The key requirements

Weightless is designed to enable machine communications in white space spectrum. This leads to two sets of requirements – those related to machine applications and those related to operating in white space. The machine requirements were introduced in Chapter 1 and are summarised below:

• Support of a large number of terminals. A typical cell might have between 100 000 and 1 million devices within it and a national network could easily contain 1 billion devices.

• Long battery life. Ten year lifetimes from one battery are needed in many cases.

• Mobility. A subset of applications has moving terminals which need to be supported as they move, potentially across national borders.

• Low-cost equipment. Costs of $2 per chip or less would appear to be necessary.

• Low-cost service. Network costs must be low and the marginal cost of each terminal very low.

• Global availability. Some applications will require global roaming. Others, like automotive, will require that one solution can be fitted into all vehicles regardless of their country of destination.

• Ubiquity. Excellent coverage, including within buildings, is needed.

• Guaranteed delivery. Some applications require certainty that messages have been delivered. This may also require strong authentication and encryption.

• Broadcast messages.

• Efficient transmission of small bursts of data. Most machines send data packets of the order of 50 bytes.

• Accommodating sub-optimal terminals. In many cases terminals will be small and low cost and will have a poor-quality antenna and limited power supplies.

• Event-stimulated loading peaks. The network needs to be able to accommodate and control the resultant peak in loading.

As discussed in the previous chapter, white space operation leads to the following requirements:

• Very low levels of out-of-band emissions. This minimises interference caused to licensed users and so maximises spectrum availability.

• Avoid interference caused by other unlicensed users. This can be random and sporadic.

• Reduce the impact of interference where it cannot be avoided. Where interference cannot be avoided the system needs to be able to continue to operate.

• Reduce power where there are few white space channels available. It is often possible to increase availability by transmitting with lower power and hence causing less interference.

Some of these requirements have immediate design implications as discussed in the section below.