Design Considerations for Robust, Low-Power RF Products (Part 3)

RF Designline
(05/11/2009 7:00 H EDT)

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)
  
  

Link Quality
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.

选择工业无线技术最重要的五个准则

文章出处:与非网

无线应用把众多的新能力(如实现新型的监控、增加现有设备的灵活性、降低运营和流程管理成本)带入它们所服务的行业。反过来，许多不同类型的无线技术和应用迅速涌现出来以满足这个日益增长的需求。

需要强调的是，工业领域存在一些特有的挑战，但许多传统的无线技术并非是专门为应对这些挑战而设计的。这些挑战包括要求可靠性高、系统功耗低并具有在RF干扰严重的物理环境中良好工作的能力，当然，同时还须具有高性价比。

无线技术应用数量的不断增加也已成为一个挑战，众多的无线应用争夺同一个RF空间，导致频谱过于拥挤，并与业已存在的挑战交织在一起。要为某个特定的应用 选择一个合适的、足以应对这些挑战的技术，工程师需要考量可靠性、简单性、功效、传输范围和成本等多个重要指标。

可靠性

在这里，可靠性是指无线系统在存在各种工业障碍的情况下成功完成通信的能力。我们可以根据无线系统的某些特征参数来评价其可靠性。

* 所使用的RF频谱：无线系统通信所使用的物理RF频谱

* 接收灵敏度：收发器完成通信需接收到的最小信号水平

* 输出功率：能够输出的信号水平

* RF捷变能力：为避免干扰而在RF频谱中移动的能力

* 抗干扰度：在存在干扰的情况下在给定频道中通信的能力。

RF波的物理特性造成其频谱使用高度依赖于环境。频率越低，波长越长，越不容易被液体和加固混凝土等典型的生活与建筑材料所吸收。

但是，为减小与其它无线通信技术的干扰，RF频谱及其使用受到很强的管制。在RF频谱中，地方或国际组织只为免许可通信保留了几个频段，并称之为工业、科 学和医疗(ISM)频段。在这些频段中，主要使用的频段是2.4GHz部分。在这个频段，由于波长短，信号迅速被不利于RF传输的工业环境所吸收，我们需 要对其它可靠性评估指标给予更多的关注。

我们可以把接收灵敏度、输出功率和抗干扰度结合起来，形成一个更为宏观也更为重要的可靠性指标——链路预算。链路预算是接收灵敏度、输出功率和抗干扰度的 综合值。系统的接收灵敏度越高、输出功率和抗干扰度越大，则链路预算越大。链路预算越大，RF吸收和RF干扰给系统带来影响的可能性越小，实现可靠通信的 潜力越大。

收发器的接收灵敏度和输出功率强烈依赖于所使用的元器件，较易于进行评估和比较，但抗干扰度在很大程度上依赖于无线收发器为提高其生命力而使用的技术。目前使用的可直接改善抗干扰度的最好技术之一是直接序列扩频(DSSS)调制。

DSSS调制的本质是通过向传输信号中引入前向误差校正，来减少因信号干扰而造成的数据损失。特别地，DSSS基于发射器和接收器共享的伪随机噪声码，把数据编制成规模更大的比特流。

图1展示了把8比特数据编制成32码片(Chip)数据的过程，在这里，4个码片等价于一个比特。

这些码片被调制成RF信号并发射出去。接收器从接收到的信号中解调出这些码片并逆向执行DSSS编码方案。尽管由于信号噪声或干扰而存在解调错误，我们仍可以复现原始数据。

图1：直接序列扩频(DSSS)。(原始数据、编码、码片错误、原始数据、复现)

RF捷变通过干扰避免技术(在RF频谱内跳动或移动等)来改善可靠性。系统在频谱中移动的自由度越大，找到干扰较小的RF静谧环境的能力越强。包括伪随机 跳频方案或基于算法的跳频方案在内，目前使用的各种RF捷变技术通过不断地在频谱中跳变来最大限度地减小干扰(见图2)。

图2：RF捷变技术通过不断地在频谱中跳变来最大限度地减小干扰。

从可靠性的角度来说，不断跳频存在的一个问题是，在繁忙的RF频谱中，系统可能会无意地连续跳到频谱中包含强干扰的频道。更智能的方案只在遇到干扰时跳频。一旦找到安静无干扰的频率时则停止跳频。

不管使用哪种捷变方案，RF捷变同样依赖于所使用的RF频谱和通道规模。所使用的RF频谱会影响可用捷变空间。例如，由于受到频率分配的制约，同工作频率 较高的系统相比，工作频率较低的系统的捷变空间较小。例如，2.4GHz系统约有100MHz的可用频谱，而900MHz系统只有大约26MHz。

通道宽度也是影响RF捷变能力的一个重要因素。通道宽度越小，则在频谱中捷变的空间越大，RF捷变能力越强，避免干扰并在干扰之间找到合适位置的能力越 强。例如基于802.15.4的系统的通道宽度为5MHz，只有16个可用通道，而通道宽度为1MHz的系统通常有80个可用的通道，因而有更多可用的避 扰位置。

