Microwave Engineering Europe - December 2007 - (Page 20) 20 TEST & MEASUREMENT — OFDM (–21, –7, +7 and +21) are the pilots. The packet structure is Preamble – Header – Data Block, and the sub-carrier modulation types are BPSK, QPSK, 16-QAM, or 64-QAM. The original WLAN standard is 802.11b, which is not based on OFDM. Also, a and g are the same: a works in the 5 GHz ISM band and g works in the 2.4 GHz ISM band. Standard j is a slower symbol rate version of g for the Japanese market, and n is based on MIMO technology, which is covered in another white paper. Table 2: WLAN summary. Several organizations are involved with WLAN: WiFi is an industry consortium that defines a required subset of 802.11 to ensure better operation between different vendors’ equipment, while EWC is an industry consortium that took the unfinished n standard, agreed upon a version, and is attempting to field solutions prior to 802.11N ratification. Test equipment requirements for WLAN Test equipment for WLAN must have a frequency range up to about 6 GHz and be able to modulate or demodulate OFDM signals with a bandwidth of up to 16.25 MHz for all types apart from 802.11n, which has a maximum bandwidth of 40 MHz. So far we’ve looked at OFDM. In OFDM all the carriers are used to facilitate a single link. OFDMA (Orthogonal Frequency Division Multiple Access) assigns different groups of sub-carriers to different users in a similar fashion as in CDMA. OFDMA’s bestknown use is in WiMAX. Table 3: Fixed and mobile WiMAX. Figure 12: Mobile WiMAX uses both frequency division multiplex and time division multiplex. Groups of sub-carriers represent individual data streams. Each group of sub-carriers also has a frame structure. Figure 13: The frame structure equates to a packet. There is a timing gap between the uplink and downlink called the transition gap. WiMAX WiMAX, or the Worldwide Interoperability for Microwave Access, is very similar in concept to 802.11, but the demands of multiple simultaneous users make the implementation much more complex. There are two major variations of WiMAX: fixed and mobile. The mobile version, 802.16e-2005 (often called 802.16e), facilitates the link between mobile devices. It uses SOFDMA (Scalable OFDM Multiple Access), which interoperates with OFDMA but requires new equipment. 802.16e also adds MIMO (Multiple-Input Multiple-Output), which is the subject of another white paper. The fixed version of WiMAX, 802.162004 (often called 802.16d), uses OFDMA and operates from 2–11 GHz (no regulatory approval above 5.9 GHz); it delivers a practical data rate of 10 Mbps over 2 km. The differences are summarized in Table 3. Fixed WiMAX is similar in some respects to WLAN, i.e., it has an OFDM physical layer. Mobile WiMAX is based on an OFDMA physical layer. It uses both frequency division multiplex and time division multiplex. Groups of sub-carriers (Figure 12) represent individual data streams. Each group of sub-carriers also has a frame structure. Time division characteristics are shown in Figure 13. There is a timing gap between the uplink and downlink called the transition gap. Microwave Engineering Europe ● December 2007 ● www.mwee.com 016_017_018_020_022_MWEE.indd 20 22/11/07 16:53:52 http://www.mwee.com
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