After introducing and comparing common wireless systems, Part II of this article will now take a look at 'Wireless system requirements'. The key part of any wireless design is determining the required system performance. In this article the term 'radio' is used to refer to the RF receiver and transmitter circuitry, realising that a total wireless solution also requires a baseband, microcontroller, and software (to be discussed later). Many books and papers have been written on wireless performance parameters but fundamentally the primary parameters that affect size/power/cost are: frequency; range; throughput; and link reliability and interference rejection.
Frequency
Choosing the desired frequency (in MHz) of operation is generally the first thing a system designer does. In the US there are the unlicensed ISM bands at 900, 2400, and 5800 MHz while in the UK the narrowband 868 MHz has proven popular for low-datarate solutions. Presently, the 2,4 GHz band comes closest to a true worldwide solution and therefore has the largest number of component vendors. Other issues the systems designer should evaluate are RF propagation issues (ie range), regional regulatory issues, and possible interference from other users.
Range
Range (in metres) is generally defined as the maximum distance between transceivers which provides a minimum signal-to-noise ratio (SNR). Range has forever been the most important parameter to the systems engineer and the most difficult to specify for the radio manufacturer (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. Antenna gain is (unfortunately) 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. Besides designing a good low-noise radio front-end, the designer should also seek to use the minimum channel bandwidth necessary to support the required throughput.
Throughput
Designing a radio to support the required throughput (in Kbps) is one of the most important aspects of the system design. Mismatching the radio channel bandwidth to the actual data throughput unnecessarily reduces sensitivity (and therefore range). Since receiver sensitivity is dependent on 10logBW, using a 1 Mbps radio for a 250 Kbps application reduces sensitivity by 10log(4) or 6 dB causing a 50% reduction in range! To get the range back requires increasing Tx power by 6 dB, effectively quadrupling the power consumption. Rarely is this something the system engineer is willing to accept.
Link reliability and interference rejection
Proper receiver operation requires accurately knowing the intended environment. Are there other users on the same or adjacent channels? Any unintended radiators like microwave ovens, etc? Can the system tolerate infrequent packet losses due to interference? Obviously the most robust radio includes enough filtering to reject out-of-band interferers and enough dynamic range to operate with co-channel users. Additionally, datalink protocols intended for wireless links usually have provisions for errors (due to the inherently unreliable wireless channel versus wired). These range from simple CRC checks and ACQ capability to more complex forward error correction. Bottom line is that the systems designer must try and estimate the worst-case RF environment the radio must work within and the tolerable level of interference.
While there are numerous other parameters that can be important to a wireless systems engineer these four items tend to be the most important in a majority of applications.
After determining the basic radio system requirements, the designer must then choose a radio IC. In Part III, we will take a look at transceiver architectures. Many different transceiver architectures have been used in the past along with many different modulation types, but which transceiver IC is best for the application can sometimes be a difficult decision.
For more information contact Components & System Design, 011 979 4274.
© Technews Publishing (Pty) Ltd | All Rights Reserved
printer friendly version