14 February 2012
Maximising range and battery life in cost sensitive wireless networks
Given the high profile of 2.4GHz wireless standards such as Bluetooth and Wi-Fi, many manufacturers assume this is the de facto transceiver frequency. While this is true for some applications, many others have relatively low data rate requirements and operate within a closed wireless network. In these cases, a proprietary protocol can reduce system cost significantly.
For example, a ZigBee radio node costs around $2 and requires the system to support a 128kbyte stack. A sub GHz radio node using a proprietary protocol can cost less than $1.20 and requires only an 8kbyte stack. Many applications use proprietary protocols for cost reasons, since few markets require vendor interoperability.
While certain markets may not use a standard wireless protocol, they have consolidated around a particular frequency (see Fig 1). Automated utility meters, for example, use sub GHz as a long range wireless backhaul network and 2.4GHz for meter to home communications.
Home security systems use sub GHz for low data rate sensors and 2.4GHz for high data rate video cameras. Some markets, meanwhile, remain undecided.
Wireless developers must determine whether a sub GHz or 2.4GHz transceiver will best serve their application needs. Transceivers based on 2.4GHz offer data rates in excess of 1Mbit/s and require a small antenna, which makes for a good fit with short range consumer electronics devices. However, a 2.4GHz radio has limited range – environmental losses are approximately 9dB more than at 900MHz. High data rates require a wider receiver channel bandwidth, which further limits sensitivity and range. A 2.4GHz radio has higher power consumption, and the 2.4GHz spectrum is crowded and subject to significant interference from Wi-Fi devices, Bluetooth nodes and microwave ovens.
In contrast, sub GHz radios offer a range in the order of kilometres, have low power consumption and can operate for years on a single battery. These factors, combined with low system cost, make sub GHz transceivers ideal for low data rate applications that need maximum range and multiyear operating life.
One disadvantage of sub GHz wireless designs is that antennas are larger than those in 2.4GHz designs. The antenna for 433MHz applications can be up to 17 cm long. If node size is an important design consideration, developers can increase the frequency (to 950MHz, for example) and employ a smaller antenna.
The 2.4GHz band has the advantage of enabling one device to serve all major markets worldwide. However, this advantage is often overstated. Each country has regulations that can force manufacturers to certify products for specific countries, as well as provide regional SKUs. Developers need to understand these regulations early in the design process to avoid costly late cycle redesigns.
For many applications, 433MHz is a viable alternative to 2.4GHz, and designs based on 868 and 915MHz radios can serve the US and European markets. In general, high data rate applications will need a 2.4GHz radio, while long range, low power applications will suit a sub GHz transceiver.
Battery lifetime is a critical design constraint in many wireless networking applications. Optimising battery life, however, requires that developers look beyond transceiver efficiency. Operating life also depends on the node's range, radio sensitivity, data rate and the number of nodes in the network.
For many sub GHz applications, range is the most important design constraint. Increasing transmit output power extends range and coverage, but consumes more power. Greater range can result in lower system cost, since the longer the range, the fewer nodes there need to be to provide coverage.
Even if range is not an issue, many wireless system manufacturers will consider overdriving the antenna. In this way, range requirements can be met with a less expensive, inefficient antenna, resulting in lower system cost. Having high output power as an option gives manufacturers the flexibility to decide this for themselves.
Sensitivity and longer battery lifetime
Range is determined by transceiver sensitivity and output power – the link budget. A primary factor affecting sensitivity is data rate: the lower the data rate, the narrower the receive bandwidth and the greater the sensitivity.
Selecting the optimal data rate can involve complex choices. The higher the data rate, the less time the system needs to expend power when transmitting – a radio operating at 200kbit/s will need to be active for half the time than a radio operating at 100kbit/s. A faster data rate will also enable more nodes to be colocated without adverse contention. However, higher data rates reduce sensitivity, leading to a need for higher output power to achieve sufficient range. Ideally, developers select a low data rate that provides sufficient system responsiveness while minimising transmit time.
Sub GHz transceivers, such as Silicon Labs' EZRadioPRO devices, support a programmable data rate. In the case of the EZRadioPRO ics, developers can select data rates of up to 500kbit/s in 2GFSK modulation and up to 1Mbit/s when using 4GFSK modulation. Because programmable transceivers can be configured to operate across a range of data rates, this allows developers to fine tune the data rate so the radio transmits for the least amount of time, while taking range and sensitivity into consideration.
Developers also have the option of improving system robustness in sub GHz designs through frequency hopping. Silicon Labs, for example, offers reference designs with and without frequency hopping to enable developers to maximise range with the available output power.
These reference designs also take into account regulatory issues that limit output power.
The transceiver's current consumption can impact the system's overall power consumption. Every transceiver offers a variety of active and power down modes. The node's transmit/receive frequency will determine which mode has the greatest impact on power. For example, if a node is transmitting frequently, transmit power will be very important. Similarly, if a node transmits only once a day, shutdown efficiency will be the most influential factor.
Different standby modes also help developers to optimise battery lifetime based on specific transmit/receive requirements. For applications where responsiveness is critical, the time to transmit/receive requires a standby mode with fast wake up. Silicon Labs' newly launched Si446x EZRadioPRO wireless transceivers, for example, support standby power consumption of only 50nA and feature the lowest transmit output current for their output power.
For simple sensor based networks, a wireless mcu solution, such as Silicon Labs' Si10xx family, enables developers to implement the entire system with a single SoC, resulting in longer battery life, lower system cost and a more compact form factor compared to multichip implementations. For new designs, a wireless mcu allows developers to optimise the radio and control code using the same set of tools, simplifying design and speeding time to market.
Developers have a range of rf technology choices when developing low cost wireless networks. The availability of 2.4GHz radios using standard protocols facilitates interoperability, while sub GHz radios provide a cost effective approach, with long range and optimal battery lifetime. The combination of high power output, radio sensitivity and configurable data rate provides developers with the flexibility to optimise their implementation for range, cost, operating life and ease of installation.
Kyle Baker is Silicon Labs' wireless product line director.