Making The Most Of Machine-To-Machine Radio Communications

The Machine-to-Machine (M2M) communications market is undergoing a significant expansion phase, led by the cellular operators looking to increase the usage of their existing networks. This is an attractive market to the cellular operators as the data messages are generally short in length and can be fit in around the existing voice calls with little impact to their network loading. However, the cellular networks are not the only carrier mechanism, and in many cases may not be the best. There is still a significant market for more traditional radio carriers, especially in areas where there is no cellular coverage, where independence and security are primary concerns, or where cellular tariffs are not appropriate.

Some of the potential applications for M2M communication are:

• Security monitoring/alarm activation
• Parking meter status
• Vending machine status
• Remote meter reading
• Point-of-sale verification
• Remote telemetry logging
• Sensor networks

Short-range applications (1 to 100m) can frequently be satisfied by Bluetooth or Zigbee systems, whilst wide area and international applications can often only be satisfied by the cellular networks. Some systems fall into neither category due to range over 1km, no cellular or data network coverage, security, cost, reliability, or speed. In this case it may be worthwhile to look at a bespoke data communications link, especially if it can be built from standard components at low cost with low power consumption, and can conform to accepted international standards.

Solar powered parking meters are an obvious candidate for wireless monitoring. Coupled with the solar panel and internal batteries, the wireless setup means that these units can be deployed quickly with little or no disruption to the area around them as there is no need to dig up the road to install cables for power or communications. The wireless monitoring ensures that any faults with the machine can be quickly reported to a central control room to allow repairs to be carried out quickly and without annoyance to the public or loss of revenue.

There are a number of frequency bands that are available for license-free use (in the EU, 433MHz for instance). However, any equipment used in this band must still satisfy certain regulatory requirements as set down in ETSI EN 300 220. Operation in other bands is usually subject to a fee-chargeable operator license as well as requiring the equipment to meet ETSI EN 300 113 (for 12.5- or 25-kHz operation) or EN 301 166 (for narrower channels). The significant differences are shown in the table. Regulatory requirements in other parts of the world vary, but generally similar license principles apply.

The reduced Tx power of the license-free operation limits the effective range of the devices, but does ease the deployment of the system in the field. The conformity to specific Rx parameters is not always required under the Harmonised Standards in the EU or FCC Rules in the U.S.A.; however good receiver performance is often necessary as the units are likely to be used in bands where many other devices are operating. Thus, receiver immunity to interference is important in order to ensure that the system can continue to function reliably. Devices operating the 2.4-GHz band must also contend with interference from Bluetooth and Wireless LAN devices.

Using a licensed radio channel has the advantage of having guaranteed access to the airwaves and the ability to transmit at higher power levels and so achieve greater range. However this does come at the cost of applying for and paying the local administration for the privilege although some administrations are attempting to make it easier to get a license.

In many countries a data network may already exist (such as Mobitex) that offer additional services (such as guaranteed delivery and maximum delivery times), which the cellular operators don’t provide. The downside to this service is the cost and the restriction to using equipment authorised by the data network operator.

Like most technology products, the design of data radio modems has evolved over the years. The early designs were essentially existing PMR radios, re-housed in a metal box with a modem daughter card added to provide a data interface. Although reasonably cheap, they suffered from a low level of integration and less than optimal performance as the data rate was often constrained by the design of the radio unit being used. A typical modem device for these units was the CML Microcircuits CMX589 IC, which offered a GMSK “data pump” in a very small package and equally offered very low power consumption. Later devices were designed from the ground-up as data-only devices and could then take advantage of an optimised radio and baseband design to improve both the levels of integration and the data rates supported.

In the late 1980s, a number of radio data networks were deployed (Mobitex and Paknet for instance) which promoted the demand for higher-volume, lower-cost products that still met the requirements of the radio regulators. Most of the major radio manufacturers produced their own designs, but the Maxon DM200 for the Mobitex network was a typical example that included a flexible radio design and a baseband modem with on-board packet processing hardware coupled with a powerful processor to control the higher protocol layers. In this design, the CML Microcirucits CMX909 IC was used because it combined not just a data pump but also included the bulk of the data formatting, encoding, and error correction that was a major factor in the success of the Mobitex network, providing secure and reliable data communications.

