink and uplink are shown in Figure 3.2-7-1.On the downlink,this shall result in the sequence 11111111111000000000110… where the 3rd 1, i.e.,w2 =1,shall be used in the first OFDM downlink symbol following the frame preamble.For each pilot (indicated by frequency offset index),the BPSK modulation shall be derived as follows

DL: c₋₈₈ =c₋₃₈ =c₆₃ =c₈₈ =1−2w_k and c₋₆₃ =c₋₁₃ =c₁₃ =c₃₈ =1−2w_k UL: c₋₈₈ =c₋₃₈ =c₁₃ =c₃₈ =c₆₃ =c₈₈ =1−2w_k and c₋₆₃ =c₋₁₃ =1−2w_k

Figure 3.2.7-1 PRBS for pilot modulation

3.2.8 Preamble structure

The first preamble in the downlink PHY PDU,as well as the initial ranging preamble,consists of two consecutive OFDM symbols.The first OFDM symbol uses only subcarriers the indices of which are a multiple of 4.As a result, the time domain waveform of the first symbol consists of four repetitions of 64-sample

fragment,preceded by a CP.The second OFDM symbol utilizes only even subcarriers,resulting in time domain structure composed of two repetitions of a 128-sample fragment,preceded by a CP.The time domain structure is exemplified in Figure 3.2-12.This combination of the two OFDM symbols is referred to as the long preamble.

Figure 3.2.8-1 Downlink and network entry preamble structure

The frequency domain sequences for all full-bandwidth preambles are derived from the sequence：

-1-j, -1-j, -1-j, 1+j, 1-j, 1-j}

The frequency domain sequence for the 4 times 64 sequence P4x64 is defined by：

( ) ( ( ) )

the factor of 2 equates the Root-Mean-Square (RMS) power with that of the data section.The additional factor of 2 is related to the 3 dB boost.

The frequency domain sequence for the 2 times 128 sequence PEVEN is defined by：

( ) ( ( ) )

In the uplink,when the entire 16 subchannels are used,the data preamble,as shown in Figure 3.2.8-2 ,consists of one OFDM symbol utilizing only even subcarriers.The time domain waveform consists of two 128 samples preceded by a CP. The subcarrier values shall be set according to the sequence . This preamble is referred to as the short preamble. This preamble shall be used as burst preamble on the downlink bursts when indicated in the DL-MAP_IE.

PEVEN

Figure 3.2.8-2 P_EVEN time domain structure

3.2.9

A frame consists of a downlink subframe and an uplink subframe.

(a) Time-Division Duplex(TDD) Frame structure

Figure 3.2.9-1 illustrates an example of OFDM frame structure with TDD.

Downlink

– A downlink subframe consists of only one downlink PHY PDU.

– A downlink PHY PDU starts with a long preamble,which is used for PHY

synchronization.

– The FCH burst is one OFDM symbol long and is transmitted using BPSK rate

1/2 with the mandatory coding scheme.

– Each downlink burst consists of an integer number of OFDM

symbols,carrying MAC messages,i.e., MAC PDUs.

Uplink

– A uplink subframe consists of contention intervals scheduled for initial

ranging and bandwidth request purposes and one or multiple uplink PHY PDUs,each transmitted from a different SS.

– An uplink PHY burst,consists of an integer number of OFDM symbols,carrying MAC messages,i.e., MAC PDUs.

In each TDD frame (see Figure 3.2.9-1),the Tx transition gap(TTG) and Rx transition gap (RTG) shall be inserted between the downlink and uplink subframe and at the end of each frame,respectively,to allow the BS to turn around.

Figure 3.2.9-1 example of OFDM frame structure with TDD

z Frequency-Division Duplex (FDD) Frame structure

Basically,the frame structure of FDD is same with the frame structure of TDD.

Figure 3.2.9-2 illustrates downlink and uplink frame structure of an OFDM system.

Figure 3.2.9-2 (a) OFDM frame structure with FDD for downlink subframe

Figure 3.2.9-2 (b) OFDM frame structure with FDD for uplink subframe