Pulse or digital communications – Systems using alternating or pulsating current – Angle modulation
Reexamination Certificate
1997-12-12
2001-03-27
Chin, Stephen (Department: 2634)
Pulse or digital communications
Systems using alternating or pulsating current
Angle modulation
C375S317000, C375S334000, C329S300000
Reexamination Certificate
active
06208696
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to low power density radio systems. In particular, the invention relates to systems suitable for use in the first hop between a meter transponder and a receiver in an automatic meter reading (AMR) system. The system is also suitable for other applications.
2. The Prior Art
It is difficult to find unused radio channels throughout the world. This is particularly true for low power, short range devices (SRD), which operate under license exemption. In many countries, particularly in the United States, the licensing authorities have encouraged the use of various types of spread spectrum modulation devices to allow for the uncoordinated sharing of channels. The normal methods of reducing the power density, and hence interference potential on any channel is to use either frequency hopping (FH), or direct sequence spread spectrum (DSS).
In frequency hopping, the transmitter skips around a series of randomly selected narrow band channels and spends only a short time on any one frequency. For example, FCC Part 15.247 specifies the use of a minimum of 50 channels, with a maximum time of 0.4 seconds spent on any one channel in a 20 second period. In this way, the average power density on any channel from a given transmitter is reduced to 2% of the power that would be present if it was not channel hopping. The alternative method, DSS, requires that the narrow band carrier be modulated by a high speed pseudo-random sequence, which has the effect of spreading the signal over a wide bandwidth. Again, in FCC Part 15.247, the spreading gain is specified as a minimum of 20 dB with a minimum bandwidth of 500 kHz. Thus, the average power density of a given transmitter is reduced to 1% or less.
AMR is a low power, low cost radio system that allows utility meters to be read from receivers that are hand held or in mobile computers. A radio device, normally referred to as a transponder, is attached to the meter, so that it can either count pulses or access the meter's internal registers. The transponder is then interrogated, i.e., activated by the reader, so as to send the desired information to the reader. The transponder will then be deactivated until the next interrogation sequence. The major problem with this approach is the high cost, both in terms of the components used and power consumed by the receiver in the transponder.
Low power density is particularly significant for AMR since the meter transponder is required to be a low maintenance and reliable device. For electric meters, the power supplied is not a problem. However, for transponders used with water, gas or other non-electric utilities, it is generally necessary for the transponder to be battery powered. This requires that continuous power consumption be kept in the 10-20 &mgr;A range. A low power density operation therefore enables long battery life.
The approach taken by most AMR systems for non-electric meters is to provide a transponder that responds to a wake-up signal and can therefore be powered down for long periods of time. The use of a wake-up signal requires the transponder to have a receiver. This considerably increases the cost and complexity of the transponder. Prior art solutions also use spread spectrum devices to achieve low power density.
SUMMARY OF THE INVENTION
The invention provides a device for reducing the average power density by occupying the target channel for only a very short time. In the proposed system, the transmission will last only 1 mS and occur not more frequently than every 4 seconds. This represents an average power density of only 0.025% as compared with a continuous transmission. In the past, it has been technically difficult to limit transmissions to only 1 mS because of problems in detecting the presence of this signal, locking on to it so that tuning errors are eliminated and allowing the clock and then the data to be recovered. The method of detecting the presence of the required signal and removing the tuning errors is described below.
Contrary to the power saving teachings of the prior art, the present invention provides a low power density transmitter in the transponder that repeatedly transmits short data bursts containing the meter reading data. In this invention, no receiver is needed. In a preferred embodiment, the meter sends its short burst of data every 4 seconds. The average current consumption of the meter radio transponder will be approximately {fraction (1/4000)} of the peak transmitter current. This means that the target battery consumption can be easily achieved. This eliminates the need for a wake-up signal receiver.
By suitable selection of the period between bursts, it can be ensured that at least one burst will be present when the receiver is within range of the transponder. Because the low power density is achieved by very short bursts, it is not necessary to use spread spectrum solutions, further reducing the complexity of the transponder circuitry.
The specific embodiment of the invention described relates to its use in an AMR transponder. However, there are other possible applications for such a low power density radio systems. These include data transmissions from parking meters and traffic information data collection transponders on the roadside. Industrial uses in manufacturing and process control may also be found.
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Haykin, Simon; Communication Systems; 1994; John Wiley & Sons; pp. 697-698.
Chin Stephen
Collard & Roe P.C.
Deppe Betsy L.
Ramar Technology Limited
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