Microwave Engineering Europe - April 2008 - (Page 12)

12 TEST & MEASUREMENT station/client and the base station/Access Point. This bi-directional channel is used as part of the normal communication that takes place between these devices. Sometimes these channels are half duplex as in the case of WiMAX Time Division Duplexing (TDD), but sometimes they are full duplex as is the case with WiMAX Frequency Division Duplexing (FDD). These channels are often described as downlink and uplink relative to the base station/Access Point. Both the bi-directional downlink and uplink channels undergo fading and multipath conditions. For a system test that closely represents realworld channel conditions, a channel emulator must provide bi-directional channels with full fading and multipath in both the downlink and uplink paths. The real channel from the base station to mobile station is reciprocal with the real channel from the mobile station to the base station. For a channel emulator to accurately reproduce real-world conditions, the emulated downlink and uplink channels must be reciprocal. Adaptive Antenna System (AAS) technologies such as beam forming rely on this principal to work properly. This further implies an inherent “balance” between these channels. Without the emulator providing such balance, accurately testing these beam forming techniques is not possible. To accurately represent the real-world over-the-air conditions in which WiMAX and Wi-Fi systems will operate, an effective channel emulator must be dynamic and provide very long intervals of non-repeating channel conditions. In addition, the channel emulator must have the ability to use both built-in standard channel models (ITU M.1225 Pedestrian B and Vehicular A for WiMAX or IEEE 802.11n channel models A through F for Wi-Fi) as well as custom, user-deﬁned models. Finally, accurate system testing of current and future MIMO techniques requires bi-directional (with full fading and multipath in both downlink and uplink paths) and reciprocal channel emulation technology. Multiple antenna connection support is critical for most radio technologies Most next-generation wireless data systems make use of multiple antenna technologies to improve range, performance and capacity. There are different techniques used to achieve these enhancements. Some examples include MIMO spatial multiplexing, AAS, Space Time Coding (STC) and Maximal Ratio Combining (MRC). Such techniques are often described by their multiple antenna conﬁgurations, such as SISO, MISO (MultipleInput Single-Output), SIMO (Single-Input Multiple-Output), and MIMO. Figure 1 shows SISO, MISO, SIMO and MIMO antenna conﬁgurations and indicates the performance enhancing techniques that each conﬁguration may enable. Spatial multiplexing (MIMO) typically provides performance improvements by increasing the capacity of the system, deﬁned effectively as bits per second per hertz (bps/hz). AAS improves the range of the network by steering the signal power to the user. STC, a form of transmitter diversity, Why is beam forming a critical feature to WiMAX and Wi-Fi devices? Beam forming is believed to be a critical enabler of cost effective WiMAX network installations. Without taking advantage of the range extension capabilities of beam forming, typical WIMAX installations will require a large number of WiMAX basestations to provide adequate network coverage for a given area thus increasing the capital and operating costs of a WiMAX network. Similarly, beam forming is an optional feature that also increases the range of Wi-Fi devices may prove to be a critical feature in many enterprise and home infrastructure and client devices. Figure 1: From the top: SISO, MISO, SIMO and MIMO antenna conﬁgurations. and MRC, a form of receiver diversity, are techniques that respectively transmit and receive multiple copies of the same user data in an effort to combat impairments, such as fading. Employing any of these techniques requires multiple antenna connections for proper testing of the system. Further, as the antennas are often correlated on the equipment under test, a system that accurately provides for considerations such as cross correlation, angle of arrival and departure, and angular spread is necessary. For WiMAX systems, AAS techniques that use many antennas at the base station are common to extend the range of the system, reducing the number of base stations required. A WiMAX MISO system with four antennas and MIMO-enabled Wi-Fi systems with a minimum of three antennas is not uncommon. At mobile stations, where battery life is a major concern, techniques like MRC help improve overall performance without costly multiple transmitters (SIMO). Both of these techniques result in the need for channel emulation with a large number of antenna connections. Effective channel emulation requires at least 4x4 capabilities to handle the many modes that are being deployed today, as well as to be ready for the technologies deﬁned by the IEEE 802.16e and draft IEEE 802.11n standards upon which mobile WiMAX technology and MIMO-enabled Wi-Fi are respectively based. Microwave Engineering Europe ● April 2008 ● www.mwee.com 011-012-014-016_MWEE.indd 12 26/03/08 17:56:39 http://www.mwee.com

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Microwave Engineering Europe - April 2008 News Contents Comment Test and Measurement: Comprehensive WiMAX and Wi-Fi Product Design Demands Effective Channel Emulation Military/Aerospace Focus: Hardware Needs Limit Software Radio Interview — Mitsubishi Electric Europe: GaAs Technologies Spanning High-End Space and Radar Through to Cost-Sensitive Handset and LNB Applications How Do You Test ZigBee Transmitters? Advanced Receiver Design Boosts Performance CMOS PAs Pave the Way for One-Chip Phones Products Calendar

Microwave Engineering Europe - April 2008

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