2.2 Standard 802.16: physical layer

Broadband wireless networks, a broad frequency spectrum, which can only be found in the range from 10 to 66 GHz. Millimeter waves have one interesting property that no longer microwaves: they are distributed in all directions (as a sound), and straight lines (as light). Consequently, the base station must be installed multiple antennas covering different sectors of the surrounding area, as shown in figure 4. Each sector will have its own users. The sectors do not depend on each other, which is not true of mobile radio communication, in which signals propagate in all directions.

Figure 4 - operating environment data 802.16 networks.

Since the power signal is transmitted millimeter waves is greatly reduced with increasing distance from the transmitter (base station), and the ratio signal/noise is also reduced. For this reason 802.16 uses three different modulation schemes depending on removal of subscriber stations. If the subscriber is located close to the BS, applied ZAHM 64 with six bits per sample. On average, the deletion is used CDM-16 and 4 bits/baud. If the subscriber is away then the circuit operates CRC with Two bits per sample. For example, in a typical spectrum band 25 MHz CDM-64 gives a speed of 150 Mbps, SEAM-16 - 100 Mbps, and SFC 50 Mbit/s. Thus the farther the subscriber from the base station, the lower the data rate. Phase diagrams of all three methods were shown on risk 5.

Figure 5 - Phase diagrams of methods used.

The 802.16 standard provides flexible allocation of bandwidth. Apply two modulation schemes: FDD (duplex communication frequency division) and ТDD (full-duplex communication with time division). The latter method is shown in figure 6. The base station periodically transmits the frames are separated by time intervals Ia. The first part of the time intervals allocated for incoming traffic. Then, the guard interval (separator), allowing the stations to switch between transmission and reception, followed by intervals of outgoing traffic. The number of allocated cycles can change dynamically, allowing you to adjust the bandwidth under traffic each direction.

Figure 6 Duplex communication with time division: frames and time slots

Incoming traffic is split into time slots to the base station. She completely controls the direction of transmission. Outbound traffic from subscribers is controlled by more complex and depends on the required quality of service. We will return to the distribution of time intervals, when we discuss the MAC sublayer.

Another interesting property of the physical layer is its ability to pack multiple adjacent frames MAC in one physical transmission. This gives you the opportunity to increase the efficiency of spectrum allocation by reducing the number of different preambles and headers, so loved by the physical layer.

For immediate correction of errors at the physical layer uses a Hamming code. All network technologies simply rely on checksums and encounter bugs with their help, requesting retransmission of the corrupted fragments. But with broadband wireless connectivity at greater distances there are many mistakes that their treatment have to deal with the physical level, although at higher levels and method of checksums. The main task of error correction at the physical level is to make the channel look better than he actually is (exactly the same CDs seem so reliable carriers only due to the fact that more than half of the total number of bits allocated for error correction at the physical level).