There are a number of chip sets and module designs on the market at the moment. However, most of them offer limited data rates or un-licensed operation only. Licensed radio data units offer higher power (and consequently longer range) and potentially higher throughput, but at a higher cost. In order to reduce the cost of these units, it is necessary to improve the integration of the underlying design, which in turn will usually lead to consequent improvements in reliability and power consumption.

A suitable device that integrates most of the radio modem functionality is CML’s CMX990 integrated modem device, which is aimed at M2M (Fig. 1). It includes both RF and baseband processing for modems operating up to 16kbps. A significant amount of formatting and error correction is performed on-chip, thus reducing the processing requirements of the host micro-controller and hence cost (a cheaper micro can be used) and power consumption.

The conventional method of modulating a radio was to apply the modulator signal to the VCO and then pass it to a PA and out to an antenna. However, the use of phase-locked-loop based VCOs means that the lower frequency content (vital to the data transmission) is removed by the loop. To compensate for this loss, an additional modulation path is provided to the loop’s reference oscillator, which re-instates the low frequency component. The design of these circuits to achieve the required level of matching usually results in a production line trim. However, when implemented correctly, this two-point modulation scheme can be an effective and reliable transmission method.

An alternative method uses signals that are up-converted to RF by a pair of quadrature mixers. This has the advantage of accurately defining the modulation characteristics. When necessary, corrections can be applied to the signal in the digital domain to compensate for any imperfections in the hardware path. The downside to this approach is increased complexity.

The CMX990 uses an offset-PLL approach that, with the benefit of silicon integration, combines many of the advantages of both architectures, achieving accurate digital modulation while still retaining the cost and performance benefits of direct VCO modulation.

Traditional super-heterodyne receiver architectures offer a low-cost, low-power solution. Conventional analogue filtering is used prior to a frequency demodulator, however the channel selecting components must be carefully chosen to ensure that the data signal is not degraded. This problem becomes more significant as data speeds increase (for a given radio channel bandwidth). Component variation across batches, as well as time and temperature, can become problematic. As in the transmitter, the signal bandwidth must be preserved almost to DC, which requires the modem to track and correct for the input DC levels.

An IQ demodulator from a suitable IF combines the benefits of a conventional super-heterodyne design with the ability to perform the final channel filtering in the digital domain, thereby removing the component variations inherent in a analogue approach. The digital filters can also be adjusted under software control to achieve an optimal response for a particular channel bandwidth.

The CMX990 uses the super-heterodyne and IQ down-converter approach to minimise the component count and allow for digital implementation of the final filtering stages. In addition, it employs DC tracking algorithms to negate any inaccuracies inherent in the IQ down-converter hardware. A further feature of the CMX990 is the use of an “image reject” first mixer, which allows the RF filtering to be simplified. All these features combine to provide an optimal solution to minimise cost and component count while maintaining maximum performance.

The radio environment is subject to a multitude of disturbances and interference, some due to natural causes, some man-made. The likelihood of bit errors increases rapidly with the distance over which the signal has to travel. This is due not only to the reduced signal level seen at the receiver (and hence a reduced signal-to–noise ratio), but also the interference due to multi-path (reflections) and Doppler (if either end of the link is moving). Additional interference can be caused by other radio or electrical equipment in the vicinity, either directly or through mixing together to produce an in-band interferer.

Good radio design can reduce many of these effects. Ideally the data modem should be designed so that a momentary loss of signal will not cause the entire message to be lost causing a re-transmission, as this would adversely affect the overall data rate and increase the power consumption of the devices.

The CMX990 system implements a suite of forward error correction (FEC) and cyclic redundancy codes (CRC) with data scrambling and interleaving to minimise the effects of short-loss signal and so maintain the integrity of the data link.

The primary market demand for M2M solutions of this type is for a low-cost solution without compromising performance. To enable manufacturers to achieve this, high levels of functional integration onto a semiconductor device offer an improvement on previous generations of technology.

The CMX990 combines as much of the RF and signal processing as possible within a small 64-pin VQFN package. With the addition of a host microcontroller, some VCOs, a simple IF filter, and a PA section, it is now easy to implement a minimum cost, low complexity, small footprint radio modem. A block diagram of a radio modem based around the CMX990 is shown in Figure 2.

Combining all the above features into one device ensures that the performance of the final design is optimised for the application and allows the equipment designer to concentrate on the higher level protocols, applications layer, and user interfaces of the final design, which can be significant market differentiators.

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