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Monday, July 2, 2012

Current And Future Prospects For MIMO - A BIRD’S EYE VIEW

The use of multiple antennas at both the transmit and receive ends has become one of the most important paradigms for the deployment of existing and emerging wireless communications systems. The importance of Multiple-Input Multiple-Output (MIMO) systems is witnessed by their presence in many recent standards, such as IEEE802.11n, Worldwide Interoperability for Microwave Access (WiMAX) and Long Term Evolution (LTE). Interestingly, like many other brilliant solutions developed in these years in the field of digital communications, the MIMO concept is not a new idea but dated back to the 1970s and was studied as a model for multi-pair telephone cables.

It is part of the “natural” evolution of systems that a first version is standardized and implemented, further versions following, improving specific aspects of the standard, including additional features, toward an increase of performance in various directions. These improvements can be dealing with very specific aspects of the system, hence, not being directly usable for other systems, or can be the result of integrating some general technology into the system, thus, taking advantage of a broader knowledge and of a larger number of suppliers. The use of MIMO in cellular networks and WLANs is clearly in the latter case. Sometimes, the implementation of features in a system requires only the effort of infrastructure manufacturers, in the sense that these features do not affect mobile terminals; the use of diversity in a base station is a good example of this situation.

However, many other times, a joint work and collaboration between infrastructure and mobile terminals manufacturers is required and essential, in order to achieve the desired target. Obviously, the latter situation increases the complexity of system improvement. In the case of MIMO, the latter rule also applies, since it requires the use of multiple antennas in both base station/access point and mobile terminal.

The increase of system performance can also be seen from two different perspectives. On one hand, some features are totally independent of users, being fully controlled by operators; the use of half-rate in GSM is a good example, among many others. On the other hand, many performance parameters do depend on the type of terminal users have, and on how they use it; of course, this creates problems in taking full advantage of the technology and in achieving the goals of optimum/maximum performance. Again, MIMO is the latter perspective. It is clear that, although MIMO is a very powerful technology for the increase of performance (i.e., capacity, data rate, quality of service, etc.) of mobile and wireless systems, really profiting from it can be a complex and nonguaranteed process. Still, the potential of MIMO is so high that, regardless of these barriers, it is being implemented in current upgrades of these systems; for example, LTE, WiMAX, and WiFi.

Nowadays, major manufacturers for cellular networks infrastructure already deliver MIMO products for base stations; four antennas are already made available, and trials with eight antennas have also been announced. However, the situation is quite different as far as mobile terminals are concerned: at this end of the link, terminals are severely size constrained and the space available to locate multiple antennas is often desperately limited. Fortunately, there is a lot of work being done on co-location of multiple antennas in mobile phones with a relatively low correlation, especially for orthogonal linear polarizations.

On the other hand, one should not look into this problem by taking only (current) mobile phones as user’s terminals, other devices being real alternatives as well; personal computers (to be used not only for WiFi, but also for cellular networks) may include two or four antennas in a relatively decoupled way (there are a number of publications on this topic), and even other hypothesis that do not present space problems can be considered; for example, the use of cars as the ”interface” between the mobile phone and wave propagation (this is already a reality today, for high-end cars). So, although there may currently be some real problems in the implementation of MIMO in mobile and wireless systems, by taking advantage of multiple antennas at both ends of the link, the conditions for the efficient deployment of MIMO systems will definitely be favorable in the near future.

In the recent standard [IEE09a], up to four antennas are foreseen, but most of the proposed schemes apply only to two, like the two schemes that are mandatory for use in downlink, but are optional for uplink, i.e., Space Time Block Coding and Spatial Multiplexing; still, a combination of these two schemes is being considered. Again, MIMO improves system performance, as expected.
LTE has been designed having MIMO in mind from the beginning [3gp09]. The effort in standardization was focused on specifying efficient schemes for downlink single user MIMO (that is, no indications are given for a transmission to multiple users such as broadcast), while for uplink the system was designed on the assumption that the mobile terminal front-end comprises a single signal chain; hence, for the latter, the support of single user MIMO is limited to adaptive transmit antenna switching; multiuser schemes are not excluded but are left to manufacturers implementation. The techniques and schemes considered in LTE for MIMO are not that much different from the ones taken in the previous systems, namely because they all share basic aspects of the multiple access technique.

However, MIMO development and usage is not going to be finished by the standardization of the previously mentioned systems, and one may see quite a number of developments in the future, some of them listed below;

MIMO Moves to MM-Wave Bands............

