Conformity Magazine - April 2008 - (Page 42)

while IEEE 802.11n channel models A through F provide the baseline for testing Wi-Fi devices. As an example, Table 1 provides key parameters of the ITU M.1225 Pedestrian B and Vehicular A channel models, including changes by the WiMAX Forum for spatial considerations to make these models relevant for multiantenna configurations. Table 2 provides key parameters of IEEE 802.11n channel models A through F. Many test organizations, product development labs and others may have their own models or permutations of these standard models that they feel better represent the conditions in which devices are expected to operate. A channel emulator must have the ability to use standard models as well as custom, user-defined models. In the real world, a bi-directional path exists between the mobile station and the base station. This bi-directional channel enables the normal communication that takes place between these device pairs. Sometimes these channels are half duplex, as in the case with WiMAX time division duplexing (TDD), and sometimes they can be full duplex, as in the case of WiMAX frequency division duplexing (FDD). These channels are most often described as downlink and uplink relative to the base station/access point. The bi-directional downlink and uplink channels undergo fading and multipath conditions and, for a system test to closely represent actual channel conditions, the channel emulator must offer bi-directional channels, with fading and multipath in both the downlink and uplink directions. The real channel from the base station/access point to mobile station is reciprocal, that is, the paths traversed by the signal from an access point transmit antenna to a mobile station receive antenna are identical, but reversed to the signal launched at the mobile station and received at the access point. In some cases, such as beamforming, the radio relies on this reciprocity principal to work properly. For a channel emulator to accurately reproduce real-world channel conditions, the emulated downlink and uplink channels must also adhere to the reciprocity principal. This further implies an inherent “balance” between these channels. An emulator that does not provide such balance cannot accurately test AAS techniques. Beamforming requires not only reciprocity but, since its operation is based on reporting received signal conditions, the reciprocal channels must be calibrated to have balanced phase, amplitude and delay. Multiple Antenna Connection Support is a Key Component Next-generation wireless broadband systems make use of multiple antenna technologies to improve range, performance and network capacity. Examples of these technologies include spatial multiplexing, and AAS schemes like beamforming, space time coding (STC) and maximal ratio combining ITU (MRC), all loosely described Vehicular A (long tap) as MIMO or by their multiple 120 antenna configurations, such 530 as SISO, multiple-input singleoutput (MISO), and single-input 10000 multiple-output (SIMO). 6 5o 70o Models Parameters Max Doppler speed (km/h) RMS Delay Spread (ns) Maximum Delay (ns) Number of Taps Total Angular Spread (BS) Total Angular Spread (MS) Antenna Configurations ITU Pedestrian B 3 375 3700 6 5 o ITU Vehicular A 60 595 2510 6 5 o 68o 70o BS 4λ, MS λ/2 (High Correlation) BS, MS Cross Polarized (Med. Correlation) BS 4λ, MS λ/2 Cross Polarized (Low Correlation) Table 1: Parameters of ITU M.1225 Pedestrian B and Vehicular A channel models Models A 5 0 0 1 N/A B 5 15 80 9 2 C 5 30 200 14 2 D 10 50 390 18 3 E 20 100 730 18 4 F 30 150 1050 18 6 Figure 1 shows SISO, MISO, SIMO and MIMO antenna configurations, and indicates the performance enhancing techniques that each configuration may enable. Spatial multiplexing provides performance improvements by increasing the capacity of the system, defined effectively as bits per second per hertz (bps/Hz). Beamforming improves the range of the network by steering the signal power to the user. STC, a form of transmitter diversity, and MRC, a form of receiver diversity, are techniques that respectively transmit and receive Parameters Avg 1st Wall Distance (m) RMS Delay Spread (ns) Maximum Delay (ns) Number of Taps Number of Clusters Rx and Tx Antenna Spacing 1/2λ, 1λ, 4λ Table 2: Parameters of IEEE 802.11n channel models 2 Conformity April 2008

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Conformity Magazine - April 2008 NewsBreaks Editor’s Note Mandatory Analog Cellular Service Ends FCC Releases Quarterly Reports on Consumer Inquiries and Complaints New Standards List Released for The EU’s Low Voltage Directive FDA Issues Notification on Unretrieved Device Fragments FDA Offers Guidance on Waivers for In-Vitro Diagnostic Devices ESD Open Forum Emerging Issues in Standards Challenges in Testing Design Fundamentals Focus on... New Products Product Reviews Premarket Approval Application Guidance Issued by FDA New Jersey Enacts Electronic Waste Recycling Regulations A New U.S. Standard for Workstation and Computer Design Might the CPSC Hold Retailers Accountable for Safe Products? CPSC Loses “Sole Toy Tester” CPSC Actions in the News IEC Standards Update UL Standards Update Telcordia Standards Update From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - April 2008

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