The Internet of Things is poised at a tipping point where market demand and technological capability are rapidly converging to enable an enormous shift in connectivity. All of the jigsaw pieces are in play now with the final hurdle of M2M optimized connectivity being realized via white space spectrum. White space spectrum offers wide area network reach equivalent, and in many respects superior to, traditional telephony based cellular architectures but at a terminal cost and power consumption more usually associated with LAN technologies such as Bluetooth and Wi-Fi. This is the catalyst for a revolution in capability that could have been created serendipitously for M2M.
Traditional approaches simply cannot tick all of the critical boxes that are necessary to realize the potential. 2G, 3G and LTE whilst exceptionally good for high bandwidth human communications such as voice and video streaming to expensive cellular handsets with a battery life measured in hours are simply not appropriate for vast numbers of low cost remotely located wireless sensors. The low price point and long battery life associated the latest LAN technologies such as Bluetooth 4.0 tick two of the critical boxes – cost and power consumption – but cannot provide the range necessary for the majority of M2M applications leading to tiny cells with large gaps between them.
We’ve all been promised the Internet of Things and billions of connected devices. To get to that point rapidly, however, means producing cheap RF chips (at $2 and below) that can carry data up to 10 kilometers and run 10 years on single battery. TV white spaces can provide that. And the Weightless Special Interest Group (SIG) is providing the standard.
The Weightless SIG just released its complete standard specification for the Weightless technology in April. The Weightless standard is an open-source, royalty-free specification for machine communications within white space. High quality white space spectrum is the globally harmonized frequency range freed up through the switchover of TV transmissions from analog to digital. White space spectrum offers exceptionally good signal propagation characteristics enabling long range and superior in-building penetration without excessive power consumption leading to long battery life and low terminal and network deployment and maintenance costs. White space spectrum is the best hope for getting rapid growth in the Internet of Things (IoT).
Designing a standard
The Weightless SIG carefully designed theWeightless Standard to meet the requirements of machine-to-machine(M2M) applications and overcome the issues associated with operation inwhite space spectrum.
We started with what M2M required:
- Low cost, both of the hardware and the service. Many machines are individually relatively low value — imagine for example a temperature sensor. Chipset costs need to be in the region $1 to $2 and annual service charges less than $10 to make it worth embedding wireless technology.
- Excellent coverage. To make applications such as smart metering viable there needs to be coverage of near 100% of all meters. With many meters deep within the home or even in basements this implies vastly better coverage than achieved with today’s cellular networks.
- Ultra low-power operations. Many machines are not connected to the mains and so have to operate on batteries. Having to change the battery is at best an annoyance and at worst a significant expense. Battery life from a single primary cell of ten years or more is essential.
- Secure and guaranteed message delivery. While machines rarely need ultra-rapid transmission, they do need to be certain that messages have been received and that security of the system has not been compromised in any way.
While the key issues with white space are:
- Relatively low output power. The FCC has specified 4W EIRP for base stations and 100mW EIRP for terminals. These are an order of magnitude lower than cellular technologies.
- Stringent adjacent channel emissions. White space devices must not interfere with existing users of the spectrum, predominantly TVs. Hence, the energy that they transmit must remain almost entirely within the channels they are allowed to use. The FCC has specified that adjacent channel emissions need to be 55dB lower than in-band emission, a specification much tighter than most of today’s wireless technologies.
- The need to frequently consult a database to gain channel allocation. Devices may need to rapidly vacate a channel if it is needed by a licensed user. They must consult a database to be informed as to the channels they can use and must quickly move off these channels as required.
- Interference can be problematic in white space. Many channels have residual signals from TV transmissions. These can either be in-band emissions from distant, powerful TV masts that are too weak for useful TV reception but still significantly above the noise floor. Alternatively, they can be adjacent channel emissions from nearby TV transmitters some of which are transmitting in excess of 100kW. In addition, since the band is unlicensed, other users might deploy equipment and transmit on the same channels as the machine network, causing local interference problems.
Designingagainst these criteria requires many trade-offs and iterations. A keystarting point is the conflict between excellent coverage requirementsand yet low-power constraints both due to white space regulation and theneed for long battery life in terminals. The only way to achieve longrange with low power is to spread the transmitted signal. Hence,variable spreading factors from 1 (no spreading) to 1024-fold are a corepart of the Weightless specification. Spreading is essentially amechanism to trade range against throughput so using high spreadingfactors can achieve significant range extension but at the cost of lowerdata rates. Happily, there is sufficient bandwidth in the white spacefrequencies, and M2M data rates are sufficiently low that more thanadequate capacity and throughput can still be achieved even with highlevels of spreading.
Machines have more patience
Use of the white space spectrum does not provide guaranteed uplinkand downlink pairing, making TDD operation essential. This in turn leadsto a frame-structure with a downlink part then an uplink part thatrepeats periodically. The maximum spreading factor informs what thisrepetition should be since the header information at the start of theframe needs to be spread by the maximum factor in order that allterminals in the cell can decode it. If this header takes up more thanaround 10% of the frame length, the system starts to becomeinefficient as signaling becomes a significant percentage of the totaltraffic. Simple calculations show frame lengths of around 2s areoptimal. This would be overly long for person-to-person communications,with such a delay being highly annoying, but is not an issue for M2Mcommunications (machines do not generally get annoyed!).
The need for stringent adjacent channel emission levels suggest theuse of single-carrier modulation (SCM) rather than OFDM as the latter ismore difficult to filter tightly without distorting the transmittedsignals. OFDM also has a high peak to average power ratio that does notfit well with very low powered devices. Because the terminals are oftenvery low power (eg 40mW) compared to base stations (which can be up to4W) the link budget needs to be balanced. This is achieved with anarrower band uplink such that the noise floor is lower. Using around 24uplink channels for each downlink has the effect of balancing the linkbudget.
Operation in white space requires good interference tolerance. Thisis achieved primarily using frequency hopping at the frame rate (2s) sothat the impact of any interference is restricted to a single hop ratherthan degrading the entire transmission. Frequencies with persistentinterference can be removed from the cell hopping sequence. Othermechanisms to remove interference include the base station directingantenna nulls towards strong sources of interference, careful schedulingof transmissions to terminals to avoid the frequencies where theyperceive the strongest interference and the use of spreading to make thesignal more resistant to interference when all these other techniquesare insufficient.
Finally, M2M traffic is often characterized by very short messages,for example a 30-byte smart meter reading. The MAC protocol is designedto add minimal signaling overhead to such messages to avoid highlyinefficient transmission. This is done through flexible small packetswith highly optimized header information.
A detailed discussion of the technological choices made is providedin the Weightless standards documentation available to download fromweightless.org.
William Webb is CSO and one of thefounding directors of Neul, a company developing Weightless M2Mtechnology and network and is CEO of the Weightless Special InterestGroup. Prior to this, William was a director at Ofcom, the UK Telecomsregulator equivalent to the FCC, where he managed a team providingtechnical advice and performing research across all areas of Ofcom’sregulatory remit. He also led some of the major reviews conducted byOfcom including the Spectrum Framework Review, the development ofSpectrum Usage Rights and most recently cognitive or white space policy.He has published 12 books, 100 papers, and 18 patents. He is a visitingprofessor at Surrey University, a member of Ofcom’s Spectrum AdvisoryBoard (OSAB) and a Fellow of the Royal Academy of Engineering, the IEEEand the IET. He has a first class honours degree in electronics, a PhDand an MBA. He can be contacted at .