iMAT: MIMO without multiple antennas
* Date: February 25th, 2008
* Author: Michael Kassner
* Category: Wireless
* Tags: Situation, MIMO, Antenna, iMAT, Antenna System, RF, EM, EM Isolation, SkyCross, iMAT Antenna System
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9
Antenna systems are not normally a preeminent topic when it comes to discussions about wireless equipment. That attitude is completely understandable; antennas are just not that interesting to most people. To some, including myself, RF propagation and antenna technology are very worthy discussion topics. My fascination started 40 years ago when I earned my first amateur radio operator’s license. I still remember the excitement of talking to as many fellow amateurs as I could on an old home built CW transmitter. Not having a great deal of money for new equipment, I focused on improving my antenna farm. It did not take long for me to notice the difference a good antenna made. Poorly designed or poorly built antennas—I learned the hard way—can literally destroy transmitter RF circuits. Whereas a well built antenna afforded me the opportunity to communicate with other amateurs anywhere in the world.
Antenna systems finally get some respect
Wireless equipment manufacturers will always have the challenge of trying to equal or surpass the capabilities of wired voice or data networks. Helping to close that gap are recent innovations using multiple—same frequency—antenna systems. Researchers and equipment manufacturers now understand that using MIMO antenna technology means greater receiver gain, increased data rates, larger network throughput, and improved reliability through antenna diversity. It all sounds good, but as that saying goes “nothing’s ever free” applies here as certain challenges surface when using MIMO antenna systems.
EM coupling and isolation
One of the more important challenges facing RF engineers developing MIMO antenna systems is how to negate the effects of a phenomenon called electromagnetic (EM) coupling. EM coupling according to Wikipedia occurs when a change in current flow through one device induces current flow in the other device. The two devices may be physically contained in a single unit, as in the primary and secondary sides of a transformer, or may be separated as in the antennas on a transmitter and a receiver. One other related term that appears in conversations about EM coupling is EM isolation which is the opposite of coupling. EM isolation is generally achieved in two ways, either by increasing physical distance between RF radiators or by creating a barrier with RF shielding between the two radiators.
It is important to understand that intentional EM coupling is a good thing when it is occurring between multiple antenna/radio systems that are supposed to communicate. On the flip side, unintentional EM coupling between two or more antenna/radio systems not meant to be communicating is detrimental to all of the involved systems. The receiving radio has to deal with additional in-band interference and the transmitting radio loses efficiency as some of its valuable RF output is not reaching the appropriate antenna/radio combination. Figure 1 represents the two extremes of coupling and isolation.
figure-1.jpg
Figure 1. Antennas on the left have coupled near-radiation fields and highly correlated inputs, coupling energy from one into the other. The ideal situation is shown for the antennas at right where little radiation is coupled between antennas, and the correlation coefficient is zero. Courtesy of SkyCross.
Knowing this, one has to wonder if it is such a good idea to have all those antennas so close together on a single radio. Wouldn’t most of the RF energy from one antenna just radiate to the antenna only a few centimeters away? This is certainly one area of concern and compromise, as it forces MIMO equipment developers to delicately balance improved data rates and signal reception against the loss of RF signal performance due to unintentional EM coupling.
Metamaterials to the rescue
A recent post of mine “Metamaterial: the unnatural composite is transforming wireless technology” describes how antennas made from metamaterial are revising RF propagation theory. Metamaterial composite antennas are physically smaller yet electrically more efficient. This creates a win-win situation as it allows more antennas to be located in the same physical space, yet they are less likely to be affected by unintentional EM coupling. Metamaterial antennas can also be constructed to focus the RF wave front away from nearby antennas which also reduces the effect of unintentional EM coupling.
Finally getting to iMAT
Over the past few years I have been following a very interesting company called SkyCross. What initially aroused my curiosity was SkyCross being picked by Samsung to produce antenna systems for the popular Blackjack and Blackjack II cell phones. Being selected is fairly significant especially since SkyCross has only been around since 2000. The company’s forte is described by Chris Morton, CEO:
“SkyCross enables manufacturers to make any wireless device a reality. Whether you’re designing a low-end, cost-conservative product or a high-end device packed with features, SkyCross has the right solution for you.”
The newest project at SkyCross is iMAT (Isolated Mode Antenna Technology) which is the topic of this post and what can be called a radical departure from typical multiple antenna methodology. An iMAT antenna system is a single antenna with multiple feed points that has performance characteristics similar to a MIMO antenna system. The significance of this can be seen immediately. Having only one antenna mitigates the power-robbing effects of unintentional EM coupling. It also gives equipment and PC board designers more freedom as there is no need to reserve space for several antennas and their supporting PCB components.
Researchers at SkyCross use a rather complicated metric called the correlation coefficient to determine the ability of a multiple antenna system to produce independently received signals. Complete signal overlap which is bad, results in a channel correlation coefficient of one. The graphs in Figure 2 show that the conventional two antenna system’s coefficient is more than double the coefficient of the iMAT antenna system.
figure-2.jpg
Figure 2. Illustration of the advantages of the iMAT single antenna over multiple individual antennas showing enhanced efficiency and reduced inter-element coupling. Courtesy of SkyCross.
How is this significant?
Using a single antenna to achieve multiple antenna results has to excite equipment developers. It gives them the chance to achieve MIMO performance levels in devices that have space or size limitations. As I understand there are no real frequency restrictions, so iMAT will support all legacy RF protocols and be available for use with HSxPA, LTE, WiMAX, and 802.11x
I had the good fortune to ask Mr. Paul Tornatta, managing director for SkyCross USA and VP of Operations a few questions about iMAT. I was really curious to learn if my Blackjack phones used iMAT technology. Mr. Tornatta replied:
“Regarding the Backjack and Blackjack II; these designs do not use iMAT, as the design work was completed some time ago. We are currently working on Smartphone implementations, but will not make a specific announcement for several months.”
I then made mention that iMAT must only work if the multiple feeds were on the same frequency, but was pleasantly surprised when Mr. Tornatta corrected my error:
“The comment about iMAT only working for multiple multi-feeds at the same frequency is not quite right. Certainly a key feature is the ability to run multiple feed points from the same multi-band feed structure, at the same frequency, for diversity and MIMO applications, but another key feature is the ability to create isolation nulls to other bands as well.
An example is GPS. In many of our diversity designs, we include GPS on the Diversity antenna port. In this case we are able to create an isolation-null to prevent the radiation from the primary antenna port from effecting the GPS receive function.
We are also doing work using iMAT to isolate transmit and receive functions normally associated with a single antenna (for GSM). By creating isolation between transmit and receive frequencies, we can relieve the filter specification for down-stream components with a potential cost saving. So in this case, the two ports of the antenna structure are at different frequencies.”
To their good fortune, SkyCross readily admits to discovering the underlying technology behind iMAT while trying to solve a completely different issue. As I understand it, an engineer was trying to place several antennas on a miniature circuit board and accidentally created a path between two adjacent antennas. When running some initial tests the engineer determined that the two antennas had a high degree of isolation. After some development SkyCross realized that they now knew how to develop a multiple feed single antenna with efficient RF characteristics.
Final thoughts
It appears that iMAT technology has a very bright future. The next step would appear to be determining how to simultaneously transmit on different frequency bands or even different protocols. |
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