因而，可靠性是由链路预算、RF捷变能力和所使用的RF频谱所共同决定的。在相同的RF频谱上，无线系统的可靠性与链路预算和RF捷变能力正相关。另外， 尽管低频技术对某些特定环境有出众的表现(如某个较低频率的技术应用于遍布水管的工厂时)，但同可提高链路预算和RF捷变能力的频率较高的技术相比，仍相 形见绌。

简单性

理想地，工业领域的无线系统应能够完成与有线系统相同的功能，且要能简单地加以实现。我们应该从两个不同的角度来考虑简单性问题：其一是从设计终端产品以替代有线产品的工程师的角度；其二是从安装和使用这些产品的用户的角度。

从工程师的角度，可以把简单性定义为设计、开发和实现无线系统的容易程度。在这一点 上，简单性涉及所包含的元件易用性、有哪些可用于设计和开发的辅助工具、以及是否有经认证的元件以消除或减小令人畏惧的当地无线认证过程。灵活的可编程技 术可使工程师最大限度地调整所设计的系统，提高无线系统的易用性。

然而，灵活性和可编程能力通常会增加复杂性；因而，开发环境和工具(包括硬件工具和软件工具)必须便于使用和理解。包含开发和评估套件的工具可帮助全面地评估和理解硬件及软件。理想地，工程师应获得完整的无线协议栈及应用示例库、帮助文档和实例代码以便加快学习进程。

从用户的角度来说，简单性涉及在目标环境中安置和激活无线设备的简便性及对相关业务流程的影响。例如，系统可靠性和传输范围影响无线技术的试运行故障率。存在试运行故障的系统将最终要通过现场考察来确定最优的位置和通信路径。

另外，适应业务流程的技术可以使用户迅速地把该技术带来的好处整合到日常操作中。这些技术包括用于监视和遥控无线执行器的可编程的灵活性接口及其对自动响 应系统的支持逻辑。这些接口通常被称为仪表盘(dashboard)，可以方便地把无线网络的状态信息整合到现有的报告和分析过程中。

总的来说，无线系统最终须变得与有线系统同样易于管理和使用。从工程师和用户角度对系统做出定性的评价有助于理解和实现这个目标。

功效

功效是对无线系统功耗水平的一种度量。衡量无线方案优劣的较为传统的方式是测量系统 所用元件的功耗，但这并不是问题的全部。例如，一个大部分时间处在最低功耗状态(睡眠模式)的高可靠性系统通常将比只在收发状态功耗低但不太可靠的系统功 效更高，因为这些不太可靠的系统处在睡眠模式的时间较少，而更多时间处在高功耗的转发模式。因而，可靠性是衡量系统实际功效的一个重要指标。

除可靠性之外，主动电源管理(动态地控制输出功率)等系统行为也可以降低功耗和提高 功效。一直致力于把输出功率降低到通信所必需使用的最低水平的系统将不仅是可靠的而且是高功效的。这种意义的功效，尽管对无线电技术而言并不是新概念，但 从保证系统切实减小系统功耗方面，却是一个新概念。

传输范围

传输范围是无线电信号可以传输且被接收器可靠编译成数据的距离。考虑到工业领域不断 改变且不利于RF传输的环境，在确定什么技术可以得到最佳传输范围时，最重要的指标是链路预算和可靠性。无线系统也可以通过片上和非片上功率放大器来提高 其链路预算。如果实现过程中使用上述功率放大器，高度可靠系统的传输范围会更大。另需指出，高功效系统只在绝对必要时才使用这些导致功耗增大的放大器。

其它扩展无线系统传输范围的方法包括使用中继器、路由器和对等通信等技术。由于在延 迟和通信路径方面存在较大的不确定性，这些基于无线协议的技术增加了复杂性，提高了功耗并降低了可靠性。因而，保持所有指标不变而提高传输范围的最佳方式 是提高可靠性或用功率放大器进一步提升信号的幅度，但这受到当地频率使用条例的限制。

成本

由于无线系统涉及众多指标，对某个特定的操作环境，最低成本并不总是最优的。相反， 我们关注的应该是整个系统的成本。例如，如果由于可靠性低而使用了高成本的权宜方案(如由于链路预算低而增加功率放大器的数量、使用有线后援系统等)，那 么，在评价无线系统时，也需把这些成本包含进来。另外，如果系统可以完成更多的功能和从总体上给系统带来进一步的好处，也应该把这些新能力的价值考虑进来 ——实际上，应从该系统的总成本中减去这个价值。

假定所有其它指标不变，降低无线系统成本的一种技术方式是提高系统对用户的价值或降低系统的实际获取成本。元件成本通常需要由开发商和供应商协商解决，但某些应用系统可能对性能(闪存、RAM、处理能力等)的要求相对较低，因而可降低元件的成本。

例如，同简单的星形联网协议相比，复杂的网格联网协议通常需要更多的闪存，因为网格 网络需要在整个网络中为通信分配路由，星形网络则是简单的点到点协议，只有集线器真正需要某种水平的路由功能。这个比较是在其它指标不变的条件下进行的。 如果网格型方案比星型方案更为可靠或反之，这就不是一个合理的比较。

可靠性、简单性、功效、传输范围和成本是比较、评估和选择工业无线技术的五个最重要的指标。每个指标都以独特的视角展示了某个技术的优点。为确保针对给定应用选出了最合适的无线技术，在比较不同技术的优缺点时，须对这些指标逐一进行考察。

Avoiding Interference in the 2.4-GHz ISM Band

By Ryan Winfield Woodings and Mark Gerrior, Cypress Semiconductor
Wireless Net DesignLine
(07/01/2006 1:34 H EDT)

As more and more companies produce products that use the 2.4-GHz portion of the radio spectrum, designers have had to deal with increased signals from other sources. Regulations governing unlicensed parts of the spectrum state that your device must expect interference.

