WMTS Medical Telemetry System

Wireless patient monitoring can offer both clinical and economic benefits. With this in mind, a leading medical device manufacturer commissioned Plextek to undertake the design and development of a high capacity wireless patient monitoring system, which would be used to provide Wireless Medical Telemetry Services (WMTS) in hospitals.

Plextek was required to provide a complete system solution including the detailed development of all RF and baseband electronics. The equipment developed needed to interface directly to the client's existing data collection and display system . Other key requirements of the system included:

• Includes both mobile (patient worn) and bedside monitor units
• Both patient worn and bedside monitors must communicate with a common base unit
• Low BOM cost and long battery life for patient worn monitors
• Tolerance to fading as the patient moves around the hospital
• High capacity - typically 188 simultaneous users (patients)
• Operation in the US WMTS bands

One of the early stages of the system level design was to perform detailed in-hospital propagation measurements. The results of these measurements were used as the basis for path budget calculations, which were used to determine the required performance parameters of the system. Detailed simulation of the complete system, including both circuit performance and environmental parameters, allowed the appropriate design trade-offs to be made, and gave confidence that the system capacity targets could be met.

To achieve the required battery life for the patient worn monitors, it was determined that a ‘transmit only’ approach would be most appropriate. The lack of a receiver helped to reduce the Bill Of Materials (BOM) cost but also removed the ability to synchronise the system for Time Division Duplex (TDD) operation. This resulted in the development of a wideband multi-channel receiver in the base, using Frequency Division Multiple Access (FDMA).

Conventional multi-channel receiver technology, using analogue frequency division at IF, is complex, can be sensitive to performance variation with component tolerance and is expensive to implement. Plextek’s in depth knowledge of Digital Signal Processing (DSP) and previous experience in the design of complex multi-channel receivers led to the implementation of an alternative, all digital solution. The required channel filtering for the wideband receiver, the final downconversion stages and the signal demodulation, was all implemented digitally.

The digital receiver incorporated ADC conversion at IF. The whole 6 MHz WMTS band is downconverted, decimated, and split into four 1.5 MHz sub-bands. Each channel is then filtered, downconverted and decimated, which reduces the data rate that must be handled and therefore reduces the associated processing costs.

Received signal strength monitoring on an individual patient basis is used to implement diversity combining. This combats the effects of fading as the patient moves around the hospital wards and corridors. Following diversity combination and demodulation, the synchronisation block extracts the frame and symbol timing information. Each patient monitor has an associated frequency offset, and the receiver monitors and adjusts the modulation parameters on a frame by frame basis to compensate for this. Forward Error Correction (FEC) and a Cyclic Redundancy Check (CRC) are used to verify each data packet.

Plextek successfully defined and developed the air-interface, the radio and the signal processing technology used on this project. It is now in volume production and successfully deployed in a number of US hospitals. This work provides a good example of our capabilities across a wide range of disciplines, involving a custom solution where standard chipset solutions were not readily available.

If you would like to discuss how Plextek could assist with your system development please contact us