CTO Series: 2 | mmWave and massive spectrum availability

In the second blog in our ‘mmWave and the 5G wireless revolution’ series, CTO and Founder Ray McConnell explores the complexities caused by a lack of spectrum and the limitations faced by sub-mmWave technologies (such as Wi-Fi and Bluetooth), in a direct contrast to the opportunities posed by mmWave technology.

In recent years the search for more spectrum for bandwidth has seen organisations move to extremely high frequency millimetric bands (24-100 GHz) where it is much easier to find usable spectrum.

To compliment this, in 2003, in the United States, the Federal Communications Council published recommendations that mmWave bands could be allocated to non-military purposes, thus enabling access to vast amounts of untapped spectrum to address the lack of available spectrum.

As bandwidth increases, capacity increases too

To fully understand the challenges of spectrum availability, we need to look back to the mid-1940s when Claude E. Shannon laid out the fundamentals of information theory. He produced an equation known as the Shannon-Hartley theory which provides the maximum rate at which data can be transmitted over a communications channel of a specific bandwidth in the presence of noise. Capacity has two components, bandwidth, and signal to noise ratio.

Capacity and bandwidth are roughly linear, meaning 10 times more bandwidth equals 10 times more capacity. When it comes to increasing the Signal to Noise Ratio (SNR), it does not have as strong a return. If you increase SNR 10 times, it only equates to 3.45 times more capacity.

In fact, to increase Capacity 10x using SNR alone, it would need to be increased by over 1000x, these levels of diminishing return can only be improved upon by increasing TX power or antenna gain.

Increasing bandwidth scales better than improving Signal to Noise Ratio

Over the years, some fantastic innovations have been developed that help circumvent some of the limitations of the Shannon-Hartley theory. Conventionally, the spectrum squeeze in the lower spectrum bands has required layering of complex methods to help optimise the use of a narrow channel. These methods use high modulations and Spatial Multiplexing (MIMO).

MIMO, which provides additional data capacity by increasing numbers of antennas, gets higher performance using rich scattering in the environment, creating multiple separate streams utilising unique delays caused by multiple reflections. This has drawbacks in that its performance varies dramatically with environmental features which are highly variable and or limited range.

Usage of high modulation and MIMO require higher power usage of transmitters, leading to legalities of licensed management to ensure safe usage.

The high performance of advanced silicon is behind the success of mmWave

The increased capacity of mmWave technologies is intimately related to the speeds and feeds of advanced CMOS integrated circuits. These cheap CMOS devices use very high-speed circuits and parallel SoC design, directly utilising high sample rate processing of the wide spectrum band and tapping into the linear capacity benefits of Shannon-Hartley.

As we start to push the limits of high frequency signalling, we start to witness some fundamental limits that need to be addressed.

Firstly, phase noise, a random variation of phase caused by clock generation circuitry (PLL or other) which roughly increases 6 dB for every doubling of the carrier frequency (For example, going from 28 GHz to 90 GHz causes a ten-fold increase). This is mitigated with specialised frequent phase tracking reference signals.

Secondly, output of power amplifiers (PAs) that drive the antenna roughly degrades by 20 dB per decade. We ensure the PAs are not driven to distortion using a power PA back off limit and use waveforms that have low peak-to-average-power-ratios. This means using Single Carrier SC waveforms (rather than more conventional OFDM waveforms) and places limits on depth of modulation.

So, as wider bands become available in the mmWave spectrum, and more direct usage of high-speed circuitry suited to advanced silicon devices that tap into linear capacity increase, there is a very big win in overall capacity and power efficiency.

In fact, mmWave frequencies are being utilised to provide unique ultra-fast, resilient, carrier-grade wireless networks in scenarios that previously has not worked for sub-mmWave technologies. These ultra-fast and extremely low latency networks are now finding significant market traction in providing low cost backhaul for sub-mmWave access technologies.

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