How can designers get the best performance out of their 2.4-GHz solution under these hostile conditions? Often the product works in a controlled lab environment but then suffers performance degradation from the storm of interference from other 2.4GHz solutions in the field. With existing standards like Wi-Fi, Bluetooth, and ZigBee there is little that can be done beyond what the architects of the standard provide. But when the designer controls the protocol there are procedures that will minimize the interference from other sources.

In this article,we'll examine the various interference management techniques provided by 2.4 GHz wireless systems. We'll then show how low-level tools can be used to create frequency-stability in a 2.4 GHz design.

Wi-Fi
The two methods for radio frequency modulation in the unlicensed 2.4 GHz ISM band are frequency-hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS). Bluetooth uses FHSS while WirelessUSB, 802.11b/g/a (commonly known as Wi-Fi), and 802.15.4 (known as ZigBee when combined with the upper networking layers) use DSSS. All of these technologies operate in the ISM frequency band (2.400"2.483 GHz), which is available worldwide (Figure 1 below).

The primary motivation for Wi-Fi is data throughput. Wi-Fi is typically used to connect computers to the local LAN (and indirectly to the Internet). Most Wi-Fi devices are laptops that are recharged daily or wall-powered access points and are therefore not power-sensitive.

Wi-Fi uses DSSS, with each channel being 22 MHz wide, allowing up to three evenly-distributed channels to be used simultaneously without overlapping each other. The channel used by each Wi-Fi access point must be manually configured; Wi-Fi clients search all channels for available access points.

802.11 uses an 11-bit pseudorandom noise (PN) code known as a Barker code to encode each information bit for the original 1 and 2 Mbit/s data rates. In order to achieve higher data rates 802.11b encodes six information bits into an eight-chip symbol using complementary code keying (CCK).

There are 64 possible symbols used in this CCK algorithm, requiring each 802.11b radio to contain 64 separate correlators (the device responsible for turning symbols into information bits), which increases the complexity and cost of the radio, but increases the data rate to 11 Mbit/s.

Figure 1: Signal comparison of wireless systems operating in the 2.4-GHz band.

Bluetooth
The focus of Bluetooth is ad-hoc interoperability between cell phones, headsets, and PDA's. Most Bluetooth devices are recharged regularly.

Bluetooth uses FHSS and splits the 2.4 GHz ISM band into 79 1 MHz channels. Bluetooth devices hop among the 79 channels 1600 times per second in a pseudo-random pattern. Connected Bluetooth devices are grouped into networks called piconets; each piconet contains one master and up to seven active slaves. The channel-hopping sequence of each piconet is derived from the master's clock. All the slave devices must remain synchronized with this clock.

Forward error correction (FEC) is used on all packet headers, by transmitting each bit in the header three times. A Hamming code is also used for forward error correction of the data payload of some packet types. The Hamming code introduces a 50% overhead on each data packet, but is able to correct all single errors and detect all double errors in each 15-bit codeword (each 15-bit codeword contains 10 bits of information).

WirelessUSB
WirelessUSB has been designed as a cable cutter for computer input devices (mice, keyboards, etc) and is also targeting wireless sensor networks. WirelessUSB devices are not recharged regularly and are designed to operate for months on alkaline batteries.

WirelessUSB uses a radio signal similar to Bluetooth but uses DSSS instead of FHSS. Each WirelessUSB channel is 1 MHz wide, allowing WirelessUSB to split the 2.4 GHz ISM band into 79 1 MHz channels like Bluetooth. WirelessUSB devices are frequency agile, in other words, they use a "fixed" channel, but dynamically change channels if the link quality of the original channel becomes suboptimal.

WirelessUSB uses pseudo-noise (PN) codes to encode each information bit. Most WirelessUSB systems use two 32-chip PN codes allowing two information bits to be encoded in each 32-chip symbol. This scheme can correct up to three chip errors per symbol and can detect up to 10 chip errors per symbol. Although the use of 32-chip (and sometimes 64-chip) PN codes limits the data rate of WirelessUSB to 62.5 kbit/s, data integrity is much higher than Bluetooth, especially in noisy environments.

ZigBee
ZigBee has been designed as a standardized solution for sensor and control networks. Most ZigBee devices are extremely power-sensitive (thermostats, security sensors, etc.) with target battery life being measured in years.

