from the transmission rate supported by a subcarrier. Example 12.1 Calculate the maximum transmission rate for the 802.11g standard using the system parame- ters shown in Table 12.4. Table 12.4 Some IEEE 802. 11n physical layer parameters Modulation BPSK, QPSK, 16-QAM, 64-QAM Channel coding rate 1/2, 2/3, 3/4 OFDM symbol length 3.2 ms Guard interval 0.8 ms Carrier bandwidth 20 MHz Number of subcarriers Pilot subcarriers Broadband Telecommunications Technologies and Management Answer The maximum transmission rate is when all subcarriers are transmitted using the highest order modulation and highest coding rate. Total number of data subcarriers = 52-4=48 Total OFDM symbol length = 3.2 + 0.8 = 4 ms; and therefore total symbol rate = 1/4 ms = 250,000 symbols/s Higher order modulation = 64-QAM, which supports 6 data bits per symbol Highest coding rate = 3/4, which means of every 4 transmitted bits 3 are data bits Total maximum transmission rate = 250 000 X 6 X 48 X 3/4 = 54 Mbps Wi-Fi Networking Mobile communications systems such as 3G generally use a centralised control system for resource allocation. For example a radio network controller allocates wireless resource to users as the need arises. It ensures multiple users access a network's resources in a fair and efficient manner. This centralised resource allocation can lead to maximal network usage efficiency. In addition, other centralised nodes keep track of where a user is located SO calls can be forwarded. In contrast most personal communications systems operate without centralised nodes. Call set-up is carried out in an ad-hoc manner, and collisions are avoided through special mecha- nisms. Resource access in Wi-Fi systems is carried out using a channel sense multiple access (CSMA) with collision avoidance (CA) technology. In CSMA-CA, a wireless LAN device wishing to send a packet to another device first listens to see if the channel is currently being used. If the channel is silent, the device sends a short message 'Request to Send - RTS' to the intended dest