Signal propagation, or the range that can be achieved by a radio signal, is key to M2M applications. This is because the M2M ecosystem depends on ubiquitous coverage. Different systems for different locations are not an option.
Cellular and many short-range technologies - including Wi-Fi, Bluetooth, ZigBee and others - aspire to play a role in the M2M ecosystem. However, islands of coverage, such as Wi-Fi provides, are not an option. Short range technologies cannot provide the coverage needed for applications such as automotive, sensors, asset tracking, healthcare and many more. Instead, they are restricted to machines connected within the home or office environments.
Even in these environments there are many good reasons why a wide-area solution is preferable. For example, an electricity supply company is unlikely to accept that its meter is only connected via, e.g. Wi-Fi, into a home network, which in turn connects to the home broadband. Were the home owner to turn this network off, fail to renew their broadband subscription or even just change the password on their home router, then connectivity could be lost. Short range technologies are not the solution.
Coverage is typically more important for M2M than data rate. Radio signals in the lower frequency bands go further. Weightless operates in TV white space, the perfect low frequency band – lower than cellular – and this enables excellent propagation, at low power, and without needing inconveniently large antennas in the devices.
Better propagation than cellular networks is a good starting point, but M2M signals need to penetrate deep within homes and other buildings. Smart meters, for example, are often in basements. It is more critical that all smart meters can be read than what the data rate of transmission is. A good M2M system design will trade off data rate against range. Weightless uses the same technology as low-power, long-range satellite networks. This technology involves spreading the data to be transmitted. Spreading multiplies the data by a pre-defined codeword so that one bit of transmitted data becomes multiple bits of codeword. The receiver can then use correlation to recover the codeword at lower signal levels than would otherwise be possible. In the most extreme case, spreading allows an extra gain of 30bB on the link budget. Most buildings have a penetration loss for signals entering them of around 15-20dB, so spreading provides indoor coverage to machines where only outdoor coverage previously existed. Variable spreading factors are a core part of the Weightless Specification, ensuring deep coverage inside homes and buildings.
Range not only affects network performance, but overall costs, too. Weightless’ ubiquitous coverage and extended range directly impacts on the cost of establishing M2M networks. As a result of the need to deploy relatively few cells, set-up costs are minimised for network operators.
These design considerations make M2M networks radically different in many respects from traditional telephony based technologies. The Weightless standard for white space spectrum meets all of the requirements for M2M communications, including range and in-building penetration.
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