Many of today's systems engineers still have difficulty specifying detailed radio requirements and evaluating possible performance tradeoffs. At times, it seems easier to select a popular wireless system, such as 802.11 or Bluetooth, rather than sweat the specifics of a customized wireless implementation. For applications that demand interoperability, this is a logical and necessary choice. But many other wireless applications exist. Among them are cordless phones, industrial control, consumer game controllers, meter reading, wireless audio, and security. For these types of applications, systems designers can usually reduce size, cost, and power by using highly integrated RF transceivers along with a simple radio protocol optimized to the specific application.

For anything other than a high-speed, Ethernet-compatible wireless network, the use of an 802.11 solution will almost certainly result in unnecessary cost, size, and power. Likewise, a Bluetooth solution—while much lower in cost and power than 802.11—carries additional overhead. Such overhead is associated with any complex peer-to-peer personal-area network (PAN) that is designed with interoperability as its primary feature.

In a lot of applications, standards-based solutions have many features that aren't required. The result is unnecessary cost and power (see table). The key part of any wireless design is determining the required system performance. In this article, the term "radio" refers to the RF receiver and transmitter circuitry. A total wireless solution also requires a baseband, microcontroller, and software. Fundamentally, the primary parameters that affect size/power/cost are:

• Frequency (in MHz): Choosing the desired frequency of operation is generally the first thing done by a system designer. In the U.S., there are the unlicensed ISM bands at 900, 2400, and 5800 MHz. In the U.K., the narrowband 868 MHz has proven popular for low-data-rate solutions. Presently, the 2.4-GHz band comes closest to a true worldwide solution. As a result, it has the largest number of component vendors. The system designer also should evaluate other issues, including RF propagation (i.e., range), regional regulations, and possible interference from other users.
• Range (in meters): Generally, range is defined as the maximum distance between transceivers that provides a minimum signal-to-noise ratio (SNR). Range has forever been the most important parameter to the systems engineer. For the radio manufacturer, it has been the most difficult one to specify (due to the wide range of performance in different environments).

  Range is theoretically dependent on Tx power, antenna gain (both Tx and Rx), Rx sensitivity, and signal processing gain (if any). Obviously, longer range can be achieved with higher Tx power, but at the expense of shorter battery life. Unfortunately, antenna gain is directly related to size, so longer range can be achieved if more space is available. Rx sensitivity depends on both receiver noise figure and IF bandwidth. Aside from designing a good low-noise radio front end, the designer should try to use the minimum channel bandwidth needed to support the required throughput.

• Throughput (in kbps): Designing a radio to support the required throughput is one of the most important aspects of system design. Mismatching the radio channel bandwidth to the actual data throughput unnecessarily reduces sensitivity and range. Receiver sensitivity is dependent on 10logBW. Using a 1-Mbps radio for a 250-kbps application will therefore reduce sensitivity by 10log(4) or 6 dB, causing a 50% reduction in range. Getting the range back requires a 6-dB increase in Tx power, which effectively quadruples power consumption.
• Link reliability and interference rejection (in dBc): For proper receiver operation, one must accurately know the intended environment. Are there other users on the same or adjacent channels? What about unintended radiators? Can the system tolerate infrequent packet losses due to interference? Obviously, the most robust radio includes enough filtering to reject out-of-band interferers. It also has enough dynamic range to operate with co-channel users. Additionally, the datalink protocols that are intended for wireless links usually have provisions for errors. Such provisions must be included due to the inherently unreliable nature of the wireless channel. The provisions range from simple CRC checks and ACQ capability to more complex forward-error correction. The system designer must try to estimate the worst-case RF environment in which the radio must work as well as the tolerable level of interference.

Once the basic radio system requirements are determined, the designer must choose a radio IC. Many different transceiver architectures have been used in the past. The same can be said for modulation types. However, choosing which transceiver IC is best for the application can sometimes be a difficult decision.

In the world of wireless, three major types of receiver architectures are widely used. The classic double-conversion superheterodyne architecture is utilized in nearly all high-performance radios. Though it has excellent performance, it needs many off-chip components. Generally, it is not used in low-cost applications. Within the last few years, the more advanced direct-conversion (also called zero-IF) architecture is again garnering a lot of attention. Recent advances seem to be overcoming the problems with DC offset and LO isolation. But for now, the primary users of this architecture remain the more complex I/Q modulation approaches like 802.11.

For low-cost applications, the most popular receiver architecture is the low-IF architecture. This architecture avoids the DC offset problem by downconverting to a low IF frequency instead of all the way to baseband. This approach has proven to be very popular in low-cost, highly integrated transceivers including Bluetooth. It is the recommended receiver architecture for the applications discussed here.