Wireless personal area networks, or WPAN, are used for sensing, monitoring and control applications because they have the unique potential to implement cost-effective wireless connectivity in many end products where this functionality has been previously unconsidered. Sensing, monitoring and control solutions drive specific consideration factors for WPAN implementation. Ranges for low cost wireless networks in sensing, monitoring, and control applications encompass those distances of 300m or less and data rates of 250kbit/s or less. In WPAN end node designs there is often the necessity to extend battery life to the optimum in order to meet product needs.

A variety of implementation alternatives for low cost wireless networking can offer a high level of flexibility in the design process. Consider solutions from vendors that offer various configurations of stand alone transceivers to be used in conjunction with a wide selection of microcontrollers. As a second and equally effective alternative, consider the newest solutions which offer integrated transceiver/microcontroller (MCU) products. Reusing design components and engineering investment may be important as designers work on multiple, yet similar, end products. Therefore, a structured evaluation of solution options can be both cost and resource efficient.

Wireless Networking Technologies

The 2.4GHz ISM band supports multiple short range wireless networking technologies. Each alternative has been developed to optimally serve specific applications or functions. The networking topologies most commonly associated to the 2.4GHz frequency range are Bluetooth, WiFi, and ZigBee, as well as other proprietary solutions. Each of these solutions are suitable for wireless personal area networks, however, some offer extended capabilities that align best with sensing, monitoring, and control application needs. Nonstandards- based proprietary solutions may be considered; however, these solutions may are vendor dependent and could be subject to change. ZigBee, an IEEE 802.15.4 standards based solution, was created to address networks which require low power consumption, low data rates, reliability and security. The ZigBee solution accommodates network-specific support mesh networking, network recovery and healing, device interoperability and vendor independence. Frequencies are typically in the 868/915MHz or 2.4GHz spectrums. Multiple levels of stack capability give the engineer the opportunity to reuse design software for a variety of WPANs, including those with varying levels of complexity. Multiple stack solutions act as the foundation on which the engineer can easily set up the radio and focus the majority of design effort on the application software.

Radio (Transceiver) or RF Modem
Several RF modem features should be considered for implementation of low cost wireless networking systems. Most cost effective WPAN RF modem solutions recommend power supplies from 2.0 - 3.4V. For lightweight wireless networks, low data rates are adequate to support monitoring, sensing, and control functions and also help manage system power consumption. 250Kbit/s or less O-QPSK data in 2MHz channels (with 5MHz spacing between channels) with full spread-spectrum encode and decode is most often selected for these application types. In these environments, the transceiver wakes up, listens for an open channel, transmits small packets of data at lower data rates, then shuts down until the next event is indicated. It is important to review the number and types of transceiver channels available in relation to the planned design. Selectable transceiver channels offer the option to take advantage of channels which minimize noise, particularly staying away from the more crowded 2.4GHz WiFi channels.

For noisier operating environments, experienced vendors will offer three to four suggested 2.4GHz channels where there is less noise potential. It is recommended that designers look for typical transmit output power in the 0-4dBm range. Receive sensitivity typically in the -90dBm range will offer adequate capabilities for sensing, monitoring, and control functions. Buffered transmit and receive data packets simplify data management for the low cost MCUs which will be used with the transceiver. The radio or transceiver should also offer link quality and energy detect functions for network performance evaluation. Multiple power-down modes offer power saving features to minimize system power consumption.

These typically include off current, hibernate current and doze currents in the single digit microamp ranges. Programmable output power also allows the designer to reduce power consumption where range or environment require less power to achieve transmit and receive objectives. Ensuring these functions are offered in the selected solution will aid in maximizing battery life in battery operated full-function/ coordinator or end node devices.

Look for additional essential peripherals, such as internal timer comparators which are available to reduce MCU resource requirements. General purpose input/output ports (GPIO) are available in various different configurations and counts. In solutions which offer the flexibility of a transceiver with separate MCU, the communications is handled through the serial peripheral interface (SPI) port. Also, integrated solutions which include low noise amplifiers (LNA),power amplifiers (PA) with internal voltage controlled oscillator (VCO), integrated transmit/receive switch, on board power supply regulation, and full spread-spectrum encoding and decoding reduce the need for external components and lower overall cost. A wide array of system clock configurations provides the designer flexibility in end system design.

Options which allow either an external clock source or crystal oscillator for CPU timing are most suitable. A 16MHz external crystal is typically required for the modem clocking. Capability to trim the modem crystal oscillator frequency helps to maintain the tight standards required by IEEE 802.15.4.

Depending on the complexity and requirements of the end design, the designer is best served by vendors who offer multiple network software topology alternatives and multiple hardware configurations, which are often based on memory sizes. These may include a simple MAC configuration which utilizes MCU flash memory sizes from 4Kbyte and up. Fully 802.15.4 compliant MAC and full ZigBee compatible topologies often utilize MCU fl ash memory from approximately 20Kbyte to 128Kbyte. Reference designs, hardware development tools, and software development tools allow the designer to set up a network easily and evaluate network and solution performance. In the past some software design tools have been extremely difficult to use. Antenna design can be a complex issue, particularly for digital designers who have limited to no experience in RF design. Typically designers would take into account factors such as selecting the correct antenna, antenna tuning, matching, gain/loss, and knowing the required radiation pattern. It is advisable to gain a basic knowledge of antenna factors through application notes. However, most digital engineers prefer to work with a vendor solution where antenna design is provided, allowing them to focus on the application design. Look for antenna solutions where the antenna design is offered in completed Gerber files, which can be provided directly to the manufacturer for implementation.

Figure 1: Example, stand alone transceiver for use with various microcontrollers

It is recommended the engineer consider review of these key factors in relation to WPAN design requirements: integration, wireless networking topologies, radio (RF modem or transceiver), performance, operating voltage, data rates, range, channel flexibility, output power, sensitivity, power management, peripherals, clocking, multi-tier software, ease of hardware and software design, antenna design, and packaging. Whether considering an integrated solution or a discrete solution, the MCU should be evaluated considering: CPU features, performance, memory options, power management, clock source options, analog to digital conversion, peripherals, packaging, in-circuit debug and programming capabilities, and ease of software and hardware design. Such analysis will provide an organized perspective for engineering decisions.

Source: Technology First online.