How can Shared Spectrum operation be possible?
In public discussions we see more and more comments and viewpoints that sharing resources and co-operating in the future is the next challenge for mankind. Sharing recourses has been reality in wireless communication for decades as there is only one radio spectrum to operate in. For this reason, the regulative rules and technology solutions have been developed to define how to access and use the spectrum.
One of the most popular and used spectrums are Industrial, Science and Medical (ISM) band at 2400- 2483.5 MHz and 5725- 5875 MHz bands. The use of variety of applications is truly happening at these frequency bands as they are probably the most used bands for many consumer and industry applications, setting high requirements for resource sharing and co-operation.
Regulation sets rules to spectrum access
In general, the operation in the ISM bands boils down to few spectrum access rules which define the transmitter maximum effective radiated power (ERP) level, power spectrum density, and the maximum transmission times with different combinations of these parameters. The regulation in Europe, also considers the minimum receiver performance, as well as polite spectrum access rules when transmitter is using higher ERP level than a threshold defined in the regulation.
From our perspective the most important regulations are: in North America FCC Part 15.243 – 15.249 and ANSI C63.10-2013 defining FCC compliance, and in Europe the ERC/EEC recommendation 70-03 and EN 300 328, which defines the essential requirements for equipment to RED compliance.
Effective use of spectrum
The regulative requirements provide a minimum requirement for equipment, but they do not guarantee interference free operation within these bands. In Europe, the efficient use of the spectrum also considers the receiver behaviour requirements, such as polite spectrum access, to improve fair use of the spectrum between users. In the US the focus is more on limiting power spectral density values and the maximum single user transmission time.
Polite spectrum access covers a set of technology options, one being where the receiver is sensing the channel before transmitting, and when a channel is found to be used withdraw the transmission for later point in time which is known as Clear Channel Assessment with Collision Avoidance (CCA/CA). Wirepas Mesh supports a set of polite spectrum features as it enables us to exploit the best and less interfered part of the spectrum in our communication and therefore improve the communication reliability. The key features for this are the capability for selecting less used channels and avoiding using channels in which other systems are active. In the 2.4 GHz ISM band we employ adaptive frequency agility for the complete band, so each device has a pool of channels from which to select locally the most optimum one.
Another important aspect for efficient use of the spectrum is to minimize the transmission time. The shorter the transmission time, the less channel is reserved thus minimizing collision probability and maximizing the opportunity of others to use the scarce spectrum resource. Wirepas Mesh maximum transmission time is 1.232 ms, and when the payload is less than the maximum size, the activity time is less. Both WLAN and Bluetooth have the same order of magnitude frame length, reducing the probability of overlapping transmissions. For the case when channel sensing would occur at exactly the same time and multiple transmitters would be sending at the same time, the physical distance and actual radio channel will attenuate interference and determine the impact to each of them. Especially WLAN radio has multiple technologies to increase its reliability of reception under challenging channel conditions or interference, such as adaptive modulation and channel coding, fast retransmission schemes, multiple transmitters and receivers to name few of them. All these features are increasing their reliability to receive data correctly.
The third aspect for efficient spectrum use is adaptive power control. For Wirepas Mesh this means that we use only the necessary power for conveying the data to the next hop, and save battery-life, but more importantly it also inherently enables us to reduce interference and reuse frequencies more often in larger deployments to increase total system capacity. This provides both better reliability for communication and better system capacity in a given area.
When considering our system transmission activity in low energy mode, we have estimated the maximum activity at a sink over 1 second is around 2.5 %. As sinks are receiving all data to be delivered from nodes to cloud services, the sink is clearly the most active entity in our system. In our system the sinks transmission is scattered over the whole 1 second period into several hundreds of transmission events, which each of them is few hundreds of microseconds long, leaving spectrum available for other systems to use.
Physical deployment and radio channel behaviour has a major role in interference management
The comparison in time domain between each system is of course interesting, but the final impact in real deployments is derived by the physical distance and the operational environment and radio channel characteristics. Even though it is tempting to use free space propagation models, Line of Sight (LOS), the channel seldom behaves like this. This is especially the case indoors where Non-Line of Sight (NLOS) models are prevailing thanks to walls, roofs and floors with different material between devices. The propagation characterises indoors are significantly harsher (i.e. higher attenuation) compared to outdoors. For pathloss analysis, we have developed a tool where we apply the latest channel models which are based on measurement results collected by the academia and telecommunication industry.
The physical distance (even few meters or so) reduces the interference levels - several tens of dB’s attenuation are likely.