Understand the techniques you can use to minimize power dissipation while achieving performance goals
By Iboun Taimiya Sylla, Manager, Low-Power RF Business Development, Texas Instruments
RF Designline
(05/11/2009 7:00 AM EDT)
(Editor's Note: This is the third part of a three-part series of articles from Iboun Taimiya Sylla. The first part is The ISM Revolution: The Next Big Thing; the second part is To ZigBee or Not to ZigBee? Factors to consider when selecting ZigBee Technology.)
The Industrial Scientific & Medical (ISM) wireless revolution that created the transition from wireline to wireless has brought into sharp focus the vast market the wireless industry represents in today's economy. The combined wireless medical industry, automatic meter reading (AMR) and alarm & security represent an 18 billion-dollar market.
With the move from wireline to wireless comes a paradigm shift in system design considerations. The robustness of a wireless system is not characterized by the same parameters as the wireline systems. The robustness of a wireless system is primarily characterized by three parameters: power consumption, link quality and link security. In this article we will discuss these three parameters, as well as different techniques that can help an engineer improve the robustness of his design.
Power Consumption
Today, the end customer's satisfaction depends largely on power consumption, among other things. Low-power consumption translates into longer battery life and, therefore in the long run, a cheaper system. In the wireless e-meter industry, it is common to target an average battery life of 20 years. One can clearly say that power consumption is a key parameter that wireless product designers need to keep in mind, unlike with wireline systems where power consumption is of little concern because the system is mains-powered. We suggest three techniques than can help the designer optimize the power consumption of his product:
- Use low-duty cycle: We recommend minimizing the transmitter's (TX) and receiver's (RX) "on time" by sending just the amount of data needed. A way to achieve this is to use high data-rate transmission. Keep in mind that using high data rates requires a trade off on the achievable link range. High data rates yield less range for two main reasons: less energy per bit makes demodulation more difficult; and the RX filter bandwidth must be wider, therefore, allowing the presence of noise.
- Use FIFO register at RX and TX: For example, using the CC1101 (a low-power, sub-1 GHz RF transceiver) , or the CC2500 (a low-power, 2.4 GHz RF transceiver), the presence of a FIFO register allows burst mode data transmission with a high over-the-air data rate, which helps to reduce overall power consumption. If transmitting 10 kbps data using an over-the-air data rate of 100 kbps, the TX or RX contribution to the overall power consumption is reduced to approximately one-tenth, compared to 10 kbps. It is important to know that when over-the-air data rate is increased, sensitivity might drop due to less energy per bit and wider RX channel filter bandwidth.
- Implement receiver polling: The RX goes to sleep and wakes up periodically to see if any packets need processing. Given the short awake time the average current consumption is minimal. This helps to reduce power consumption and thereby significantly extends the system's battery life. Figure 1 illustrates an example of a receiver polling implemented on a highly integrated multichannel RF transceiver such as the CC1100. Notice the time distribution of the current consumption. This technique of programmable wake ups, receives and sleep times offers some flexibility on the battery life.
Figure 1: Current consumption time distribution in the receiver polling implemented on the CC1100
(Click on image to enlarge)
The link quality of a wireless system is mainly dependent on three key factors: the transmitter's output power; receiver sensitivity; and the propagation environment, which includes the level of interference, especially when targeting the 2.4 GHz band. Output power and sensitivity are two parameters that a design engineer can control. Using an external power amplifier and low-noise amplifier can help improve these parameters and, ultimately, the link budget.
The most critical part of the link quality is the propagation environment, especially when facing the challenge of increasing the immunity to numerous interference sources. With several applications (Bluetooth', Wi-Fi', ZigbeeTM, 802.15.4, microwave oven, etc.) using 2.4 GHz band, designing in that band presents a serious challenge, as you have to ensure that the design is robust enough to be jammed by external signals. To help against this type of interference, spread spectrum modulation techniques are widely used and have proven to be very efficient.
These techniques consist of spreading the energy across a number of frequency-band channels. They reduce output and power spectral density and help limit the interference on other users in the band.
The Federal Communication Commission (FCC) allows wireless systems using spread spectrum techniques to output more power. There are two spread spectrum techniques:
- Frequency hopping spread spectrum (FHSS): as depicted in Figure 2, to lower the average power spectral density. Frequency hopping utilizes a predetermined set of frequencies with either a repeating hop pattern or a pseudorandom hop pattern. Note that FHSS is also used in military applications to prevent eavesdropping.
Figure 2: Frequency hopping spread spectrum technique
(Click on image to enlarge) - Direct sequence spread spectrum (DSSS): illustrated in Figure 3, DSSS spreads its energy by rapidly phase-chopping the signal in such a way that each bit is represented by multiple bits using spreading code.
Figure 3: Direct sequence spread spectrum technique
(Click on image to enlarge)
Frequency agility techniques are also used against interference when working on a robust system. As illustrated in Figure 4, frequency agility can be considered as an extremely slow frequency-hopping system. The frequency is changed when the link performance is degraded and the measured packet error rate (PER) exceeds a predetermined threshold.
Remember that in both FHSS and frequency agility techniques, the phase-locked loop (PLL) lock time is very critical, as the system needs to hop to the next frequency in a very short period of time.
Figure 4: Frequency agility technique
(Click on image to enlarge)
Another technique that helps maintain the quality of the wireless link is to implement forward error correction (FEC). This method helps reduce the effect of bit errors in the packets. With the FEC, a bit error doesn't necessarily result in a packet error.
Link Security
Usually, the two most important aspects of wireless link security are preventing eavesdropping and preventing an attacker from inserting his own packets in the link. To solve these issues, the designer has access to advanced encryption standard (AES) and the asymmetric cryptography.
The AES algorithm uses one of three cipher key strengths: a 128-, 192-, or 256-bit encryption key (password). Each encryption key size causes the algorithm to behave slightly different, so increasing key sizes not only offers a larger number of bits with which you can scramble the data, but also increases the complexity of the cipher algorithm. The asymmetric cryptography allows the keys to be encrypted and is much more processor-intensive. Therefore, it is usually used only to encrypt keys.
Conclusion
In this article we have discussed the main design considerations for a robust RF product. Power consumption, link quality, and link security are the main parameters that characterize the robustness of a wireless product. These parameters must be at the forefront of any consideration. They can not be ignored without affecting the robustness and consequently the reputation of the product.
References
·For more information about low power RF and Zigbee solutions, click http://www.ti.com/zigbee-ca.
About the Author 
Iboun Sylla is currently managing business development for low-power RF products for Texas Instruments. Iboun brings to this role his extensive experience as a Sr. RF Design Engineer. Iboun received his Bachelor in Telecommunications Engineering from ESPT (Tunis-Tunisia), and his Master's and PhD in Electrical Engineering from Ecole Polytechnique de Montreal, Canada. Iboun also holds a Master's in Business Administration from the University of Texas at Dallas with focus on Corporate Finances and Strategic Leadership. Iboun can be reached at ti_ibounsylla@list.ti.com.
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