MIMO antennas enhance wireless communication links for modern applications
MIMO basics
Taking full advantage of mobility in wireless communications is a complex and ever-evolving issue with the problem of fading ― random attenuation and phase instabilities due to multipath components from signal obstacles (e.g., walls, rain, mountains, brush, etc.). When the user is mobile, updates are triggered often because of the changes in topology and decay in received signal strength. These updates change the channel state information (CSI) at the receiver and/or transmitter ― instantaneous channel gains that enable either the receiver (open loop MIMO) or both the receiver and transmitter (closed loop MIMO) to update its transmission/reception strategy. It is beyond the scope of this article to dive into stochastic modeling that goes into realizing a multiple MIMO data streams although there is a plethora of research covering this topic.
There are generally three different diversity techniques to alleviate the issue of fading: frequency diversity, time diversity, and space diversity. As shown in Table 1, most diversity techniques are a function of the coherence bandwidth or coherence time. This parameter expresses the bandwidth over which two signals will experience similar fading environments. For instance, in the case of a signal in deep fade with a signal with a coherence bandwidth of 1.25 MHz, shifting 1 MHz away from the carrier frequency will likely maintain the deep fade environment. Moving beyond the 1.25 MHz bandwidth yields a higher likelihood of a different fading environment.
MIMO in particular, takes advantage of spatial diversity with multiple antennas at both the transmitter and receiver. The benefits yielded from spatial diversity actually increase with the number of antennas employed. However, there is the tradeoff between transmission rate and spatial diversity. The multitude of antennas elements can either send different data over different paths (multiplexing gain), or the same signal over different paths (diversity gain). Commonly known as the diversity-multiplexing tradeoff, there is a balancing act between these two methods of transmission (Figure 1). Some standard MIMO configurations are 2×2, 3×3, 4×4 (802.11n/ah), and 8×8 (802.11ax/ac).
Multi-user MIMO
Also known as multi-user MIMO (MU-MIMO), the concept of spatial multiplexing can also be applied towards multiple receivers with or without antenna diversity (Figure 2). This increases system efficiency while maintaining link reliability. Wi-FI has support for MU-MIMO in the 802.11ac/ax standards (downlink only) while 4G LTE (rel 8) and LTE-A (Rel. 10) include MU-MIMO. As opposed to SU-MIMO systems, MU-MIMO systems are more susceptible to co-channel interference, often requiring nearly perfect CSI and a low relatively SNR.
Antenna options
Polarization diversity
In many cases, MIMO technology can exploit both spatial and polarization diversity. Polarization describes the direction of an electric field emanating from an antenna element as it propagates through the atmosphere. Linear polarizations will have an electric field in either the vertical (90º) or horizontal (0º) planes whereas slant polarizations propagate -45º or +45º from the horizontal reference plane. Some polarization diversity systems employ circular polarization where the signal rotates forward in three dimensional space in either a clockwise (right hand) or counter-clockwise (left hand) fashion. Antennas with linear polarizations are generally omnidirectional monopoles or dipoles, while patch or helical antennas can exhibit circular polarization. Generally speaking, orthogonality between antennas is highly desirable to provide high isolation between the antennas across a specific frequency band.
Regardless of an indoor or outdoor environment, two parallel channels can be achieved through polarization diversity creating another level of freedom in a given environment. This is particularly useful for backhaul infrastructures that must consider the tradeoff of link distance and throughput as well as solutions operating in the unlicensed ISM bands with tight EIRP restrictions. The physical separation between antennas for adequate spatial diversity presents a space efficiency problem that can be overcome with two polarizations for equivalent, and in some cases better, system throughput [2]. Experiments have shown that the use of two polarizations in environments with typical scattering can yield capacity gains from 10% to 20% over spatially separated antenna elements [3]. Figure 3 shows a number of antenna types with their respective radiation patterns that utilize dual polarization diversity. Some also leverage spatial diversity for indoor/outdoor wireless LAN systems.
Typically, dual polarization is accomplished at -45º and +45º using a linear or planar array of antenna elements that can include dipoles, monopoles, patch, and slot antennas. Polarization is often controlled through a SPDT switch, exciting either the +45º polarized wave or the -45º polarized wave. A sector panel antenna, for instance, could include three ±45° polarized panel antennas where each antenna can also be leveraged for MIMO techniques. Other examples include 360º coverage through the use of four dual polarized antennas with separate feeds for horizontal and vertical polarities.
Separate antenna or combined in one package
Typically, linearly polarized omnidirectional antennas are used in MIMO systems. The choice of antenna depends on the available size where chip and/or PCB antennas would be required in highly integrated user equipment (e.g., handsets, tablets, etc.) while, higher gain, rubber duck antennas could be leveraged in for equipment such as access points and routers (Figure 4). The higher the frequency of operation, the closer the antennas can be moved towards one another while still being able to take advantage spatial diversity. For more industrial applications, custom-built radomes can encase an antenna assembly while protecting the internal circuitry from harsh weather elements (e.g., wind, moisture, salt fog, etc.).
Passive intermodulation distortion (PIM), or the nonlinear intermodulation distortion (IMD) products from the mixing of two or more signals in passive media, is a concern for high power multi-band systems. As technology progresses, these systems grow in antenna complexity with the introduction of more channels and antennas with MIMO techniques. The need to maintain low noise floors by mitigating PIM in the antenna and connector heads is important particularly for distributed antenna systems (DAS) or cellular installations.
Conclusion
Understanding the fundamentals of MIMO wireless communications can better illuminate the current and future methods leveraged to overcome fade and multipath in various environments. In the diversity-multiplexing tradeoff, designers have to decide where it is best to put resources depending on the bit error rate (BER), data rate, and signal-to-noise ratio (SNR) of the channel. A system with a high SNR and low BER could then take better advantage of diversity gain by spreading resources over multiple channels. While a system with a high BER would be able to take advantage of spatial diversity to increase the data rate of the system. The antennas used in various MIMO applications can vary in polarity and radiation pattern depending upon system requirements.
Resources
[1] Kshetrimayum, Rakhesh Singh. Fundamentals of MIMO Wireless Communications. Cambridge University Press, 2017.
[2] Jensen, M.a., and J.w. Wallace. “A Review of Antennas and Propagation for MIMO Wireless Communications.” IEEE Transactions on Antennas and Propagation, vol. 52, no. 11, 2004, pp. 2810–2824., doi:10.1109/tap.2004.835272.
[3] J. W. Wallace, M. A. Jensen, A. L. Swindlehurst and B. D. Jeffs, “Experimental characterization of the MIMO wireless channel: data acquisition and analysis,” in IEEE Transactions on Wireless Communications, vol. 2, no. 2, pp. 335-343, March 2003.
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