ZigBee also uses a DSSS radio signal in the 868 MHz band (Europe), 915 MHz band (North America), and the 2.4 GHz ISM band (available worldwide). In the 2.4-GHz ISM band sixteen channels are defined; each channel occupies 3 MHz and channels are centered 5 MHz from each other, giving a 2-MHz gap between pairs of channels.

ZigBee uses an 11-chip PN code, with 4 information bits encoded into each symbol giving it a maximum data rate of 128 Kbps. The physical and MAC layers are defined by the IEEE 802.15.4 Working Group and share many of the same design characteristics as the IEEE 802.11b standard.

2.4-GHz Cordless Phones
2.4 GHz cordless phones are becoming increasingly popular in North America and do not use a standard networking technology. Some phones use DSSS; most use FHSS. The phones using DSSS and other fixed channel algorithms typically have a "channel" button on the phone allowing users to manually change the channel. FHSS phones do not have a "channel" button, because they are constantly changing channels. Most 2.4 GHz cordless phones use a channel width of 5 to 10 MHz.

Collision Avoidance
Along with understanding how each of the technologies work, it is also important to understand how each technology interacts in homogeneous and heterogeneous environments.

Wi-Fi's collision-avoidance algorithm listens for a quiet channel before transmitting. This allows multiple Wi-Fi clients to efficiently communicate with a single Wi-Fi access point. If the Wi-Fi channel is noisy the Wi-Fi device does a random back off before listening to the channel again. If the channel is still noisy the process is repeated until the channel becomes quiet; once the channel is quiet the Wi-Fi device will begin its transmission. If the channel never becomes quiet the Wi-Fi device may search for other available access points on another channel.

Wi-Fi networks using the same or overlapping channels will co-exist due to the collision avoidance algorithm, but the throughput of each network will be reduced. If multiple networks are used in the same area it is best to use non-overlapping channels such as channels 1, 6, and 11. This allows each network to maximize its throughput since it will not have to share the bandwidth with another network.

Interference from Bluetooth is minimal due to the hopping nature of the Bluetooth transmission. If a Bluetooth device transmits on a frequency that overlaps the Wi-Fi channel while a Wi-Fi device is doing a "listen before transmit", the Wi-Fi device will do a random back off during which time the Bluetooth device will hop to a non-overlapping channel allowing the Wi-Fi device to begin its transmission.

Interference from 2.4 GHz cordless phones can completely stop a Wi-Fi network, even if the cordless phones use FHSS as opposed to DSSS. This is partially due to the wider channel (5 to 10 MHz) compared to Bluetooth (1 MHz) and also due to the higher power of the cordless phone signal. An FHSS cordless phone that hops into the middle of a Wi-Fi channel can corrupt the Wi-Fi transmission, causing the Wi-Fi device to repeat its transmission. 2.4 GHz FHSS cordless phones will most likely cause interference with all Wi-Fi devices in close proximity; therefore, these phones are not recommended for use around Wi-Fi networks. If the cordless phone is DSSS the channels used by the cordless phone and Wi-Fi access point may be configured to not overlap, thus eliminating interference.

Handling Interference in Bluetooth
In Bluetooth, interference from other Bluetooth piconets is minimal, because each piconet uses its own pseudo-random frequency-hopping pattern. If two co-located piconets are active the probability of a collision is 1/79. The probability of a collision increases linearly with the number of co-located active piconets.

Bluetooth originally relied on its frequency-hopping algorithm to handle interference, but people have realized that a single active Wi-Fi network can cause heavy interference on 25% of the Bluetooth channels. Packets lost due to overlap have to be retransmitted on quiet channels, thereby greatly reducing the throughput of Bluetooth devices.

Bluetooth specification version 1.2 addresses this issue by defining an adaptive frequency hopping (AFH) algorithm. This algorithm allows Bluetooth devices to mark channels as good, bad, or unknown. Bad channels in the frequency-hopping pattern are then replaced with good channels via a look-up table. The Bluetooth master may periodically listen on bad channels to determine if the interference has disappeared; if so, the channel is then marked as a good channel and removed from the look-up table. Bluetooth slaves, when requested by the master, can also send a report to the master informing the master of the slave's assessment of channel quality. For instance, the slave may be able to hear a Wi-Fi network the master cannot. The Federal Communications Commission (FCC) requires at least fifteen different channels be used.

The AFH algorithm allows Bluetooth to avoid channels occupied by DSSS systems such as Wi-Fi and WirelessUSB. 2.4 GHz FHSS cordless phones may still cause interference with Bluetooth since both systems are hopping over the entire 2.4 GHz ISM band, but since the Bluetooth signal is only 1 MHz wide the frequency of collisions between the FHSS cordless phone and Bluetooth is significantly less than the frequency of collisions between Wi-Fi and FHSS cordless phones.

Bluetooth also has three different packet lengths that translate into different dwell times on a given channel. Bluetooth has the option to reduce the packet length in an effort to increase data throughput reliability. In this scenario it is better to get smaller packets through at a slower data rate than losing larger packets at the normal data rate.

