The chart demonstrates the relative performance of normalized throughput for a variety of solutions. A performance level of 1 represents the theoretical maximum performance of a MIMO system, which consists of two completely independent channels. In reality, a performance level of 1 cannot be achieved. A performance level of 0 means that multiple antennas are effectively behaving as a single antenna, which defeats the purpose of MIMO entirely.

The test results show that ceramic chip and short monopole flex antennas offer the worst performance. Printed-circuit board (PCB) and planar inverted-F antenna (PIFA) stamped metal designs exhibit somewhat better performance. These four design implementations share many of the same characteristics. Each of these solutions is entirely embedded and therefore enables a sleeker design at the expense of performance. They also exhibit spatial diversity, meaning that the different signal paths are achieved from physical separation between the antennas.

Standard ceramic, flex, PCB, and stamped metal solutions commonly use two antenna structures in MIMO implementations and tend to be sold as “standard” parts. This leaves the problem of placing the antenna in the optimal position for high isolation, low correlation, and high efficiency up to the device designer, who may not be able to adequately control the industrial design. In practice, the data sheets for these parts may show great performance, but the performance cannot necessarily be achieved in a production device. Integration support from suppliers may also be difficult to obtain.

As shown, the external whip antenna offers relatively high performance. In this configuration, one external antenna is typically used in conjunction with one internal antenna, each offering different performance levels. Figure 2 shows an example of this implementation with external and internal SkyCross antennas for the Samsung SPH-H1100 WiMAX PC card. The SPH-H1100 card operates on the WiBro network in South Korea, which was the first deployed WiMAX-compliant network. Although effective, external antennas limit the aesthetic appeal of a device and occasionally break.

The highest-performing MIMO solution shown in Figure 1 is obtained using the iMAT antenna from SkyCross. iMAT is an acronym for isolated mode antenna technology, which enables a single antenna element to behave like multiple, equally high-performing antennas through the use of multiple feed points.

In contrast to the other antenna solutions, iMAT offers pattern diversity rather than spatial diversity. Pattern diversity is a term suggesting that the antenna exploits independent signals from differences in the 3D shape of the antenna’s radiation field versus spatial separation. As a result of these benefits, embedded iMAT antennas usually outperform external antennas and enable a more aesthetically appealing device.

In addition to performance benefits, the iMAT solution offers integration benefits. The single antenna element eliminates the guesswork for device designers who must implement MIMO. The iMAT solution is a single part with high isolation, low correlation, and high efficiency built-in.

Performance is important, but in reality it may not be enough to convince a device manufacturer to select one solution over another. Cost is also a key driver in this competitive, high-volume industry. As a design technique, iMAT isn’t married to a particular material or antenna style. Therefore, SkyCross can apply iMAT cost-effectively using standard manufacturing methods such as stamped metal (Fig. 3). iMAT also reduces the number of parts needed on the bill of materials while being cost-effective in its own right.

WiMAX in Applications

Many regions around world are currently planning or rolling out WiMAX networks in select bands. In general, a single WiMAX band for a region covers at least a 200-MHz frequency span. This relatively broadband solution enables WiMAX to offer high capacity for densely populated areas such as large cities. Developing regions are also implementing WiMAX as a wireless replacement for last-mile connectivity typically served by DSL or cable lines today. The high frequency of WiMAX makes this a more challenging implementation than large city coverage. 

The MiMAX Q-Series quad-band USB product from Airspan Networks covers 2.3 to 5.9 GHz and supports all global WiMAX frequencies plus Wi-Fi (Fig. 4). It is the first USB device certified by the WiMAX Forum, and it uses an iMAT antenna from SkyCross. Figure 5 shows key performance metrics for a derivative antenna design in a similar form factor: best isolation of less than –20 dB, correlation coefficient of nearly 0 across most of the band, and 50% efficiency from either feed point.

The United States is also enjoying the benefits of WiMAX in select areas. Sprint XOHM launched its first WiMAX service in Baltimore last September. Its first client device, the Samsung SWC-E100 ExpressCard, is powered by SkyCross antennas. Independent tests in Baltimore prove the superior link performance provided by WiMAX antennas from SkyCross.

While we all have to wait and see how WiMAX and LTE unfold, the WiMAX launches to date give us a taste of the 4G wireless future. Regardless of the protocol, MIMO will be a staple of the next-generation devices, and developers will be challenged to design solutions that effectively address the space constraints in very small form factors.

Joe Gifford has more than 20 years of entrepreneurial and corporate leadership experience as a business strategist and tactical operating manager in the wireless industry and venture-backed community. He completed his undergraduate studies at Rutgers University and graduate studies at the Wharton School, University of Pennsylvania.