Design Article
What's Required for RF4CE?
Peder Rand, Texas Instruments
8/4/2008 2:45 PM EDT
The main advantage of RF4CE is that it ensures interoperability between remote controls and audio/visual (A/V) devices that IR never achieved. The protocol's upper layers are defined specifically for remote control, while the lower layers (including physical layer) uses IEEE 802.15.4, on which ZigBee and 6LowPAN are also based. IEEE 802.15.4 is used in a wide variety of proprietary protocols in both consumer and industrial markets. Advantages include low power consumption, robustness, long range, worldwide deployment in the 2.4GHz band, and a mature market of devices. When evaluating a radio for any given application, there are a multitude of parameters to consider. This article highlights those with the greatest impact on the user experience in a typical remote control application.
Achieving Longer Battery Life with RF Remote Controls
In a remote control system, usually the remote control will be battery-driven and the controlled device mains powered. This imposes different power consumption requirements for the two devices.
Remote Control
A remote control is usually powered by batteries that can easily deliver the modest peak currents used by an IEEE 802.15.4 radio (≈30mA). More significant is the average power consumption. A normal requirement for TV remote controls is that the battery life should be at least one year with 2xAA batteries. But an RF remote control with low-power implementation can easily achieve several years of battery lifetime. The IEEE 802.15.4 radio has three main power consumption categories: receive, transmit, and sleep mode.
These power consumption modes affect the average by the product of their magnitude, and the time spent in the respective mode. In effect, the time it takes a device to send a packet is as important as the current consumed in the TX mode. Remote controls are considered low-duty-cycle devices. That means that they are usually asleep, and wake up occasionally to use the radio, for example to send a command.
However, two-way RF remote control protocols like RF4CE enable advanced features such as displaying device status on the remote control and paging (e.g., user presses a button on the TV which causes the remote to beep making it easier to locate). These features require the remote control to wake up autonomously at regular intervals to poll the controlled device for data.
The calculation example in Figure 1 shows that a feature like paging can dominate the remote control's power consumption. This simplified calculation is based on numbers from the CC2430 IEEE 802.15.4 SoC, which has a CPU and Flash and can be used to implement a single-chip remote control. The example assumes a simple, generic RF remote control used for 50 button pushes per day. It also implements a paging function which requires the remote control to wake up every five seconds to check if it is being polled by the controlled device. When not active with a command or polling, the device goes into the PM2 sleep mode and waits to be woken by a button push interrupt or the sleep timer polling timeout.
When paging is not implemented, then power is consumed by the current drawn during sleep. In this scenario, the radio has an average power consumption of less than 1μA. To put things in perspective, the self-discharge of two alkaline AA batteries is approximately 1μA. Another point of comparison is the power consumed by a traditional IR remote. The modulated IR LED draws around 50mA when a button is pressed, and draws that current for as long as the button is pressed. This can easily be hundreds of milliseconds. Power consumed during a button push for IR is around 1000mAmS, while an RF-based remote control would not even use 1/10th of that.
Controlled Devices
When a controlled A/V device is on, the power consumed by the RF device generally is not significant. This means that it can be left on continuously to minimize latency in receiving commands. However, when the controlled device is in standby mode, the RF device's power consumption becomes significant.
To achieve Energy Star certification, a TV must not, on average, consume more than 1W in standby mode. Future standards and regulations will have even stricter requirements. This leaves few mA in the power budget for the RF transceiver. Limiting the RF transceiver's power consumption means that it cannot be in a continuous receive state. Thus, the turn-on command's latency issued from the remote control will be higher than for commands sent when the controlled device is on.
There is in fact an almost linear trade-off between power consumption and latency exemplified by Figure 2, which shows a sample calculation of an A/V device in standby mode polling every 50ms. To put this latency into perspective, the minimum latency of a typical IR implementation is 110ms.
Coexistence
Figure 3 illustrates a typical domestic setting consisting of A/V equipment surrounded by other 2.4GHz RF devices. Such devices typically include WiFi, cordless phones, microwave ovens, Bluetooth devices, ZigBee networks, etc. When many devices operate in the same frequency band, there is likely to be interference between them. RF interference causes lost packets, increased power consumption and communication latency. There are several ways of mitigating these effects in an RF remote control network.
From a software remote control protocol standpoint, the most efficient way to alleviate the effects of interference is frequency agility. This is an algorithm that responds to an impaired RF link by changing channels until it finds a channel with little interference. IEEE 802.15.4 defines 16 channels in the 2.4GHz band numbered 11-26. Changing to a channel with little interference is usually possible; however, there will always be a risk of interference in the neighboring channels. An example is IEEE 802.11 radios (WiFi) that uses wider channels than IEEE 802.15.4. Where several WiFi networks are present, Figure 4 shows that even if your remote control has moved to channel 20 where there is little interference from WiFi channels, the adjacent channels will have interference if an active WiFi network uses channel 6.