Handling Interference in WirelessUSB, ZigBee
In WirelessUSB, each network checks for other WirelessUSB networks before selecting a channel. Therefore interference from other WirelessUSB networks is minimal. WirelessUSB checks the noise level of the channel at least once every 50 ms. Interference from a Wi-Fi device will cause consecutive high noise readings causing the WirelessUSB master to select a new channel. WirelessUSB peacefully co-exists with multiple Wi-Fi networks, because WirelessUSB is able to find the quiet channels between the Wi-Fi networks (Figure 2).

Figure 2: Diagram showing the frequency agility of a WirelessUSB design.

Interference from Bluetooth may cause WirelessUSB packets to be retransmitted. Due to the hopping nature of Bluetooth, WirelessUSB retransmissions will not collide with the next Bluetooth transmission because the Bluetooth device will have moved on to a different channel. Bluetooth networks will not cause enough consecutive high noise readings for the WirelessUSB master to change channels.

ZigBee specifies a collision-avoidance algorithm similar to 802.11b; each device listens to the channel before transmitting in order to minimize the frequency of collisions between ZigBee devices. ZigBee does not change channels during heavy interference; instead, it relies upon its low duty cycle and collision-avoidance algorithms to minimize data loss caused by collisions. If ZigBee uses a channel that overlaps a heavily used Wi-Fi channel field tests have shown that up to 20% of all ZigBee packets will be retransmitted due to packet collisions.

What Can Be Done?
When developing Bluetooth, Wi-Fi, or ZigBee, designers must use the methods provided in the specification. When developing a proprietary system based on 802.15.4, WirelessUSB or other 2.4 GHz radio, designers can use lower-level tools to create frequency agility.

DSSS systems have the most to lose because of the danger of overlapping with another DSSS system. But there are things DSSS systems can do to obtain the frequency agility of FHSS systems. One approach is network monitoring. If the DSSS system uses a polled protocol (where packets are expected at specified intervals) then the master can switch channels after a number of failed transmit attempts or bad received packets.

Another approach is to take a reading of the energy level on the air if the radio has this capability. A receive strength signal indicator (RSSI) can be used to proactively measure the amount of energy on the air and if that level is too high over a period of time switch to a clearer channel. A period of time is taken into account so as not to change channels if a FHSS system is passing through.

Network monitoring and RSSI readings assume that both radios are transceivers — they can transmit and receive packets. In a DSSS system, in which one side is a transceiver and one side is a receiver, a multiple transmit approach can be used to obtain frequency agility. The transmitter sends the same packet at multiple frequencies and the receiver cycles though the receive channels at a much slower rate. This system works when the receiver is connected to power and the battery-powered transmitter is used less frequency. A wireless remote might use this approach.

Wrap Up
Each of the standard 2.4-GHz networking technologies has made design tradeoffs to mitigate the effects of interference or to avoid it altogether. Designers can create their systems to be frequency agile either by using the procedures provided by the standard being implemented or by building their own protocol using the methods mentioned here in conjunction with radio features like RSSI when available. While it will never be possible to completely eliminate interference from outside 2.4-GHz systems, designers can create their systems to be frequency agile and give their product the best chance to survive in today's hostile 2.4-GHz ISM band environment.

About the Authors
Mark Gerrior is a principal software engineer in the Consumer and Computation Division of Cypress Semiconductor. He holds a MA in computer science from Marlboro College and can be reached at mgt@cypress.com.

Ryan Winfield Woodings is a systems engineer in the Consumer and Computation Division of Cypress Semiconductor. He holds an MS and BS in computer science from Brigham Young University and can be reached at rww@cypress.com.

Understand Radio Architectures, Part 1

By Christopher Bowick
RF Designline
(02/11/2008 1:35 H EST)
The following is excerpted from Chapter 8 from a new edition of the book, RF Circuit Design, 2e by Christopher Bowick. (If you order a copy of this book before March 30, 2008 you can receive additional 20% off. Visit www.newnespress.com or call 1-800-545-2522 and use code 91603. )

Click here for another excerpt: "What's in an RF Front End?"
Click here for Radio Architectures, Pt 2: Receivers, LOs, and Mixers.
Click here for " Radio Architectures, Pt 3: Intermodulation and Intercept Points"
Click here for "Radio Architectures, Pt 4: Sensitivity, Noise, Front End Amps"
Click here for "Radio Architectures, Pt 5: ADCs and Receivers"

The fundamental operation of an RF front end is fairly straightforward: it detects and processes radio waves that have been transmitted with a specific known frequency or range of frequencies and known modulation format. The modulation carries the information of interest, be it voice, audio, data, or video.

The receiver must be tuned to resonate with the transmitted frequency or frequencies in order to detect them. Those received signals are then filtered from all surrounding signals and noise and amplified prior to a process known as demodulation, which removes the desired information from the radio waves that carried it.

These three steps—filtering, amplification and demodulation—detail the overall process. But actual implementation of this process (i.e., designing the physical RF receiver printed-circuit board (PCB)) depends upon the type, complexity, and quantity of the data being transmitted. For example, designing an RF front end to handle a simple amplitude-modulated (AM) signal requires far less effort and hardware (and even software) than building an RF front end for the latest third-generation (3G) mobile telecommunications handset.

