In addition to reviewing the methods of distribution of bearing, the standard and optional mechanisms - in particular, the so-called optional fusc, not fundamentally different from that considered.

Ascending channel

Rising subframe immediately follows the top-down through the TTG interval. It contains the packets from the subscriber stations and the interval for the access-request/initialize. The minimum size of a single message in an upward subframe (slot) - 3 OFDMA symbol within the subchannel. This has led to the appearance in document IEEE 802.16 term "fragment" (mosaic element, tile).

Figure 15 - structure of the “fragment” of a rising channel.

A fragment is a set of three symbols and four load-bearing, in which the position of the pilot frequencies are rigidly defined (figure 15). The entire frequency range of the channel (1680 carriers) 420 is divided into sequential fragments, 4 carriers in each. Provided 70 subchannels. Each of them consists of 6 pieces - i.e. 24 bearing on the character in one subchannel. The distribution of fragments on the sub-channels is as follows. All 420 of the fragments are broken into 6 groups of 70 fragments. In each subchannel is enabled by one segment from each group in accordance with the equation:

Tile (n, s) = 70n + {P[(n + s) mod 70] + UL_IDcell} mod 70, (8)

where Tile(n, s) - a fragment of the n subchannels s, n = [0...5], s = [0...69].

P(x) is a permutation sequence,

UL_IDcell - variable in the range of 0-69, asked BS to the MAC layer.

As a result, each subchannel in each symbol has its own set of carriers.

After the distribution of the sub-channels occurs numbering information carriers in each slot making a total of three characters 48. Information frequency subchannels are numbered starting with the lowest carrier of the fragment with the smallest index first in the first character, then second and third. Then the information carriers in each slot are renumbered in accordance with the formula:

subcarrier (n, s) = (n + 13s) mod 48, (9)

where s is the number of subchannels, n = [0...47] (i.e., there is a cyclic shift of numbering information bearing on 13s in each subchannel s).

Note that in the text of the document IEEE 802.16 is substitution of terms: a subchannel in an upward subframe songwriters of IEEE 802.16 is called slot information structure the size of the bearing 24 on 3 characters. And when the document is English and white - it is written that in the subchannel 48 information carrier, it should be remembered that from the point of view of correct terminology it is not about the subchannel, but about slot. Real carriers (i.e. natural frequencies) in the subchannel only 24. Multiplying them by 3 (the number of OFDMA symbols in the slot) and subtracting 24 pilot bearing, just obtain 48 information carriers.

Optional in the ascending channel has a mode in which in a fragment of one pilot frequency (figure 16), 6 fragments on subchannel, 96 subchannels (1728 used frequencies).

The mechanisms of the first initialization request in the network and the primary request bandwidth in OFDMA mode is similar and fundamentally different from other modes. For these requests in OFDMA uses a dedicated channel. He is assigned to the BS and consists of six consecutive subchannels, the indices of which are given in the UL-MAP. The query is a 144-bit CDMA code transmitted by BPSK, i.e. 1 bit per carrier in one symbol. As a result of the transfer of such code is 6 subchannels (24 information carriers in each). The code itself is generated in the generator of the SRP - 15-bit shift register with defining polynomial 1 + X1 + X4 + X7 + X15. Senior 6 bits of the initialization vector generator of the SRP is $ variable UL_IDcell, the other 9 is a constant. The code number is determined by the initial point (i.e., the number of cycles of the generator of the SRP after initialization) - there is a total of 256 codes. Moreover, the BS uses only a subset of all possible codes - first N codes first initialization, followed by M code the periodic determination of the parameters AC, then L codes of the query band. For each BS set the start point of the sequence (N + M + L).

Figure 16 - structure of a “fragment” of a rising channel in optional mode

Initialization occurs as follows: AC, taking the handle of the rising channel and the UL-MAP defines a set of CDMA codes and sends in the designated interval randomly chosen code from the group is possible. The same code is transmitted in two consecutive OFDMA-symbols. If the interval of competitive access is more than one slot, the MSS can send CDMA code for the four consecutive symbols, and codes must be adjacent (i.e., sequential fragments SRP).

Successfully adopting and recognizing the CDMA code (and this may not happen, because in the interval of competitive access possible conflict with simultaneous operation of several transmitters AC), base station does not know from which the speaker came the request. Therefore, in response to UL-MAP of the next frame, it indicates the received CDMA code, subchannel and character in which the code was sent. So the MSS determines that the request is accepted, and understands that following this broadcast message indicating the range of the query (number symbol, subchannel, and duration) is intended for her. In this message, the BS transmits the necessary parameters for the initialization process in the network (including the connection identifier CID assigned to the MAC address, a set of physical parameters, etc.). Further specified in UL-MAP interval, the MSS proceeds to the standard procedure of registration in the network.

The primary request of bandwidth in OFDMA method can occur in two ways: through the request headers of the strip, as in the other modes, and by sending a CDMA code of the bandwidth request in the interval of competitive access. The sending of a code request page (as well as the code of the periodic measurement of parameters) occurs in one OFDMA symbol. Possible and sending three consecutive codes in three characters (which you must use is specified in UL-MAP). Adopting the CDMA code, the BS in the UL-MAP repeats his room and options, and also reports the interval for sending the request header strip is already in the normal way.