Even if 4G is still actively being deployed and its technology, LTE, stands for “Long-Term Evolution”, there’s lots of discussion on the hot topic of 5G, the next generation of wireless communication technology. In recent blog posts, Nutaq shared its perspective and expertise on one potential 5G technology, massive MIMO. But what other topics are related to 5G and how does massive MIMO fit with them? This blog post provides a very short overview of the three principal topics being discussed for 5G next-generation wireless technology: densification, millimeter waves, and massive MIMO.

Densification

One of the main research topics for future wireless communication systems is the multiplication of the number of cells in a given geographical area. “Cell shrinking” involves the reduction of cell size and includes the use of femto-cells, cellular base stations that have a range similar to commonly used Wi-Fi routers. Smaller cells have two main advantages:

·         A reduction in competition for base station resources (less users share a common cell)

·         The possibility to reuse spectrum in more cells due to their increased number and smaller size. A shorter cell range avoids inter-cell interference and communication can occur on the same frequencies in more cells, thus optimizing the use of the spectrum.

We already observe a shrinking of the cells size as femto-cell devices are being developed. This trend is expected to continue in the future.

Millimeter wave

A major concern today is the scarcity of spectrum bands allocated for wireless communications. The current approach is to use frequencies ranging from several hundred MHz to a few GHz. However, the growth in spectrum demand with the ever-increasing number of users and the rise of new technology paradigms like machine-to-machine (M2M) communication and the Internet of Things (IoT) will make the currently used frequencies – often called “beachfront spectrum” – insufficient to meet the demand.

Even with new technologies being developed to increase the spectrum usage efficiency (e.g.  cognitive radio and MIMO techniques involving spatial diversity), licensed spectrum will not be sufficient. Unlicensed and relatively inactive frequency bands are therefore being investigated, with extremely high frequency (EHF), called “millimeter waves” or “mmWave” because of their wavelength (30 to 300 GHz), being a significant area of interest.

However, there are reasons why these frequencies are not already in use. Given the design of our existing communication networks, the optimal frequencies are the ones we currently use. The reasons are numerous, but a major one is the way in which electromagnetic waves propagate through space. Wave propagation depends on the frequency and mmWave has rather poor propagation properties. It has a high path loss, meaning that a lot of energy is lost as it travels through space. Strong atmospheric and rain absorption, low diffraction around obstacles, and poor penetration through objects are also known issues.

To overcome these obstacles, the network architecture must be rethought. A lot of research must be done to make mmWave technology practical. Its potential to help meet the always increasing demand for spectrum, however, make it a very important 5G research topic.

Massive MIMO

The idea of using many antennas to promote spectral efficiency is not particularly new. The theoretical development for MIMO systems began in the late 1990s. The first MIMO systems were used for WiFi in 2006. Technical limitations limited the number of inputs and outputs to 8. A publication in 2007 mentioned raising this number to above 16

[1]. Since then, many massive MIMO papers have been published. Technology that would use around a hundred transmitting and receiving antennas per base station is considered very interesting for future wireless development such as 5G.

The main benefits of massive MIMO (100 transceivers per base station) are as follows [2]:

·         Spectral efficiency would increase drastically without needing to increase the number of base stations. MIMO systems introduce spatial diversity, enabling the transmitter to target the energy used for wireless transmission more accurately for the receiver and to control the interference in a way that reuses the same frequencies at different locations in space without having harmful interference occur. This leads to a result similar to shrinking the cell size but without the maintenance costs and complexity of augmenting the number of cells.

·         Better channel response – the high number of antennas reduces the global effect of random parameters in individual antennas. From a statistical point of view, the probability of an error occurring at one (or a few) antennas is significantly higher than the probability of having the same error on multiple antennas at the same time. Increasing the number of antennas will therefore increase channel quality.

Conclusion

To meet the ever-increasing demand for wireless spectrum, we need to develop cutting-edge technologies that break current paradigms. In this mindset, three main topics are being considered for 5G: densification, mmWaves, and massive MIMO. For an in-depth explanation of each topic, see [2].

References

[1] T. L. Marzetta, “The case for MANY (greater than 16) antennas at the base station” in Proc., Information Theory and its Applications (ITA), San Diego, CA, Jan. 2007.

[2] Jeffrey G. Andrews et. al., What Will 5G Be?, in IEEE JSAC Special Issue on 5G Wireless Communication Systems, May 2014