Because of the enhanced performance of analog components due to IC process improvements and decreasing costs of more powerful digital-signal-processing (DSP) hardware and software functions, the ways that different RF front-end architectures are realized has changed over the years. Still, the basic requirements for an RF front end, such as the frequency range and type of carrier to be received, the RF link budget, and the power, performance, and size restrictions of the front-end design, remain relatively the same in spite of the differences in radio architectures.

Let's start by looking at the simplest of radio architectures or implementations.

AM Detector Receivers
One of the basic RF receiver architectures for detecting a modulated signal is the amplitude modulation (AM) detector receiver (see Fig. 8-2). The name comes from the fact that information like speech and music could be converted into amplitude (voltage) modulated signals riding on a carrier wave. Such an RF signal could be de-modulated at the receiving end by means of a simplediode detector. All that is needed for a basic AM receiver—like a simple crystal radio—is an antenna, RF filter, detector, and (optional) amplifier to boost the recovered information to a level suitable for a listening device, such as a speaker or headphones.

The antenna, which is capacitive at the low frequencies used for AM broadcasting, is series matched with an inductor to maximize current through both, thus maximizing the voltage across the secondary coil. A variable capacitance filter may be used to select the designed frequency band (or channel) and to block any unwanted signals, such as noise. The filtered signal is then converted to demodulate the AM signal and recover the information. Fig. 8-3 represents a schematic version of the block diagram shown in Fig. 8-2.

The heart of the AM architecture is the detector demodulator. In early crystal radios, the detector was simply a fine metal wire that contacted a crystal of galena (lead sulfide), thus creating a point contact rectifier or "crystal detector." In these early designs, the fine metal contact was often referred to as a "catwhisker." Although point-contact diodes are still in use today in communication receivers and radar, most have been replaced by pn-junction diodes, which are more reliable and easier to manufacture.

For a simpleAM receiver, the detector diode acts as a half-wave rectifier to convert or rectify a received AC signal to a DC signal by blocking the negative or positive portion of thewaveform (see Fig. 8-4).A half-wave rectifier clips the input signal by allowing either the positive or negative half of theAC wave to pass easily through the rectifier, depending upon the polarity of the rectifier.

A shunt inductor is typically placed in front of the detector to serve as an RF choke. The inductor maintains the input to the detector diode at DC ground while preserving a high impedance in parallel with the diode, thus maintaining the RF performance.

In a simple detector receiver, the AM carrier wave excites a resonance in the inductor/tuned capacitor (LC) tank subcircuit. The tank acts like a local oscillator (LO) to the current through the diode is proportional to the amplitude of the resonance and this gives the baseband signal (typically analog audio).
The baseband signal may be in either analog or digital format, depending upon the original format of the information used to modulate the AM carrier.As we shall see, this process of translating a signal down or up to the baseband level becomes a critical technique in most modern radios. The exception is time domain or pulse position modulation. Interestingly, this scheme dates back to the earliest (spark gap) radio transmitters. It's strange how history repeats itself. Another example is that the earliest radios were digital (Morse code), than analog was considered superior (analog voice transmission), now digital is back!

The final stage of a typical AM detector system is the amplifier, which is needed to provide adequate drive levels for an audio listening device, such as a headset or speaker. One of the disadvantages of the signal diode detector is its poor power transfer efficiency. But to understand this deficiency, you must first understand the limitation of the AM design that uses a halfwave rectifier at the receiver. At transmission from the source, the AM signal modulation process generates two copies of the information (voice or music) plus the carrier. For example, consider an AM radio station that broadcasts at a carrier frequency of 900 kHz. The transmission might be modulated by a 1000-Hz (1-kHz) signal or tone. The RF front end in an AM radio receiver will pick up the 900-kHz carrier signal along with the 1-kHz plus and minus modulation around the carrier, at frequencies of 901 and 899 kHz, respectively (see Fig. 8-5). The modulation frequencies are also known as the upper and lower sideband frequencies, respectively.

But only one of the sidebands is needed to completely demodulate the received signal. The other sideband contains duplicate information. Thus, the disadvantages of AM transmissions are twofold: (1) for a given information bandwidth, twice that bandwidth is needed to convey the information, and (2) the power used to transmit the unused sideband is wasted (typically, up to 50% of the total transmitted power).

Naturally, there are other ways to demodulate detector-based receiver architectures. We have just covered an approach used in popular AM receivers. Replacing the diode detector with another detector type would allow us to detect frequency-modulated (FM) or phase-modulated (PM) signals, this latter modulation commonly used in transmitting digital data. For example, many modern telecommunication receivers rely heavily on phaseshift keying (PSK), a form of phase (angle) modulation. The phrase "shift keying" is an older expression (from the Morse code era) for "digital."

All detector circuits are limited in their capability to differentiate between adjacent signal bands or channels. This capability is a measure of the selectivity of the receiver and is a function of the input RF filter to screen out unwanted signals and to pass (select) only the desired signals. Selectivity is related to the quality factor or Q of the RF filter. A high Q means that the circuit provides sharp filtering and good differentiation between channels—a must for modern communication systems.

