Framing the future for 5G wireless

Researchers are trying to tackle wireless data demand in a spectrum-poor world.

Wireless consumers’ insatiable demand for bandwidth has spurred unprecedented levels of public and private sector investment to increase network capacity. This demand is also driving wireless researchers to develop new ways to address capacity challenges and explore new network topologies that offer features and functions never before thought possible.

Industry experts universally agree that even with current and planned infrastructure rollouts, data demand will continue to outpace capacity and the debate has shifted from ‘if’ to ‘when’ this will occur.

Wireless service providers plan to furiously upgrade their networks to 4G LTE, LTE-A and beyond, adopting new innovations including multiple input, multiple output (MIMO) and carrier aggregation, along the 3GPP roadmap.

However, it is clear that the current technology trajectory still produces a capacity slope that is flatter than the demand line.

Embracing the challenge, wireless researchers around the world have begun their journey to investigate new wireless technologies that will be part of a fifth generation or 5G network.

Researchers are not only addressing capacity in their innovations, they also aim to improve coverage and reliability at the cell edges, improve energy efficiency for providing service and decrease latency — all of which will improve the overall responsiveness of the network.

To this end, researchers are focusing on four potential technologies to make 5G a reality.

Massive MIMO

Massive MIMO promises significant gains in wireless data rates and links reliability using large numbers of antennas (>64) at the base station or eNodeB. This approach radically departs from the traditional eNodeB architectures in place today that use six or eight antennas in a sectorised topology.

With hundreds of antenna elements, massive MIMO reduces the power in the channel by focusing wireless energy to targeted mobile users using precoding techniques. By directing the energy to specific users, the power in channel is reduced and also decreases interference to other users.

If the promise of massive MIMO holds true, 5G networks of the future will be faster and will accommodate more users with better reliability — all while consuming 100x less energy than today’s networks.

A massive MIMO testbed at Lund University in Sweden. Based on USRP RIO (a) with a custom cross-polarised patch antenna array (b). Courtesy NI.

Densification of networks

Given the current limited amount of available spectrum for mobile users, researchers are exploring how to increase data rates by increasing the number of eNodeBs in a particular geographic area. Rather than have one eNodeB serving as an access point in a 3 km² area, network densification would increase this number substantially.

The densification category of 5G research encompasses small cells, heterogeneous networks (‘het-nets’), pico cells, femto cells and relays. All of these approaches increase the access point density for a served region.

Although the concept is relatively simple — slice the spectrum geographically rather than by frequency — the implementation is challenging.

First, an operator that deploys multiple access points must do so strategically so that the devices do not interfere with one another.

They must also take into account the location of the devices, the power of each device and how to control it in coordination with the other access points in the region.

New waveforms

4G and 4G+ networks employ a type of waveform called orthogonal frequency division multiplexing (OFDM) as the fundamental element in the physical layer. In fact, almost all modern communication networks are built on OFDM because OFDM improves data rates and network reliability significantly by taking advantage of multipath, a common artefact of wireless transmissions.

However, as time and demands progress, OFDM technology suffers from out-of-band spectrum regrowth resulting in high side lobes that limit spectral efficiency. In other words, network operators cannot efficiently use their available spectrum because two users on adjacent channels would interfere with one another.

OFDM also suffers from high peak-to-average ratio of the power amplifier, resulting in lower battery life of the mobile device.

To address OFDM deficiencies, researchers are investigating alternative methods including generalised frequency division multiplexing, filter bank multicarrier and universal filter multicarrier.

Researchers speculate that using one of these approaches over OFDM may improve network capacity by 30% or more while improving the battery life for all mobile devices.

MM-wave communications

Spectrum availability is increasingly crucial. Shannon Theory posits that channel capacity is a function of bandwidth and signal-to-noise ratio. With current signal-processing techniques already approaching the Shannon limit in terms of signal-to-noise ratio, more bandwidth is necessary to significantly increase network capacity.

Until recently, researchers have focused on spectrum availability in the frequency bands below 6 GHz, primarily because communication at frequencies above 6 GHz was viewed as unfeasible.

However, recent studies have shown that communication at frequencies in the mm-wave frequency range — particularly at 28, 38, 60 and 72 GHz — is possible and holds promise for commercial deployments.

Although transmission at these frequencies endures more path loss, researchers are compensating for this path loss by using high-gain, phased array antennas at the base station and advanced signal processing techniques.

The potential for mm-wave communication is exciting as the available spectrum in the mm-wave bands is significant. At some frequencies, service operators can realise up to 2 GHz of continuous spectrum per use compared to 20 MHz available today.

With more spectrum, network capacity increase up to 1000x is indeed feasible.

Reallocating spectrum is not an easy task because service operators have already invested billions of dollars to acquire the spectrum already in use, and transitions are neither easy nor cheap.

Increasing demand for wireless data and the future of 5G is still evolving, but one thing is certain — unless industry, government, and associated spectrum regulating entities can agree on how and when to reallocate spectrum, there will be no remaining spectrum available below 6 GHz.