Microwave Engineering Europe - December 2007 - (Page 22) 22 TEST & MEASUREMENT — OFDM Figure 14: In this WiMAX measurement, we see a packet structure containing downlink and uplink data, DL and UL, each separated by a transition gap. The UL contains more data and would use a complex modulation format such as QAM. This is what we have chosen to demodulate, although we could also demodulate the DL portion, which is QPSK. We can even demodulate both and display a hybrid of the two modulation types in the constellation. Mobile WiMAX is a dynamic system. The amount of data transferred is a function of the modulation type and symbol rate on each set of sub-carriers. If the link quality is good, a high throughput modulation type such as QAM is used, and most of the bandwidth is consumed, thus limiting the number of users on the system. As the user moves further away from the base station, the signal quality decreases, and with it the ability to maintain a high throughput. A lower throughput modulation scheme such as QPSK would then be employed. This, of course, does not require a large group of sub-carriers, so the system can support more users. Figure 14 shows two WiMAX measurements that the Keithley Model 2820 can perform. We can see a packet structure containing downlink and uplink data, DL and UL, each separated by a transition gap. The UL contains more data and would use a complex modulation format such as QAM. This is what we have chosen to demodulate, although we could also demodulate the DL portion, which is QPSK. We can even demodulate both and display a hybrid of the two modulation types in the constellation. Conclusions In terms of speed versus mobility, the WLAN and WiMAX standards provide a marked increase in data speed over traditional cellular based communications technology. The future of wireless and of fourth generation cellular systems, such as LTE or UWB, will be based on a combination of OFDM types of modulation and MIMO radio configurations (Figure 15). When choosing test equipment for testing today’s radio standards, it’s important to consider the evolution of wireless technology and to ensure that your purchases are forward compatible. One key consideration for instrumentation is bandwidth; WiMAX and WLAN have bandwidths that can exceed 25MHz. As shown in Figure 16, the Keithley range of wireless equipment has 40MHz of bandwidth as standard, creating a new price performance point in the market place. Figure 15: The long term evolution (LTE) of wireless and of fourth generation cellular systems will be based on a combination of OFDM types of modulation and MIMO radio configurations. When choosing test equipment for testing today’s radio standards, it’s important to consider the evolution of wireless technology and to ensure that your purchases are forward compatible. Figure 16: One key consideration for instrumentation is bandwidth; WiMAX and WLAN have bandwidths that can exceed 25 MHz. The Keithley analyzers have 40 MHz of bandwidth as standard, while the generators have arbitrary waveforms available with 20, 40 and 80 MHz bandwidth, creating a new price performance point in the market place. MICROWAVE ENGINEERING EUROPE Free subscription at: www.mwee.com/subscribe Microwave Engineering Europe ● December 2007 ● www.mwee.com 016_017_018_020_022_MWEE.indd 22 22/11/07 16:54:07 http://www.mwee.com/subscribe http://www.mwee.com
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