Unfortunately, tuning the center carrier frequency of the filter across a large bandwidth while maintaining a high differentiation between adjacent channels is very difficult at the higher frequencies found in today's mobile devices. Selectivity across a large bandwidth is complicated by a receiver's sensitivity requirement, or the need to need to detect very small signals in the presence of system noise—noise that comes from the earth (thermal noise), not just the receiver system itself. The sensitivity of receiving systems is defined as the smallest signal that leads to an acceptable signal-to-noise ratio (SNR).

Receiver selectivity and sensitivity are key technical performance measures (TPMs) and will be covered in more detail in this chapter. At this point, it is sufficient to note that the AM diode detector architecture is limited in selectivity and sensitivity.

Part 2 of this article will cover direct-conversion, and superheterodyne receiver configurations.

Printed with permission from Newnes, a division of Elsevier. Copyright 2008. "RF Circuit Design, 2e" by Christopher Bowick. For more information about this title and other similar books, please visit www.newnespress.com.

OOK通訊協議

From EETaiwan

振幅移位鍵控(ASK)是一項廣泛用於數位數據通訊的調變技術，針對許多低頻RF應用所開發。當訊號源要發送一個‘1’時，它會傳送一個大振幅的載波；而當它要發送‘0’時，就會發送一個小振幅的載波。開/關鍵控(OOK)調變是該方法進一步的簡化方法，當訊號源要發送‘0’時不需要發送載波。

ASK 和OOK通訊協議一般用於短距離無線應用，例如家庭自動化、工業網路、無線基地台、遙控車門開關(RKE)以及胎壓監測系統(TPMS)。OOK特別適用 於電池供電的可攜式應用，因為這些系統可在傳送‘0’時節省發送功率。涉及到的載頻視應用而定會有相當大的變化，例如基地台的低頻有線通訊使用=2MHz 的載頻；而利用ISM頻段(工業、科學和醫療)的小範圍無線通訊載頻可達=433MHz。

藍牙、ZigBee、 Wi-Fi等不同的無線技術在目前的消費性電子產業已獲得進展。這些協議為設備間提供安全的通訊機制，一般利用結合了頻移鍵控(FSK)、相移鍵控 (PSK)和振幅移位鍵控技術來執行2.4GHz的ISM頻段。這些技術提供的安全性包含通訊的通道跳頻和展頻模式。由於這樣的方案難以監聽，因此可提供 業界對安全性的需求並提升抗雜訊能力。所有這些方法在發送‘0’和‘1’的時候都會消耗功率。不幸的是，這些協議還具有相當高的複雜性及硬體建置成本，特 別是當安全性和高抗雜訊不是主要需求的時候。

Wi-Fi是特別針對高數據速率及長距離應用所設計，對於簡單的控制和監控應用 而言或許過於鋪張。ZigBee被視為即將登場的感測器網路領域的理想選擇，而藍牙在消費性音訊裝置及個人無線裝置上已獲得廣泛的利用。表1簡單地比較了 藍牙、ZigBee和ASK/OOK的性能特徵。

表1：藍牙、ZigBee和ASK/OOK特性的比較。

簡 單的ASK/OOK硬體建置仍是業界首選，因為它具有低建置成本的優勢，適合長電池作業時間應用，或接取點對點有線基礎網路和無線紅外線的連結。針對不同 的應用，建置成本可能為替代技術的2到5倍。透過在發送器和接收器間利用雙向的訊問機制仍然可以覆蓋該連結的安全性，例如在需要時可透過交換特殊的程式碼 來達成。ASK相較於OOK可提供較好的抗雜訊能力，相較於FSK可提供較低的成本，但相較於OOK卻具有較高的功耗。

振幅移位鍵控

ASK 接收器前端一般由三個模組組成：一個可從寬頻輸入雜訊頻譜中察覺同行載頻的輸入帶通濾波器；一個用來節錄同行資訊的封包檢測器；以及一個用於獲得二進位輸 出的比較器。該比較器的觸發閾值電壓由封包檢測器本身的輸出電壓驅動，這使得閾值電平能依據通道長度和發送器強度等不同接收訊號電平自動調變。

一 種可行的前端建置是採用一個MAX9933射頻(RF)功率檢測器，它可利用介於2MHz到1.6GHz的45dB動態範圍讀取輸入訊號。與眾不同的是， 它提供了一個與-58dBV到-13dBV(即1.25mVrms~223mVrms)間的訊號電平成比例的對數電壓。圖1顯示了它在ASK接收器訊號鏈 中的使用。

圖1：在ASK應用中使用MAX9933的電路。

饋 入FRIN接腳的RF訊號在外部交流耦合。由於它是一個峰值回應RF檢測器，本質上具有簡單封包檢測器的功能，即使是低至mV電平的訊號也能夠探測。它用 於輸入RF電壓振幅與輸出DC電壓的記錄轉換功能產生了等比例的dB特性，使其對於微小訊號也相當敏感，允許ASK接收器能區別微小的1和0訊號電平。 CCLPF電容值可確定晶片輸出處的回應頻寬，因此是由預期的數據速率決定。圖2顯示了當功率檢測器作為封包檢測器進行測試時的輸出波形，其中比較器與一 個自適應參考一起使用，藉以產生數位輸出位元。該測試波形具有1MHz的載頻以及40kbps的數據速率。CCLPF濾波器電容為150pF，R-C濾波 器由一個100kΩ的電阻和一個0.22uF的電容組成。

