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What’s required for RF4CE?

Posted: 03 Nov 2008 ?? ?Print Version ?Bookmark and Share

Keywords:consumer electronics? A/V? IR? remote control?

Peder Rand
Texas Instruments Inc.

Radio Frequency for Consumer Electronics (RF4CE) is a radio protocol standard for remote control of consumer electronics. Standardization is being driven by a consortium that consists of Panasonic, Philips, Samsung Electronics and Sony Corp. Their goal is to provide more value to the customer through increased reliability and novel features made possible by two-way RF communication when applied to remote controls.

The main advantage of the radio protocol is that it ensures interoperability between remote controls and A/V devices that IR never achieved. The protocol�s upper layers are defined specifically for remote control, while the lower layers (including PHY) use IEEE 802.15.4, where ZigBee and 6LowPAN are also based. IEEE 802.15.4 is used in many proprietary protocols in both consumer and industrial markets. Good features include low-power use, robustness, long range, global 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. The article highlights those with the greatest impact on the user experience in a typical remote control application.

Longer shelf life with RF remote controls
In a remote control system, the remote control is battery-driven and the controlled device remains powered. This imposes different power consumption needs for the two devices.

Remote controlA 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 usage. 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 used in the TX mode. Remote controls are considered low-duty cycle devices. This means that they are usually asleep, and wake up occasionally to use the radio, like whenever sending 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 that causes the remote to beep to make it easier to locate). These features need 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 5s 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 awakened by a button push interrupt or the sleep timer polling timeout.

Figure 1: This is the remote control power consumption calculation sample.

When paging is not implemented, then power is being used by the current drawn during sleep. In this scenario, the radio has an average power use of less than 1?A. To put things in perspective, the self-discharge of two alkaline AA batteries is approximately 1?A. 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 devicesWhen a controlled A/V device is on, the power used 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, use 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. Reducing the RF transceiver�s power usage 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 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.

Figure 2: This is the controlled device standby power consumption calculation sample.

CoexistenceFigure 3 shows a general domestic setting consisting of A/V equipment surrounded by other 2.4GHz RF devices. Such devices usually include Wi-Fi, 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 use and communication latency. There are few ways of mitigating the effects in an RF remote control network.

Figure 3: This shows the typical domestic wireless environment.

From a software remote control protocol standpoint, the most efficient way to alleviate the effects of interference is frequency agility. This algorithm 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 nearby channels. An example is IEEE 802.11 radios (Wi-Fi) that uses wider channels than IEEE 802.15.4. Where several Wi-Fi networks are present, Figure 4 shows that even if your remote control has moved to channel 20 where there is little interference from Wi-Fi channels, the adjacent channels will have interference if an active Wi-Fi network uses channel 6.

Figure 4: This presents the IEEE 802.11 and IEEE 802.15.4 channel plan.

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 seen 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 causing enough interference at the receiver to bring packet errors.

Figure 5: The selectivity is measured as the adjacent and alternate channel rejection.

IEEE 802.15.4 sets the minimum requirements for receiver selectivity to 0dB for adjacent channel rejection and 30dB for alternate channel rejection. To achieve a robust system in a heavy interference environment, these requirements should be achieved. TI�s CC2520 IEEE 802.15.4 transceiver achieves 49dB 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 because of interference sources.

RangeAn 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 need line-of sight. Imagine how you can use an RF remote control now. For example, you want to hide your DVD player behind solid cabinet doors, place the STB 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 what it wants to communicate. The process of discovering this address is called binding. This 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 STB, 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, a 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 and 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. To detect what 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 received signal strength indication (RSSI). To get an intuitive and user-friendly proximity pairing, the RSSI measurement in the receiver must be accurate.

As shown in Figure 5, the CC2520 has the necessary capabilities for good proximity pairing performance. RSSI vs. power is both linear and has good dynamic range (100dBm). This is ideal for identifying the distance to a transmitter.

Global standards like RF4CE for RF-based consumer electric remote controls will 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 not possible in the past. 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 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 essential to build a great remote control product (see Figure 6).

Figure 6: The CC2520 typical RSSI value vs. input power is shown.

About the Author

Peder Rand is a low-power RF systems engineer with Texas Instruments Inc., where he is responsible for strategic marketing and systems engineering for Low Power RF IEEE 802. 15.4 and ZigBee devices.





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