Multiple input, multiple output (MIMO) RF architectures have been studied for decades, but the technology was historically reserved for high-end systems. However, contemporary standards, such as IEEE 802.11n and IEEE 802.11ac Wi-Fi variants, support MIMO connectivity – meaning MIMO is primed for broad adoption. This article will review the top three reasons why, despite added complexity and costs, MIMO is the wave of the future.
MIMO systems increase the number of antennas from one to four or more. Thus, in order to deliver MIMO functionality, an RF transceiver must be able to control multiple antennas simultaneously while transmitting or receiving. The technique requires complex processing and switching that provides control over each antenna. As expected, the added complexity increases cost and design time.
Yet, design engineers must keep a key point in mind: 5G cellular is just around the corner, and the technology will use MIMO to support enhanced bandwidth capabilities. Markets such as surveillance, automotive, transportation, robotics, augmented reality/virtual reality, and telecommunications will rely upon 5G to meet their high data rate application needs. The technology will become ubiquitous with base stations (BTS), access gateways and MIMO-supported end devices.
By enabling spatial channelization and diversity, MIMO expands bandwidth available within a given spectral bandwidth and space. There are three MIMO transmission techniques, and each offers an opportunity to selectively and adaptively optimize the space and bandwidth already in use.
- Beam steering – MIMO offers the opportunity to electronically guide the directivity of the RF signal by controlling the signal propagating phase over multiple antennas. This provides two major benefits: First, beam steering can directionally focus the RF energy on a single user, ignoring the remaining space. It is also possible to track the user, reducing interference and boosting signal to noise wherever the user is located. Secondly, beam steering can solve the problem of RF multipath by discovering the best path and targeting RF energy toward that direction. Even when transceivers are stationary, environmental changes affect the many paths that an RF signal can take, so dynamically adjusting and selecting the best path maintains best connectivity and increases range in high interference environments.
- Increased data capacity– MIMO can add data carrying
capacity without requiring additional bandwidth through spatial
multiplexing. The long-established Shannon-Hartley theorem states that
the data carrying capacity of a channel is proportional to its bandwidth
(B) in Hertz: (Data Carrying Capacity = B log2 (1 + S/N). But MIMO
offers the advantage of channelizing the space: Each spatial channel can
become independent, thus breaking through the limits posed by
Shannon-Hartley. Although an arbitrary number of spatial channels is not
practical, the ability to increase data rate by 50%, or perhaps double,
within the same bandwidth use is a major advantage.
- Diversity– Because MIMO offers the ability to
distinguish transmission over multiple paths, it is possible to encode
the signal more efficiently if the effect of those paths is considered.
Space-time encoding uses optimally encoded versions of the RF
transmission, knowing that each version will be received with varying
delays and signal-to-noise ratios. The encoding aims to compensate for
losses and added noise through each of the spatial channels within a
MIMO transmission while also adding redundancy that can correct bit
errors. This type of encoding requires the use of multiple channels,
each with independent delay and loss. Thus, a single antenna system
cannot make use of this technique.
Not every RF system will make use of these top three techniques. However, taken individually or consolidated into a complete solution, all three MIMO transmission techniques improve performance beyond the capability of a single antenna system: Controlling the path and focusing on tight spaces through beam steering allows high density RF deployments, such as those planned with 5G, to become a reality. Spatial multiplexing will allow increased data transmission through a constrained bandwidth space if the channels can maintain similar signal-to-noise ratios. Diversity is ideal for ensuring important data is not lost when transferring through a noisey channel, especially when bandwidth is not as constrained.
Collectively, these MIMO techniques enable improved signal range, reduced bit errors, lower power consumption, reduced interference and even enhanced non-line-of-sight (NLOS) or quasi-NLOS connectivity. Thus, the advantages provided quickly outnumber any disadvantages related to initial cost or design complexity, especially as 5G starts redefining wireless technology to support increased device connectivity and data speeds. MIMO systems will become a dominant RF transmission architecture in applications requiring high data rate, extreme Quality of Service (QoS), high density and increased range.
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