Due to the fact that the UHF band has excellent propagation characteristics for mobile and wireless communications, the race on higher data rates has not implied (yet) the move to higher frequency bands, and basically all mobile and wireless systems working today use this band. Moreover, with the digital dividend (the use of part of the spectrum released from broadcast systems to mobile and wireless ones), the pressure to go higher in frequency has decreased. The achievement of higher data rates are usually obtained by more efficient multiple access techniques and modulation schemes, better management of the radio resources, and so on, rather than by investigating in the area of propagation and channels. MIMO was the exception confirming the rule, i.e., by taking advantage of the randomness of the propagation channel, it was possible to profit from the “parallel channels,” and therefore, to enable higher data rates. Nevertheless, one cannot avoid going up in frequency, if higher data rates are to be obtained, especially because efficiency and performance boundaries are being attained today. The knowledge of propagation and channels up to the UHF band is almost complete, but beyond that (including mm waves and up to the THz band) there is still a lot of work to be done, and a thorough characterization of MIMO in these high frequency bands is still to be performed.

MIMO Goes Green...................

Mobile communications gave rise many years ago to the fears from (electromagnetic) radiation. Basically, the extremely fast introduction of a new technology in the mass market, without a proper explanation of the system behavior, generated a lot of health concerns, which are still present today. One can currently envision that environmental concerns, which are already of key importance nowadays (“green communications” is in the agenda of many initiatives, projects and fora), will become even more important, with a huge impact on systems and networks, concerning their development, deployment and operation. Therefore, energy efficiency has to be (is already being) taken into account in mobile and wireless communications. MIMO can play an important role in this matter, through a better exploitation of the spatial/polarization degrees of freedom toward more efficient communications, at a lower transmission power. Moreover, by combining MIMO with beamforming, one can further decrease the transmission power (given the increase in the corresponding antenna gain); hence, providing additional contribution to increase energy efficiency.

MIMO as an Enabler of the RF SIM Card..............

Another trend regards mobile terminals. The evolution has been such that one can foresee that users may carry just an RF SIM card for their identification by the network, using any terminal/device at hand for communication, ranging from a PC to a “TV screen,” encompassing terminals embedded in cars and in the office, and reaching spectacles as the replacement of today’s mobile phones. Therefore, short range communications, in the vicinity of a person’s body, will play a major role. The exploration of MIMO in such distance ranges has not deserved much attention, most probably due to the low randomness of the propagation channel, as well as to the difficulties in having more than one antenna available at either end of the link. Still, given the development of technology (in antenna design, signal processing, nanodevices, etc.), together with an increase in frequency band, one can easily envision that the conditions for deploying MIMO in these kind of applications may arise in the near future.

MIMO Technologies and the Propagation environment.............

Propagation in not so usual environments needs to be studied as well, for example: several indoor scenarios, with a differentiation between business and residential environments; for personal systems, in-, on- and off-body propagation; for car communications, propagation in between cars, from a heavy traffic situation in a motorway (addressing high speed as well) to a city street; public transport scenarios, including buses, trains, planes, and boats, among others. The characterization of all these scenarios will allow one to estimate how far they are appropriate for MIMO technologies, and how to optimize the location of antennas.

Geographical Mapping of Channel Conditions............

Location based services are already popular nowadays, where a user can know his/her location via the mobile phone, and take advantage of them to navigate or to find a nearest shop, among other applications. Basically, this means that the user is accessing the network for location purposes. However, one can invert this concept, that is, the network knowing where users are, and take advantage of it. For example, by exploiting users’ terminals (and other devices) as channel sensors, the network can establish a geographical map of channel conditions, hence, of channel quality, and with that information forecast services availabilities in an efficient way (specifically for each user). The creation of such geographical channel mapping for better MIMO usage (i.e., enabling the system to know where and when to use MIMO, and then, increasing data rate accordingly), in a given scenario (from indoors to streets, including bodies and cars, and many others), would definitely increase the overall system performance.

M2M Communication....................

Machine-to-machine communications (recently renamed as the Internet of Things) no longer requires a dedicated introduction. The increasing number of applications of this type of communications, where the user has no intervention, is obvious, and definitely will surpass human communications (voice or data) in terms of traffic volume, as data has already surpassed voice in terms of exchanged number of information bits. This also extends to car-to-car and car-to-infrastructure communications, and other types of systems. This means that one will have devices all over the surroundings, at any location, many times with low power consumption requirements, communicating not only among each other but also to a control or information network. Again, MIMO can play an important role in this type of communication, and conditions for deployment should be explored.

The Internet of Things and MIMO................

Sensors can be considered as subset of the Internet of Things, not only for personal use (e.g., on the body), but also through myriad devices that start being implanted in our surroundings (e.g., in cars or houses). Being a subset of the Internet of Things, the same type of problems apply, but one can envision in this case that power consumption and efficiency of communications will play enhanced roles. This opens a wide range of study cases, by extending the MIMO concept to the use of different multiple antennas, located at somehow random locations (e.g., buttons on clothes); hence, creating the need to analyze system behavior under those conditions.

In summary, the possibilities for deploying and amplifying MIMO technologies in the future are immense, bringing to the conclusion that the exploitation of these technologies is just in its infancy.
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