In addition to the data ofdm6символ includes a guard interval duration Tg, so that the total duration of the ofdm symbol

Ts= Tb+ Tg, (3)

Guard interval is a copy of the final fragment of the symbol. The duration Tg can be 1/4, 1/8, 1/16 and 1/32 from Tb.

Each subcarrier is modulated independently by quadrature amplitude modulation.

Figure 8 - OFDM-symbol.

The total signal is calculated by the method of inverse fast Fourier transform (OBPF) as a complex representation is convenient, since the generation of radio signals takes place with the help of the quadrature modulator in accordance with the expression

(4)

where: - a comprehensive view of the square symbol modulation (QAM symbol)

For FFT algorithms/OBPF it is desirable that the number of points in match 2 . Therefore, the number of carriers is chosen equal to the minimum number NFFT = 2 greater than N. In the OFDM mode of IEEE 802.16 N = 200, respectively NFFT = 256. 55 ( k = 6128...6101 and 101...127) form a guard interval at the boundaries of the frequency range of the channel. The Central frequency of the channel ( k= 0) and frequency protection intervals are not used (i.e. the amplitudes of the corresponding signals is equal to zero). Of the remaining 200 bearing eight frequencies - pilot (with indexes±88, ±63, ±38, ±13), the rest are divided into 16 subchannels bearing 12 each, and in one subchannel frequencies are not consecutive. For example, subchannel 1 are bearing index -100, -99, -98, -37, -36, -35, 1, 2, 3, 64, 65, 66. The division into sub-channels is necessary because in the real WirelessMAN-OFDM is provided (optionally) an opportunity to operate in all 16, and one, two, four and eight sub-channels - a type of multiple access scheme OFDMA. To do this, each subchannel and each group of sub-channels have an index (0 to 31).

The duration of the useful part Tb OFDM6символа depends on the bandwidth of the channel BW and system clock frequency (sampling rate) Fs;

Fs = NFFT/ Tb. (5)

The ratio Fs/ BW = n is normalized, and depending on the bandwidth of the channel takes values 86/75 (BW multiple of 1.5 MHz), 144/125 (BW multiples of 1.25 MHz), 316/275 (BW is a multiple of 2.75 MHz), 57/50 (multiple of BW 2 MHz ) and 8/7 (BW multiples of 1.75 MHz, and in all other cases). Guard interval in OFDM-modulation is a powerful tool to combat intersymbol interference (intersymbol interference, ISI) arising from inevitable in an urban environment reflections and multipath propagation of the signal. ISI causes the receiver right on the propagating signal is superimposed multipath signals containing the previous character. When OFDM modulation multipath signal falls within the guard interval and does not cause harm. However, this mechanism does not prevent vnutrishkolnoe interference - the superposition of signals with the same symbol that came with the phase delay. As a result, information can be distorted or completely (e.g., with a phase shift of 180°) just disappear. To prevent information loss in case of failure of individual characters or parts of the IEEE 802.16 standard provides an effective means of channel coding.

The data encoding at the physical layer is divided into three stages - the randomization, noise-immune coding and interleaving. Randomization is the multiplication of the data block in a pseudo-random sequence (SRP), which forms the PRS generator sets with polynomial 1 + x14 + X15. In the downward flow of the PRS generator is initialized with the beginning of the frame by means of code words A. Starting with the second batch frame of the PRS generator is initialized on the basis of the identification number of the base station BSID, ID profile package DIUC (downlink interval usage code) and frame number (figure 9). In an upward flow of everything is similar, with the only difference that the initialization of the generator of the SRP in the circuit shown in Fig.2, with the first package (DIUC is used at UIUC - uplink interval usage code).

Figure 9 - the Formation of the initialization vector generator of the SRP for randomization downstream OFDM.

Coding data involves a cascade code with the two stages of the encoder reed-Solomon from Galois field GF (256) and a convolutional encoder. In basic form, a reed-Solomon code operates on blocks of raw data 239 bytes, forming of them encoded block size of 255 bytes (adding 16 bytes test). This code is able to recover up to 8 damaged bytes. As actually used smaller blocks of data of length K, they are added (239 - K zero bytes. After encoding these bytes are removed. If you want to reduce the number of test words, to reduce the number of bytes restoring T, used only 2 of the first test of bytes. Mandatory to support in IEEE 802.16 options cascade code is given in table 5.

Table 5. The main modes in the IEEE 802.16 standard.

Modulation	The data block encoding byte	The Encoder Reed-Solomon	Encoding rate of the convolutional encoder	Total encoding rate	Block of data after encoding byte
BPSK	12	(12,12,0)	1/2	1/2	24
QPSK	24	(32,24,4)	2/3	1/2	48
QPSK	36	(40,36,2)	5/6	3/4	48
16-QAM	48	(64,48,8)	2/3	1/2	96
16-QAM	72	(80,72,4)	5/6	3/4	96
64-QAM	96	(108,96,6)	3/4	2/3	144
64-QAM	108	(120,108,6)	5/6	3/4	144

After Reed-Solomon encoder receives data at a convolutional encoder (Figure 3) with generators sequences (code generators) G1 = 171 (exit X) and G2 = 133 (for Y) - the so-called standard code NASA. Its basic coding rate - 1/2, ie, each of the input bits it generates a pair of coded bits of X and Y. The sequence of pairs Missing elements Xi or Yi, we can obtain various coding rates.

