One of the tools in the LTE toolbox of radio features is Multiple Input Multiple Output (MIMO) antennas. Mimo enables radio systems to achieve significant performance gains by using multiple antennas at their transmitters and receivers. In my Facts and figures on HSPA+, LTE and LTE-Advanced I comment that LTE-Advanced uses base stations and mobile devices with eight MIMO antenna elements to achieve its highest performance levels.

MIMO antennas can bring a number of potential benefits to mobile radio systems, including more reliable operation in poor signal conditions, greater spectral efficiency (and hence overall system capacity) and increased data rates for individual users. However, MIMO is a complex technology, with a number of variations on its basic principle. The LTE standard supports a range of MIMO operating modes, which are suited to different radio conditions, and it allows dynamic switching between these modes to suit the prevailing circumstances. Here I present a short review of MIMO principles and consider some of the practicalities of LTE MIMO implementation.

The figure below shows an example of MIMO operation using four transmit antennas and two receive antennas, which is generally referred to as 4×2 MIMO. In a situation where the radio waves undergo significant scattering and multipath propagation between the transmitter and receiver, a MIMO system can achieve significantly better performance than would be possible with single antennas using the same total power. For example, with suitably placed antennas, the arrangement shown in the figure could double the data throughput between the transmitter and receiver.

By means of coding at the transmitter and signal processing at the receiver, a MIMO system is able to benefit from complex radio propagation environments. A number of different techniques can be applied:

• Spatial diversity exploits the independent fading of different signal paths between the various transmit and receive antennas to improve the reliability of a communication link. The same signal stream is transmitted from each antenna but with different coding applied, which enables the receiver to benefit from the diversity of the received signals. This technique is particularly useful for system control channels and for robust operation in poor signal conditions, such as near the edge of cells.
• Spatial multiplexing takes advantage of multipath propagation to create a number of independent transmission channels between the transmitter and receiver, which enables two or more different signal streams to be transmitted simultaneously. By applying suitable coding and signal processing these can be extracted independently at the receiver. This technique can be used to increase the throughput available to an individual user or to multiplex data from different users (commonly referred to as multi-user MIMO). The maximum number of multiplex channels that can be supported corresponds to the smaller of the number of antennas on the transmitter and receiver. For example, the MIMO arrangement shown above could provide two independent transmission channels if the radio conditions were suitable.  This could be used to double the data rate available to a user or to carry two independent data streams.
• Closed loop feedback and precoding enables a transmitter to take advantage of information about the transmission channel, provided by the receiver. If feedback is available, the transmitter can modify its coding of the transmitted signals to take account of the prevailing channel characteristics, to simplify the signal processing required at the receiver and enable potentially greater performance gains.

The effectiveness of MIMO in a real network depends on a number of factors, including antenna separation on the transmitting and receiving devices, the level of scattering and multipath propagation in the radio path, the signal-to-noise ratio of received signals, and the speed of the mobile terminal. MIMO is at its most effective when there is significant multipath propagation, such as an urban environment where signals are scattered by buildings and other objects. In an open, rural location, where there is a strong line-of-sight transmission path between the transmitter and receiver, MIMO is less useful.

Different MIMO operating modes suit different circumstances. For example, closed loop operation works well with a terminal that is relatively static with high signal strength. However, its performance will be poor with a mobile terminal that is moving rapidly and experiencing low signal strength, because of delays and inaccuracies in providing channel feedback to the transmitter. In such cases it is preferable to use a simpler form of MIMO.

MIMO is a fundamental element of the LTE system design and the first version of the LTE standard (3GPP Release 8) supported 2×2 MIMO in both the downlink and uplink. Subsequent developments have extended this capability and the most recent LTE-Advanced standard (3GPP Release 11) supports 8×8 MIMO in the downlink and 4×4 MIMO in the uplink. The standard exploits the flexibility of MIMO by including a number of different operating modes, including spatial diversity, open and closed loop spatial multiplexing and closed loop multi-user MIMO, and the system is able to switch between modes to suit different operating circumstances. The algorithms for making such choices are not standardised, which provides opportunities for equipment manufacturers and network operators to differentiate their implementations.

While MIMO can bring significant potential benefits, a number of practical issues are likely to constrain its impact in the short term. On the network side, increasing the number of antenna elements involved in a MIMO implementation generally requires visits to each base station, to modify the physical antenna configuration and wiring. The initial rollout of LTE networks required site visits to implement new base station equipment and provided a natural opportunity to introduce the 2×2 MIMO configuration of the initial LTE standard. However, network operators may not be keen to revisit these sites to make further changes until they must. 2×2 MIMO already brings benefits and the additional value of higher-order implementations is dependent on the operating environment. A further challenge is that MIMO operation results in quite different network behaviour than previous systems, which creates some uncertainty in the network planning process. Consequently network operators may prefer to implement other developments first, particularly if these can be deployed by software downloads.

As for mobile terminals, upgrades to MIMO operation require the introduction of new devices with different antenna configurations and processing capabilities. Also, the benefits of MIMO are constrained by the physical size of mobile devices. Ideally, MIMO antennas should be separated by half a wavelength  to achieve good separation of the spatial channels. At 800MHz, half a wavelength is equal to 18.8 cm. There are various developments aiming to alleviate this requirement. However, in the short term there may be little benefit in implementing more than two antennas on a mobile phone, although there may be scope to implement four antenna elements in larger devices, such as notebook computers and tablets.

Equipment manufacturers and network operators have embraced MIMO as an important element of their LTE and LTE-Advanced systems and it is generally listed alongside other LTE features such as carrier aggregation, Coordinated Multipoint (CoMP) transmission and enhanced small cells as a key enabler for the capacity, performance and cost targets required. 2×2 MIMO is already widely deployed in LTE systems and there has been on-going speculation about the launch of 4×2 MIMO by T-Mobile USA in 2013. Compared with the 2×2 MIMO implemented by other operators, this configuration does not deliver greater throughput, but it can achieve more robust performance towards the edge of cells. As the pressure for more capacity increases over the coming years there is little doubt that higher order MIMO will become increasingly common, particularly in capacity hotspots in urban areas.