Clearing the confusion on battery life and range for 2.4-GHz low power RF

This article explains why these specifications are misleading and how they can lead designers to choose non-optimal approaches. It will show how to accurately evaluate power consumption and range, how to use link budgets, and how to work with current profiles to maximize sleep time, minimize retries, and avoid interference.

Wireless embedded control applications are low data rate (less than 2Mbps), low-power (can be battery-powered for years), and typically operate over 10-50m range. Some example applications include low-power sensor networks for medical (patient monitoring), remote control, industrial process automation, home/commercial building automation, asset management, and precision agriculture. In these applications, data rate is very predictable because it is ultimately limited by just protocol overhead.

The other key aspects of a wireless technology, power and range, happen to be the most misunderstood and confusing specifications to evaluate when choosing a wireless technology. Many vendors advertise their radio's power consumption and range in numbers that are not entirely useful for designers because the figures will vary depending on the application and don't take noise and protocol into account. This makes choosing a wireless technology that predictably achieves design specifications very difficult.

Chip vendors are not entirely at fault because it's difficult to agree on a standard way to specify performance under different noise environments and wireless protocols. Therefore, to make the best decisions while evaluating wireless solutions, embedded wireless firmware engineers need to consider how they should evaluate power and range, as well as how they will optimize these parameters for a low-power RF application. This article will address these issues in the context of 2.4-GHz low-power RF technologies since the 2.4-GHz ISM band is unlicensed worldwide.

Starting with Power
For power, let's first consider the misleading information found in datasheets. Then we'll look at what parameters really affect power performance the most followed by some suggestions for how to optimize power consumption in typical environments with interference.

• Some vendors go as far as quoting years of battery life with no qualification of what conditions the radio is operating under. And really, every vendor will quote different conditions so it's difficult to have an apples-to-apples comparison between radios.
• Except for transmit/receive modes, different vendors have different names for operating modes, including: Off, Hibernate, Doze, Idle, Sleep, Standby, Standby-I, Standby-II, and Power Down. Each of these modes has different functionality available depending upon the chip, so it's very difficult to figure out which radio will give you predictable performance.

This said, most 2.4-GHz radio's datasheets will highlight transmit, receive, and sleep currents for evaluating power. To give an idea of typical ranges, you may see transmit/receive currents anywhere from about 15mA to 60mA depending on output levels, and sleep/standby currents from sub-1 uA to 30uA.

At face value, one might think that a radio which transmits at 15mA is twice as power-efficient as a radio with 30mA. Drawing this comparison would be the first of many false conclusions a designer may come to that inevitably leads to disappointment in the process of designing a long-lasting low-power wireless product.

A typical radio has the following generic modes: sleep, idle, synthesizer settle, transmit, and receive. Each operating mode has varying levels of current consumption and different radios spend different amounts of time in each mode. Also, some radios can do more in certain modes than others can, such as being able to load data into buffers or accept SPI communications to wake up the radio. Thus, calculating average power consumption over a complete transmission cycle tells a more truthful story when comparing radios.

Calculating Average Power Consumption
Unfortunately, a datasheet cannot really provide this data in a standard way. Therefore, calculating average power consumption requires some upfront work from the engineer. It's useful to map out the current profile of a high performance transceiver as the following example illustrates:

Figure 1 shows that a good radio usually spends less than 2ms to complete a transmission (from mode 2 through 5 in the figure) and most of this time is spent in crystal start-up and synthesize mode. The radio actually spends less than 518us when communicating on the air (transmit with acknowledgement received) and doesn't need to re-synthesize, leaving the crystal on and just loading the next packet for transmission. In this example, synthesize mode takes 100us on fast channels, which is actually very quick. Different channels do need different synthesize times. For most radios, you can easily expect anywhere from 200us to over 400us for the synthesizer to settle, even in fast channels. Note that for simplicity, the above diagram does not take into account up and down current ramp times.

Comparing radios using current profiles gives a much clearer picture of power performance, but we still need to consider retransmits in order to tell the complete story. Looking at the above example, transmit/receive currents are more than 20,000 times higher than sleep currents. Such a significant difference between transmit/receiver and sleep modes holds no matter what 2.4-GHz low-power RF radio is being considered. For any radio, then, each retransmission is very expensive in terms of power consumption (as well as affecting latency and throughput).

Equipped with methods to navigate through the confusion of power specs, an engineer still needs to think about implementing an intelligent protocol to take advantage of a low-power radio. As illustrated in the above example, there is clear motivation for sleeping as much as possible. This begs the point that a big part of power-efficiency is driven through reliability. Put another way, ensuring successful communication with the least number of retransmissions leads to the best power efficiency.

A power efficient radio/protocol combination may have some or preferably all of the following intelligent features:

• Frequency Agility: A way to detect interference in a channel and find a new channel. The faster a radio can find/switch channels and the more channels it has to choose from, the better. As other 2.4-GHz networks like WiFi (802.11b/g/n) occupy a broad spectrum and leave little room for other networks to operate on, narrowband signals are very desirable.
• Direct Sequence Spread Spectrum (DSSS): Instead of sending data directly onto the air to directly face interference and drop bits, a radio can minimize bit error rate using DSSS, which encodes data into pseudorandom chip-sequences known as symbols. By transmitting a chip-sequence onto the air that represents the original data, the receiving side can still recover the original data even if a number of chip errors occur. This is known as coding gain and is generally adjustable. Some radios on the market appear to consume less power compared to others because of marginally better power consumption specs, but may actually consume more power if they don't have DSSS to maximize successful transmissions.
• Dynamic Data Rate: Power-efficiency is tightly linked to reliability, but there is a delicate balance needed to achieve the lowest power because reliability could come at the expense of longer air time which in and of itself uses more power. DSSS inherently spends more time on the air to transmit the same amount of data as a raw mode of transmission like Gaussian Frequency Shift Keying. Using DSSS makes sense in an environment with noise, but typically operating environments are dynamic, with periods of both quiet and noisy time. Thus a good radio not only has DSSS, but can dynamically switch it off to take advantage of quiet times on the air and transmit faster to sleep more. A good protocol has the intelligence to decide when to use which data rate, and optimize it to minimize overall on-air time.
• Dynamic Power Amplifier Levels: Most radios have a number of power output levels to choose from, so a robust protocol can intelligently reduce the output level to transmit only as loud as needed. Such a protocol will monitor noise levels and consecutive successful transmissions to decide if a lower power output level can be used to save power and still maintain a good link.