3.3 Application Features multiple access ofdma

Mode WirelessMAN-OFDMA (hereinafter - OFDMA) as its name implies, is a method of multiple access by dividing the orthogonal carriers. Unlike discussed in a previous publication [2] method, WirelessMAN-OFDM, it is not only about the mechanism of modulation, but also about the way of channel separation. This mechanism is already well known, in particular, it has found wide application in systems of digital television DVB (terrestrial, cable and satellite). One logical OFDMA channel is formed by a fixed set of carriers, generally distributed over the available frequency band of the physical channel. In simplified form this optional mechanism is used in the OFDM mode - recall the decomposition of the channel into 16 subchannels.

From the point of view of forming a modulation OFDMA symbols similar OFDM: OFDMA symbol includes the actual zone data and the preceding guard interval (repetition of the initial fragment of a symbol) that is designed to prevent seismology interference). The symbol is a combination of the modulated orthogonal carriers. In the OFDMA mode of carrying significantly more than in OFDM - 2048 instead of 256, respectively, and the number of subchannels becomes sufficient for the organization of the network in different modes from 32 to 70, 24, or 48 information carriers in each. Are not all 2048 carrying about 200 and bottom 200 of the upper frequencies make a guard interval channel and not modulated. Also, do not use a Central frequency of the channel (frequency index 1024). In addition, part of bearing - pilot intended for service purposes and not for data transfer. The exact number of pilot carriers and frequencies in the protective intervals varies slightly depending on the OFDMA modes, described below.

The system clock frequency is always 8/7 bandwidth physical channel BW. The width of the physical channel is not standard (the standard says "not less than 1 MHz), but in real applications are unlikely to be effective channels for less than 5 MHz.

The method of formation, the structure of OFDM symbols and the mechanism of channel coding in OFDMA is similar to that described for OFDM [2]. Channel coding includes the randomization, error-correction coding, interleaving and modulation. Method of randomization is almost identical OFDM, only different ways of generating the initialization vector of the pseudorandom sequence generator (PRSG).

Error-correction coding in OFDMA as binding provides only the convolutional encoder is the same as in OFDM, and with the same set of velocity encoding. Encoder reed-Solomon no. Optional provision for the use of block and convolutional turbo codes. The method of alternation is also almost identical.

In a descending channel, the first symbol is a preamble. Bearing in characters of preambles modulated by BPSK special pseudo-random code depending on the segment (in PUSC mode) and variable IDcell is defined at the MAC layer [3]. The preamble is modulated every third carrier of the entire channel (except for bearing protection intervals and Central), with the initial shift [0..2] is set to advanced. Recognizing the type of the preamble, the AU defines the value of the variable IDcell and mode of operation of BS.

For the preamble is followed by two symbols, the transmission frame header FCH and map the field distribution for the descending channel DL-MAP. The header is transmitted by QPSK with rate 1/2 coding. It contains the prefix of the descending channel (DL Frame prefix) that identifies segments used and the settings of the descending channel DL-MAP (length, encoding method used and the number of repetitions), broadcast immediately after the frame header. Also in the header flag is used, the installation of which means a change in the location of the field of competitive access in the ascending sub frame to the previous frame.

Next broadcast map upside channel UL-MAP and the downstream data packets for different AC.

The FUSC mode means that uses all physical channels (all possible carriers). This 1702 carrying frequency information and a guard interval (173 and 172 of the bearing in the top and bottom of the range, respectively). Center frequency at index 1024 is not used.

Figure 13 - the Combination of different “zones reshuffle” in the OFDMA frame.

In FUSC mode is primarily assigned to the pilot frequency. They are divided into fixed and variable. Lists and those others mentioned in the standard. The term "variables pilot frequency" means that in each even-numbered OFDMA-symbol indexes are presented in document IEEE 802.16, in each odd - climb 6 (numbered OFDMA symbol starts with 0). There are a total of 166 of the pilot frequencies, 24 of which are fixed. Both fixed and variable pilot frequencies are divided into two sets of equal volume. This break is only important when working with adaptive antenna systems in the regime of space-time coding (STC).

After determining the pilot frequencies of the remaining 1536 carriers are intended for data transmission. They are divided into Nsubchannels = 32 subchannels on Nsubcarriers = 48 bearing in each. The goal of the information-carrying sub-channels is in accordance with the formula:

where subcarrier(k,s) - carrier index k in subchannel s, s = [0...Nsubchannels - 1],

k = [0...Nsubcarriers - 1],

nk = (k + 13s) mod Nsubcarriers. IDCell

- the ID of the segment BC, which is defined at the MAC layer (specified by the base station a variable in the range 0-31). P(x) means the x-th element of the sequence of permutations {P}, given in the standard (P= {3, 18, 2, 8, 16, 10, 11, 15, 26, 22, 6, 9, 27, 20, 25, 1, 29, 7, 21, 5, 28, 31, 23, 17, 4, 24, 0, 13, 12, 19, 14, 30}). The operation x mod k is the remainder of x/k.

Obviously, before applying the formulas of information carriers shall be renumbered so that their indices within the range 0-1535 (the last value corresponds to the physical index 1702), i.e. numbered in a row, excluding the pilot frequencies. Because the even and odd symbols, the location of the pilot frequencies are different, the distribution of information carriers for them also need to be calculated independently.

In the PUSC mode entire available range is divided into 60 sub-channels. By definition, the work is only a part of them, but not less than 12. The subchannels are grouped into six segments, three basic (segments 0, 1 and 2), each includes 12 subchannels (0-11, 20-31 and 40-51 sub-channels, respectively). Obviously, based on the requirement of a minimum of 12 subchannels, the base segments can be used only in conjunction with basic. The division into segments is introduced to BS it easier to communicate, in any moment), but it works (just report the numbers of segments).

Figure 14 - structure of the cluster

The symbols in the PUSC mode is formed by the following principle. There are a total of 2048 frequencies, of which the Central (index 1024) and protective (184 lower and 183 upper) not used. The remaining 1680 bearing successively divided into 120 clusters, each contains a bearing 14. After that, consecutive physical clusters are renumbered in a "logical" in accordance with the formula LogicalCluster = RenumberingSequence [(PhysicalCluster+13 IDcell) mod 120], where RenumberingSequence (x) is the corresponding element described in the IEEE 802.16 a sequence of permutations, IDcell is defined at the MAC level identifier of the individual segment BS (base station asked a variable in the range 0-31). This operation actually means interleaving - distribution of consecutive groups of carriers across the band of the physical channel. Next logical clusters are divided into six groups(0-23, 24-39, 40-63, 64-79, 80-103, 104-119), on 24 and 16 clusters. Large groups correspond to large segments (default, group 0 corresponds to the segment 0 group 2 - segment 1, group 4 segment 2). In each cluster are determined pilot bearing - for even symbols this is the 5th and 9th are load-bearing, for odd - 1-I and 13-I (figure 14).

Thus, the set of subchannels within a segment or multiple segments is assigned to a set of carriers (12 subchannels - bearing 336, 24 of the pilot information and 288). Information carriers in the segment are numbered in a row, not including the pilot frequency, and then in accordance with formula (1) to each subchannel are assigned by bearing 24. In this case, in equation (1) uses the values Nsubchannels = 12 or 8, Nsubcarriers = 24, and also a special permutation of the sequence of P12 and P8 for the segments 12 and 8 channels, respectively (given in the standard [3]).