2.4 Standard 802.16: frame structure
All frames sublayer management media access control (MAC) begin with the same header. It is followed (or not) the data field, and the ends of the frame also optional checksum field. It is shown in Fig. 4.31. The data field is not in service frames that are designed, for example, for the query time intervals. The checksum is optional due to the fact that error correction is performed on a physical level and never attempts to resend frames of information transmitted in real time.
Consider a header field (figure 7, a). Bit the EU suggests that the data field is encrypted. The Type field specifies the type of the frame (in particular, reports about whether the frame is Packed and whether fragmentation). Field C1 indicates the presence or absence of the field the final checksum. Field EC indicates which of the encryption keys is used (if it is used). In the Length field contains information about the overall length of the frame, including the header. The connection identifier is reported, which compounds the current frame belongs to. At the end of a header Checksum field of the header whose value is calculated using the polynomial x8 + x2 + x + 1.
Figure 7 - Normal frame (a); the request frame channel (b).
Figure 7 b shows another type of frame. This is a request frame channel. It begins with the individual and not the zero bits and generally resembles a conventional header of the Frame, with the exception of the second and third bytes that make up the 16-bit number, indicating the required bandwidth for transmission of the corresponding number of bytes. In the request frame is not transmitted data field, and no checksum of the entire frame.
The 802.16 network can cover whole areas of cities and the distances thus calculated kilometers. Therefore, a received signal stations may be of different capacities depending on their distance from the transmitter. These deviations affect the ratio signal/noise, which, in turn, leads to the use of multiple modulation schemes.
In each cell broadband regional network can be much more users than in the conventional 802.11 cell, and wherein each user has a much higher bandwidth than the user of the wireless LAN. Unlicensed (15M) bandwidth is insufficient for the load, so the network 802.16 working in the high frequency range 10-66 GHz.
802.11 support the transmission of information in real time (PCP), but actually they are not intended for telephony and great multimedia traffic. The 802.16 standard, by contrast, is focused on supporting such applications because it is designed both for citizens and for business people.
3. FEATURES OF THE APPLICATION OF OFDM MODEMS AND MULTIPLE ACCESS OFDMA
At the physical layer the IEEE 802.16 standard provides for three fundamentally different methods of data transfer: a method of modulation of a single carrier (SC, in the range below 11 GHz) modulation method by orthogonal carrier OFDM (orthogonal frequency division multiplexing method and multiplexing (multiple access) through orthogonal carrier OFDMA (orthogonal frequency division multiple access).
3.1 Features of the application of OFDM modems.
Mode OFDM is a modulation of the flow of data in a single frequency channel (width 1-2 MHz or more) with a center frequency fc. The division of the channels, as in the case of SC - frequency. Recall that when the modulation data through orthogonal carriers in the frequency channel allocated to N subcarriers:
fk= fc + k∆ f, (1)
where: k-is an integer in the range of [6 N/2, N/2] (in this case k ≠ 0). The distance between the orthogonal carriers.
where: k is an integer in the range of [6 N/2, N/2] (in this case k ≠ 0). The distance between the orthogonal carriers.
where Tb-is the duration of a data symbol.