Now that PCs are old news and seemingly everyone on earth has a cell phone, the Next Big Thing promises to be machine-to-machine (M2M) communication, giving rise to the Internet of Things (IoT)—presumably a parallel universe to the Internet of People (IoP).
Whether you believe AT&T’s prediction of 50 billion connected devices by the year 2020 or IBM’s of 1 trillion devices by 2015, the numbers are huge. Every vendor with a vested interest is arguing that their wireless solution is the best way to connect these devices, at least in certain applications. Now suddenly there’s a new entrant in the race—Weightless. As one wag joked, “Weightless is not 1G, 2G, 3G or even 4G – it is ZERO G!”
Weightless is a new low cost, low power, long-range wireless protocol designed for M2M communications. The design is the brainchild of Professor William Webb, co-founder of Neul Ltd., CEO of the Weightless SIG, and author of Understanding Weightless. First announced in 2011, Weightless has picked up some serious backers, including ARM, CSR, and Cable & Wireless. The goal is to make it the first global standard for M2M communications.
Its proponents claim Weightless has a number of advantages vs. other protocols:
- Cost—Cost is comparable to Bluetooth modules, less than $2. Also the cost of the infrastructure would be a lot less than for cell phones, since the protocol can go up to 10 km—all things being equal—meaning you need a lot fewer base stations. Finally, Weightless reuses the unlicensed white space between TV channels, so there’s no upfront massive investment in spectrum (unless the FCC decides to put it up for auction).
- Power consumption—Weightless devices are designed for a minimum of 10 years battery life, since remote wireless sensors aren’t amenable to frequent battery replacement. This is possible in part because Weightless is a very lightweight protocol that spends little time in active mode. Also, it uses spread spectrum technology, which minimized output power. Finally, Weightless devices have allocated time slots, so they aren’t constantly listening to the network and can stay asleep most of the time. Weightless basestations only page connected devices every 15 minutes, varying the symbol rate based on signal strength.
- Range—Using sub-GHz frequencies, Weightless devices have very good propagation and penetration characteristics vs. Wi-Fi, Bluetooth, and other protocols that utilize the 2.4 GHz ISM band.
How does it work?
Weightless is designed to work in the so called “white spaces” previously occupied by analog TV signals; typically this is in part of the UHF band approximately spanning 470MHz – 790MHz depending on the country. The FCC has ruled that these bands can be used for unlicensed devices, but only if they can detect the presence of other users and not interfere with them. That pretty much rules out Wi-Fi, which comes up short on interference detection and frequency agility.
Weightless uses time division duplexing (TDD), so both the uplink and downlink occupy the same channel. Since Weightless devices are assigned a particular time slot, they can spend most of their time asleep and needn’t constantly poll the channel, just waking up and transmitting only at preset intervals.
Weightless uses either phase shift keying or quadrature amplitude modulation (QAM) depending on signal strength and the amount of interference. It also utilizes a “whitening” algorithm to spread the signal and make it appear more like white noise, thus reducing interference. The data rates for the downlink range from 2.5 Kbps to 16 Mbps.
As the table indicates Weightless uses a spreading algorithm to create a longer data sequence when the signal levels are weak. It reduces the data rate and shifts to a simpler modulation scheme in order to reduce the error rate or to gain additional range.
Despite relying on a TDD scheme Weightless also implements frequency hopping in order to reduce interference and maximize the data rate. This also helps to reduce the effects of Rayleigh fading. In its preferred implementation Weightless makes use of narrowband uplink channels in order to balance the link budget with relatively high power base stations and low-power terminals.
Weightless utilizes root raised cosine pulse shaping to convert the square waves from the digital baseband into sine waves that can be fed to the RF PA. This would typically be handled by a DAC, but Weightless provides a software approach should you choose to go direct from baseband to antenna.
Weightless systems operate in master-slave mode, with the basestation as the master and the terminals as slaves. Basestations have separate IP addresses and backhaul capability. When they go live they contact a master database, which knows their location, power, and estimated coverage radius. When a terminal within the coverage range of a basestation announces its presence, the basestation queries the database for a clear frequency, which it then assigns to that terminal. When the terminal starts transmitting the basestation sends that information back to the database server along with signal levels.
As conditions change the basestation negotiates changes in frequency and modulation with the terminal as needed. The central database—with a few already in place in the U.S. and the U.K.—is key to enabling this all to work, since sub-GHz signals can travel over the horizon, causing interference of which the transmitting station was unaware, since intervening mountains might prevent it from hearing the distant transmitter. If this happens the database server will be aware of the problem and instruct the basestation to shift to another frequency.
Launching an Open Standard
Webb and the Weightless SIG folks argue that only wireless protocols have been standardized will be really successful, since different ends of a wireless link will come from different vendors, and without a universal standard that connection isn’t liable to work. However, seeing an immediate market opportunity, the Weightless SIG chose to develop their own standard rather than wait years for the IEEE or ETSI to hash one out. This was the approach taken by the Bluetooth SIG, and that’s worked out.
In late 2011 Neul, a founder Member of the Weightless SIG and a member of the Weightless Promoter Group, presented v0.6 of the Weightless Specification to a small group of companies for ongoing development work to commence. The Weightless SIG currently has a draft specification (version 0.9) under review by its members, and it plans to formally release version 1.0 on April 3, 2013. The specification will be open to all but with licensing arrangements that are yet to be formalized. Once the specification is published the Weightless SIG proposes to pass it to ETSI for consideration as a formal specification; presumably an IEEE specification would follow at some point.
Weightless moved beyond the concept stage last month when Neul announced first silicon of Iceni, which it bills as “the world’s first TV White Space ASIC.” Iceni operates over the entire TV white space frequency range from 470 MHz to 790 MHz supporting both 6 MHz and 8 MHz channel bandwidths. It features the adaptive modulation schemes listed above; data encryption; programmable I/Os for controlling an external RF front end; an on-board, low-power MCU; and a memory-mapped parallel bus interface and discrete interrupt lines for waking an applications processor.
Will the M2M Future be Weightless?
The danger is that the Weightless SIG is basically a startup, with only an early specification and limited vendor support. That is changing rapidly, with over 500 members registering in the last 12 months. But then again there is no lack of capable IEEE protocols that never gained much market traction, including—fairly or not—ultra-wideband (UWB), HyperLAN, 802.22 (WRAN), and WiMAX. Technical success doesn’t ensure market success, and it’s too early to judge either in this case.
Weightless seems to be a well designed protocol for M2M communications, though it’s not without competition; and standardized or not it’s not necessarily the obvious choice for all applications. However as an alternative to cellular it makes a lot of sense. Far from being weightless in that domain, it may well turn out to be a heavyweight. But that will take time, and only time will tell. Still, the Weightless SIG is off to a good start, and we wish them well.