Pulse or digital communications – Spread spectrum – Frequency hopping
Utility Patent
1998-04-01
2001-01-02
Pham, Chi H. (Department: 2731)
Pulse or digital communications
Spread spectrum
Frequency hopping
C375S326000, C375S334000, C455S226100
Utility Patent
active
06169761
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to radio systems and more particularly to a frequency hopping quadrature amplitude modulation radio system having an improved protocol using frequency shift key modulation for avoiding busy channels.
2. Description of the Prior Art
Government regulatory agencies commonly regulate the radio frequency spectrum by designating certain frequency bands for certain uses. For example, in the United States the Federal Communication Commission (FCC) allocates the frequencies between 5.725 GHz and 5.850 GHz for use as an industrial, scientific, and medical (ISM) band. In order for many users to use the same band and minimize interference between one radio system and another, the FCC requires that all radio transceivers in this band use spread spectrum signals. Two techniques are commonly used for generating such signals—direct sequence using a spreading code or frequency hopping among frequency channels. Well-known examples of direct sequence spread spectrum are the global positioning system (GPS) and code division multiple access (CDMA) cellular phone systems. However, a problem with direct sequence spreading radio systems is that the limited isolation between spreading codes places a limitation on the number of non-interfering signals that can use the same band. On the other hand the use of frequency hopping requires that several issues that are particular to frequency hopping techniques must be resolved.
A frequency hopping radio system must have a protocol for avoiding or recovering from an interfering signal in a particular channel. Existing radios use an 802.11 Ethernet protocol that calls for testing that a frequency channel is clear before transmitting on that channel. When the channel is busy, it is re-tested at random times until it becomes clear. In existing radios, when a channel is busy, the radio dwells on the channel until the time for hopping to the next frequency channel. Such protocol limits the throughput of the radios in a communication system. A further limitation of an Ethernet type protocol is that although the channel may be clear at the transmit end of a link, it may be busy at the receive end.
The transceivers on each end of a frequency hopping radio system must hop in unison. Therefore, time must be synchronized between separated transceivers. Existing systems send time messages from a master transceiver to calibrate time in slave transceivers to match an accurate time in the master. However, maintaining an accurate time clock is expensive and such messages may need to be sent frequently, thereby decreasing throughput.
In order to maximize throughput within an allowable bandwidth, it is necessary to use a high order modulation having multiple bits of information carried by each modulation state. Existing transceivers use a type of modulation known as quadrature amplitude modulation (QAM) having in-phase (I) and quadrature phase (Q) components. However, higher order QAM modulations such as 256 QAM require more effective protocols and more accurate timing in order to acquire the QAM signal. There is a need for resolving these issues that are particular to frequency hop radio systems in order to improve the throughput of these systems without causing or being susceptible to interfering signals.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a spread spectrum frequency hopping communication system, a transceiver, and a method having an improved protocol for avoiding unusable channels by selecting a next channel in a frequency hop sequence before a specified dwell time has elapsed.
Another object of the present invention is to use a simple and robust modulation for exchanging handshake signals on a channel in preparation for using high order QAM modulation for exchanging payload signals on that channel.
Another object of the present invention is to use a differential time for frequency hopping synchronization.
Briefly, in a preferred embodiment, a communication system of the present invention includes two nearly identical frequency hopping 256 QAM transceivers communicating with each other in a radio frequency (RF) range having a low band of 5.727 to 5.768 GHz and a high band of 5.806 to 5.847 GHz. When a transceiver is receiving on the high band it is transmitting on the low band and vice versa. One transceiver is designated as a master and the other as a slave. The master and slave frequency hop in unison using the same frequency hop table including frequency channel pairs of transmit and receive frequencies. When the master is using the transmit frequency of the channel, the slave is using the receive frequency of the channel and vice versa. The channel pairs are arranged so that both the transmit frequencies and the receive frequencies occur in a pseudorandom order. The specified dwell time for exchanging signals on each channel is not longer than four-hundred milliseconds in order to meet a Federal Communications Commission (FCC) regulation for a spread spectrum signal.
A transceiver of a preferred embodiment includes an outdoor unit and an indoor unit. The outdoor unit includes an antenna for radiating and receiving signals, an RF splitter for separating the low band signals from the high band signals in order to isolate the receive signals from the transmit signals and up and downconverters for converting the frequencies of transmit (TX) and receive (RX) intermediate frequency (IF) channel signals to the frequencies of the channels of the RF TX and RX signals, respectively. An RF transmit channel is determined by an IF transmit channel and the RF band; and an RF receive channel is determined by the IF receive channel and the RF band.
The indoor unit includes an intermediate frequency (IF) section, a modem section, and a digital section. The IF section includes a TX IF upconverter coupled to a TX IF downconverter for upconverting a low IF TX signal to the TX IF channel signal; and an RX IF upconverter coupled to a RX IF downconverter for downconverting the RX IF channel signal to a low IF RX signal. A local oscillator frequency shift key (LO/FSK) generator provides a local signal to the TX IF upconverter for generating a handshake signal and upconverting the low IF TX signal to a high IF TX signal. A TX direct digital synthesizer (DDS) provides a local signal to the TX IF downconverter for selecting the TX IF frequency channels and downconverting the high IF TX signal to the TX IF channel signal. An RX DDS provides a local DDS signal to the RX IF upconverter for tuning to the RX IF frequency channels and upconverting the RX IF channel signal to a high IF RX signal. An IF local oscillator provides an LO signal to the RX IF downconverter for downconverting the high IF RX signal to the low IF RX signal. An FSK demodulator non-coherently demodulates an incoming handshake signal and a receive signal strength indication (RSSI) detector measures an RSSI from the high IF RX signal.
The modem section includes a quadrature amplitude (QAM) modulator for generating a digital TX signal having CW or four to two-hundred fifty-six IQ states and an interpolator/DAC for converting the digital TX signal to he low IF TX signal. A digitizer converts the low IF RX signal to a digital RX signal and a QAM demodulator demodulates the digital RX signal having four to two-hundred fifty-six IQ states. A reference oscillator provides a reference frequency to the QAM modulator, the LO/FSK generator, the TX DDS, the RX DDS, and the IF local oscillator.
The digital section includes a processor system including an executable code for directing a microprocessor for controlling the elements of the transceiver. The executable code includes a rendezvous protocol code, an initial frequency calibration protocol code, and a frequency hop protocol code. The rendezvous protocol code establishes initial contact and determines a transit time. The initial frequency calibration protocol code establishes communication on two channels for determining carrier frequency errors. The fr
Marcoccia Roberto R.
Sosnowski George R.
Corrielus Jean B.
Menlo Patent Agency LLC
Pham Chi H.
Wavespan Corporation
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