After encoding interleaving procedure should - mixing bits within the encoded data block corresponding to the OFDM-symbol. This operation is carried out in two stages. The purpose of the first - to make sure that adjacent bits were separated by non-contiguous carriers. In a second step adjacent bits are separated into different halves sequence. All this is done to ensure that when the group errors in the symbol damaged noncontiguous bits that are easy to recover when decoding. Interleaving is implemented in accordance with the formulas

mk = ( Ncbps / 12) – ( kmod 12) + floor ( k / 12);

jk = s – floor ( mk / s) + ( mk + Ncbps – floor (12 mk / Ncbps)) mod s, (6)

k = 0… Ncbps – 1,

where mk and jk is the number of the original bits after the first and second stage of interleaving, respectively;

Ncbps is the number of coded bits per OFDM symbol (for a given number of sub-channels),

s - 1/2 the number of bits on the carrier (1 / 2 / 4 / 6 bits for BPSK / QPSK / 166QAM / 646 QAM, respectively, for BPSK s= 1).

Function floor ( x) is the greatest integer not exceeding x; function ( x mod r) is the remainder of x/ r.

Figure 10 - Generation of the modulating sequence for a pilot bearing.

Each group is defined in accordance with the values of Q and I from vector diagrams warming (figure 10), which are then used in direct modulation of the carrier.

Pilot carriers modulated by BPSK.

After determining modulation symbols by OBPF the radio signal is calculated and transmitted to the transmitter. When taking all procedures performed in the opposite order. Mode OFDM physical layer for networks with architecture "Toccata" human transmission structure is fundamentally little different from the SC mode. As in the high-frequency region, information exchange takes place through a sequence of frames (frames). Each frame (Fig.6) is divided into two subframe - descending (DL from the BS to MSS) and ascending (UL - from the MSS to the BS). The separation of the ascending and descending channels - as a temporary (TDD) or frequency (FDD). In the latter case DL and UL are broadcast simultaneously in different frequency bands.

The downward subframe includes a preamble, a control frame header (FCH frame control header) and the sequence of data packets. Preamble in a descending channel - making of two OFDM6symbols (long preamble), intended for synchronization. First OFDM6 symbol uses carriers with indices that are multiples of 4, the second only even carriers (modulation - QPSK).

For the preamble should the control header of the frame - one OFDM6символ with BPSK and a standard encoding scheme (encoding rate 1/2). It contains the so-called prefix frame of the descending channel (DLFP - Downlink Frame Prefix), which describes the profile and length of the first (or initial) package in DL6субкадре.

In the first packet includes a broadcast message intended for all AU) - map of the location of the packet DL-MAP, UL-MAP, the descriptors of the descending/ascending channel DCD/UCD, other service information. Each package has its own profile (coding scheme, modulation, etc.) and is transmitted by means of an integer OFDM6символов. The point of beginning, and profiles of all packages, other than the first, contained in the DL-MAP.

Figure 11 - structure of the OFDM frame in the temporary depletirovannoi.

The downward subframe contains the interval of competitive access, including the periods for initializing the AU (join the network) and bandwidth request transmission. Followed by the time slots assigned to a certain base station subscriber stations for transmission. The distribution of these intervals (start point) is contained in the message UL-MAP. ACE in the time interval starts a broadcast transmission of a short preamble (one OFDM6символ, uses only even carriers). It is followed by the actual information packet generated on Masarova.

The duration of the OFDM frame can be 2,5; 4; 5; 8; 10; 12,5; and 20 MS. A given base station, the period of building frames cannot be altered, as in this case you'll need to resync all the speakers.

The request to establish the connection does not differ from those generally accepted in the IEEE 802.16 standard, except for the additional mode "concentrated" query (Region-Focused). It is intended only for stations that are able to work with individual sub-channels. In this mode, in the intervals of competitive access (defined in UL-MAP) of the AU can send short Arany code on one of the 48 sub-channels, each of which includes four load-bearing. There are a total of eight codes. Table of codes and sub-channels are given in the text of the standard IEEE 802.16. Code and channel number AC selects randomly.

After receiving a coded message, the BS provides the AC interval for the transmission of "normal" request to grant access (request header Masarova) - if possible. However, unlike other mechanisms, BS in UL6MAP does not indicate the identifier of the requesting station, and quotes the code number of request subchannels and the sequence number of the access interval during which was handed over to the inquiry. These speaker parameters and determines that the interval for the query band transmission is suitable for her. The timing of the transfer arastradero code access request is random, the above-described algorithm appeals to the competitive access channel.

Note that in mode OFDM channel resource may be provided not only in the time domain, but in a separate subchannels (subchannel groups), if the BS and subscriber stations support this. One of the most important applications of such option - Mesh-network.