RF receivers are interfered with by devices operating on the same channel as well as signals transmitted at neighboring frequencies. Selectivity is a measure of how much interference the receiver can tolerate from a strong interferer transmitting in a neighboring channel without getting packet errors. Selectivity or jamming resistance is referred to as adjacent and alternate channel rejection in IEEE 802.15.4 and is measured in dB. Good selectivity in a receiver is a hardware parameter of the receiver and can be found by looking at a device's datasheet. High selectivity is achieved in devices such as the CC2520 through a combination of high-performance analog and digital filters.
Figure 5 shows interferers in two neighboring channels on the limit of causing enough interference at the receiver to cause packet errors.
IEEE 802.15.4 sets the minimum requirements for receiver selectivity to 0dB for adjacent channel rejection and 30dB for alternate channel rejection. In order to achieve a robust system in a heavy interference environment, these requirements should be surpassed. TI's CC2520 IEEE 802.15.4 transceiver achieves 49 dB adjacent channel rejection and 54dB alternate channel rejection, which means that it can be located very close to a jammer without losing packets and incurring latency due to interference sources.
Range
An IR remote control requires line-of sight to the receiver, making it difficult to operate from an adjacent room or many locations within the same room. An RF remote control does not require line-of sight. Imagine how you can use an RF remote control now! For example, you might want to hide your DVD player behind solid cabinet doors, place the set-top box in the center of your house and stream the video to several TV's from there, mute the music in your living room when you want to answer the phone in the kitchen, etc.
Many factors influence the range of an RF design such as the PCB design, the antenna and the casing. The RF transceiver's potential range is given by the sensitivity and output power. Sensitivity is measured by how weak a signal the receiver can receive (in dBm) at a certain packet error rate (PER). Output power is measured by how strong a signal the transmitter can send (in dBm).
Signal Strength Measurement
For a remote control to operate as expected within a system, it must know the address of the device with which it wants to communicate. The process of discovering this address is called binding. This process enables a single remote control to be set up to control one or more devices in the system. For example, a universal remote could control your TV, DVD and set-top box, even if these devices were not purchased at the same time from the same vendor. In IR remotes, this process is achieved by manually entering codes, an often daunting process that often fails if a device is not supported or the code cannot be found. For RF-based remote controls there are several ways to do pairing, of which the most common is called proximity pairing.
Proximity pairing works by pressing a button on the remote when it is physically close to the device it should control. The remote control sends a broadcast packet, and any device in close proximity will reply to establish a binding. In order to detect which device it is physically close to, the remote control relies on the RF signal strength from the transmitting device when it replies to the broadcast. Calculating the distance between transmitter and receiver from the signal strength at the receiver is based on the attenuation of RF signals over distance. In an RF receiver this signal strength is measured as RSSI (Received Signal Strength Indication). To get an intuitive and user-friendly proximity pairing, the RSSI measurement in the receiver must be accurate.
As shown in Figure 6, the CC2520 has the necessary capabilities for good proximity pairing performance. RSSI vs. power is both linear and has good dynamic range (≈100dBm). This makes it ideal for determining the distance to a transmitter.
Conclusion
Global standards like RF4CE for RF-based consumer electric remote controls is going to change our expectations for the home entertainment experience. The time to switch to RF-based remote control technology has arrived. A reliable two-way RF link between the remote control and A/V equipment greatly improves the user experience and allows new features and functionality previously not possible. While RF adds significant benefits to a remote control-based system, continue to be mindful of the RF performance parameters when choosing hardware for your RF4CE remote control implementation. As we discussed, selecting a device with low average power consumption for long battery life, good selectivity for a robust link, and accurate RSSI measurements for intuitive proximity pairing are all key to building a great remote control product.
References
[1] IEEE std. 802.15.4 " 2006 : http://standards.ieee.org/getieee802/download/802.15.4-2006.pdf
To download a datasheet or request a CC2430 or CC2520 development kit, visit www.ti.com/lprf.
About the Author
Peder Rand is a LPRF Systems Engineer with Texas Instruments where he is responsible for strategic marketing and systems engineering for Low Power RF IEEE 802. 15.4 and ZigBee devices. Peder holds a Masters Degree in Computer Science from the Norwegian University of Science and Technology. He can be reached at ti_prand@list.ti.com.
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