Practical aspects of FHSS-based ISM band wireless telemetry

system developmentValentin Vasilevskiy,

PhD Student, OmSTU

Student member, IEEE

Victor Antropov,

R&D Engineer

Radioengineering Department,Omsk State Technical University,

Russia

10th International PhD Workshop on Systems

and Control

Hluboka, Czech Republic

Introduction to centralized wireless telemetry systems (CWTS)

Simplified structure of a wireless telemetry system with a centrally located base station

Applications: burglar alarms, fire alarms, power utility meters, leak detectors, environmental monitoring, temperature control, etc.

Topology: star, consists of a base station and independent numerous telemetry object devices

downlink

uplink

Limitations and advantages of conventional CWTS:

Require license for frequency band usage (costs money/usually suitable for government structures only)

Require high transmitter power to provide good operating distance (15..25 km) (results in big current drain, not for battery powered applications)

Low interference level in a frequency band High speed transmission is possible (the more

bandwidth we have the more bits per second we can transfer)

Two-way operation (high reliability, low response time)

Main idea

We can use Industrial, Scientific and Medical (ISM) band for the CWTS operation to eliminate the need of licensing

International Telecommunication Union ISM bands, Frequency range [Hz]

6.765–6.795 MHz13.553–13.567 MHz26.957–27.283 MHz40.66–40.70 MHz433.05–434.79 MHz902–928 MHz

2.400–2.500 GHz5.725–5.875 GHz24–24.25 GHz61–61.5 GHz122–123 GHz244–246 GHz

Problems

The maximum transmitter power is limited to 10 dBm (10 mW)– Impossible to compensate multipath fading inherent

to single frequency transmission– Impossible to achieve long range with conventional

equipment Very high level of in-band interference

– car alarm remote controls– radiocontrolled toys, etc)

Conventional analogs in 433 MHz Band

Hundreds of meters operating range (AT86RF211 – Atmel, ASTRX1 – AMI Semiconductor, SRWF-501F433 – Tangray infotech, …)

Up to 5 km operating range (XE1205 – Xemics, RC12x0 – Radiocrafts, СС10xx – Chipcon, …)

Example of CWTS ISM band implementation based on conventional devices (Russia, Omsk)

Solution

Frequency-Hopping Spread Spectrum (FHSS)– Eliminates multipath fading– Resistance to narrowband interference

Low speed (50 bit/second)– Raises Bit Energy to Noise Level (Eb/No)

One-way system (no downlink)– Use an advantage of proper base station location,

highly raised base station receiver high-gain antenna and high selectivity A/DSP.

Closest analog

LONTA 202, Patent RU2231458, «Radio system of information reception and processing for centralized security system…»

FHSS System Works in ISM band (433.92 MHz ±0,2%) Transmitter power up to 10 mW Operating range up to 20 km (city service)

Design considerations

Synchronization Data rate Spectrum efficiency Energy efficiency

Synchronization

Wideband receiver (LONTA 202)

FFT as a set of narrowband filters

Data rate

Digital communication system error probability is dependent on Eb/N0 ratio:

where Eb – bit energy, N0 - noise power spectral

density, S – signal power, N – noise power, W – bandwidth, R – data rate.

0

/

/bE S R

N N W

,

Low data rate raises Eb/N0 ratio

Telemetry systems often do not require high data rate (remote sensing, temperature measurements, burglar alarm system, etc).

LONTA 202 FHSS wireless security system uses 50 bps data rate

Spectrum efficiency

The FHSS system performance is strongly dependent on a spectrum efficiency of the signal during one hop.

The less signal bandwidth during a hop the less probability of collisions between different object transmitters.

Shape filters can be used (Gaussian, raised cosine) to reduce signal effective bandwidth.

Low modulation indexes

Amplitude-Frequency Modulation (Lonta-202, Patent RU2231458)

Energy efficiency

Arbitrary envelope modulation type requires linear amplifiers in transmitters

Constant envelope modulation type allows usage of energy effective D-class nonlinear amplifiers

Proposed system

Plurality of transmitters and a wideband quadrature receiver (robust frequency and time synchronization recovery)

One channel of the wideband quadrature receiver

Complex mixer Cascaded Digital Down Converters, Finite Impulse Responce filter

(Overall adjacent channel rejection not less than 90 dB) Simplified Quadrature FM-Discriminator

Spectrum and energy efficiency

Gaussian Frequency Shift Keying (GFSK)

____Patent RU2231458

(LONTA-202),

Datarate = 50 bps

____Proposed system,

Datarate = 50 bps,

GFSK BT=0.5,

Frequency deviation = 25 Hz

Simulation results, BER (Bit Error Rate) dependency on Eb/N0 ratio

Conclusions I

The proposed FHSS-based wireless telemetry system provides robust operation in an ISM band with high operating distance.

This task was achieved by means of:

- choice of spectral effective modulation technique with high level of power efficiency

- providing narrowband highly selective level of filtering

- providing robust algorithm of demodulation

- defining trade-off between baud rate of the system and spectrum width

Conclusions II

Advantages of the proposed system:

- no frequency or symbol synchronization scheme is needed as long as all possible signal positions are used to recover the message

- effective in the ISM band with high level of interference

- improved BER compared to closest analogues

- increased number of the telemetry objects

- FPGA-oriented algorithm

Practical aspects of FHSS-based ISM band wireless telemetry

system developmentValentin Vasilevskiy,

PhD Student, OmSTU

Student member, IEEE

Victor Antropov,

R&D Engineer

Radioengineering Department,Omsk State Technical University,

Russia

10th International PhD Workshop on Systems

and Control

Hluboka, Czech Republic