圖 2：MAX9933 RF檢測器對調變頻率為10MHz、數據速率為40kbps的RF輸入訊號的回應。兩個波形顯示出對-10dBm~-20dBm ASK訊號的輸入訊號(上圖藍色)和下圖-40dBm OOK訊號的輸出回應(黃色)。MAX9930的比較器輸入波形顯示在下圖，以粉紅色及綠色表示。

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

By Shone Tran, Cypress Semiconductor
RF Designline
(01/26/2009 2:01 H EST)

Designers of low-power RF portable products must address two important criteria that deeply affect their choices for a wireless technology: battery life and range. The good news is that there is a multitude of standard and proprietary 2.4-GHz wireless technologies for designers to choose from. The bad news is that evaluating how each meets the battery life and range requirements for an application can be extremely challenging. Most designers are forced to sift through misleading marketing and datasheet specifications, including transmit current, receive current, sleep current, and range.

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:

1. Example Current Profile Over One Transmission Cycle Using Cypress's CyFi Transceiver: CYRF7936

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.

The Impact of Range
Range also turns out to be linked to reliability. However, first consider the problem with how many 2.4-GHz RF chip vendors market range. Today, most vendors state range in meters. For example, one vendor will state their radio transmits from 10-50m while another will state they can achieve up to 150m range. Range quantified in meters may not mean a whole lot because it is hard to qualify and varies from application to application. It is difficult to predict how far a radio can communicate in the presence of noise, multi-path, metal, water, and other RF-obstructing materials and situations. Furthermore, moving objects or a moving wireless node will cause range to vary a great deal as well. So how can engineers objectively compare one radio to another in terms of range? The closest thing to an objective metric is link budget.

LINK BUDGET = MAXIMUM TX POWER " RX SENSITIVITY

Receive (RX) sensitivity is a negative number so link budget should always be a positive number. One important thing to note about RX sensitivity is that arguably, it is a more important metric than TX power because a high RX sensitivity enables the radio to hear quieter signals. For the same output power, a radio with higher sensitivity achieves more link budget (range). This is very important and useful for certifying wireless products in regions of the world that severely limit TX power. Consider the following table of TX Power and RX Sensitivity from popular radios on the market today. Link budget is calculated for objective comparison.

As you can see in the above table, RX sensitivity is what varies the most when it comes to link budget. It turns out Direct Sequence Spread Spectrum can add up to 9-13dBm to RX sensitivity due to the coding gain, so range also depends on reliability as well. Remember the rough rule of thumb that every 6dBm of link budget equates to about twice the range.

Choosing a radio with a high link budget is a good start for achieving desired range, but engineers must not rely on a chip specification alone. There are several factors outside of the chip that will affect range:

• Board layout: having separate ground and power planes in a 4-layer board tends to yield better performance.
• External power amplifier: can be used to extend range, but at the expense of more current consumption. Also, using an external power amplifier can raise issues when it comes to certification.
• Antenna design: there are many different antenna options, including simple random wire antennas, dipole antennas, PCB wiggle antenna, dual antenna designs, chip antennas, SMA antennas, and others. There are tradeoffs between cost, form factor / size, and range between the different options.
• Matching Network: When the impedance matching network is not designed to specification, range takes a significant hit because power transfer is not maximized and there may be significant reflections. Following the chip vendor's requirements for the matching network is very important. Sometimes ODMs take shortcuts to cut cost by using components with wider tolerances, which results in poorer-than-expected performance. In some cases, these seemingly small shortcuts can reduce range from 30 meters down to 2-3 meters. Some chip vendors offer design review services for engineers designing with their radio to ensure predictable performance. It's important that engineers take advantage of these services and work with a chip vendor that offers them.

Ultimately, range and battery life are best predicted by testing the actual application in real, dynamic environments with noise. Engineers will find that battery life can range significantly depending on how much noise a given device sees. Don't be surprised if average power consumption can increase by as much as 4-5 times with many radios on the market because they don't have interference immunity features or frequency agility. A good radio/protocol combination will perform with predictable and relatively consistent average power consumption even when facing noise. Creating a good protocol is much easier said than done, which puts into perspective why most wireless protocols are developed by post-doctorate engineers that really understand wireless communication theory. Also, chip vendors know their radios best and know how to optimize the features of their radios in their protocol. Therefore, engineers are usually much better off using off-the-shelf protocols that are optimized to a specific radio.

About the Author
Shone Tran is a Product Manager in Cypress Semiconductor's Data and Communication Division. His focus is on product marketing for Cypress's embedded wireless products. Shone earned his BSEE from the University of California, Davis and is currently based in San Jose, CA. Shone can be contacted at xut@cypress.com.