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	TABLE OF CONTENTS Home MIMO intro and hardware options Hardware checklist, continued

The WiMAX Forum's certification standards support MIMO in two baseline profiles. The first is Matrix A, which is a space-time or Alamouti coding scheme that delivers more robust links through diversity. The second is Matrix B, which is a spatial multiplexing scheme used to increase subscriber data rates. In 802.16e, client devices must be designed to support receive processing for Matrix A and Matrix B MIMO.

WiMAX client devices must also communicate information back to base stations to enable accurate in-the-moment characterization of the radio channel between subscriber and base station. This characterization, often called channel state information (CSI), enables enhanced modes of both MIMO Matrix A and Matrix B that include active interference cancellation. These enhanced modes are particularly important in interference-limited environments, where there are large numbers of users.

FPGAs or DSPs?
Although systems designers have multiple options for implementing MAS techniques such as MIMO and adaptive antenna systems, not all of them have the same benefits. One decision is whether to use FPGAs or DSPs.

FPGAs have the disadvantage of long lead times for tasks such as creating, testing and tweaking custom algorithms. These lead times hinder the vendor's ability to bring its infrastructure products to market. In a market as crowded and competitive as WiMAX—there are already more than 400 companies developing products—time-to-market can make or break an infrastructure vendor's competitive position. Long lead times also delay the point at which a mobile WiMAX product begins driving revenue.

Another drawback to FPGAs is that adding interfaces often requires custom work, which creates additional delays and costs. By comparison, a good DSP will have built-in support for a variety of interfaces, such as RapidIO.

Figure 3 illustrates an 802.16e modem card with MIMO that uses a DSP-based design. This example features Texas Instruments' new 1-GHz TCI6455 DSP.

(Click to enlarge)
3. TCI6482 Based Modem Design

A DSP checklist
When choosing a DSP for mobile WiMAX applications that use MAS techniques, systems designers should look for several features that determine both their ability to bring their product to market quickly and their product's competitive position. These must-have features include:

• Computing power – The ideal mobile WiMAX DSP should have the cutting-edge computational performance necessary to support demanding multi-antenna techniques. More computational power means better adaptive antenna and MIMO performance, which helps improve QoS to the point that it's noticeable to service providers and their customers. From a sales perspective, this clear performance advantage helps the WiMAX infrastructure product stand out from the competition, and it can be used to justify a price premium.
• Tools – The DSP's manufacturer should provide a suite of tools that let systems designers optimize and take advantage of the computational engines on the DSP. The last thing that system designers should have to do is devote resources to writing a lot of assembly code to perform those types of tasks. Systems designers also should look for DSPs that are backed by extensive smart antenna scheduler and calibration libraries. These libraries maximize resource allocations for adaptive antenna systems and MIMO, and the automated calibration scheduler ensures optimum base station performance.
• Interfaces – Many mobile WiMAX infrastructure products will have a DSP devoted just to MAS tasks. That design means that large volumes of data have to be transferred between the DSPs. This highlights the value of using DSPs that include standardized, high-speed interfaces such as Serial RapidIO, which also can be used for fast connections between components, such as the DSP and the MAC controller. Built-in RapidIO support is particularly valuable because it is standardized and has a low pin count, two attributes that both eliminate the need for glue logic and help reduce system cost. Regardless of whether the systems designer prefers RapidIO, all interfaces should be backed by the DSP's manufacturer in the sense that all of the necessary code is provided and that it has been thoroughly debugged.

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