The present invention relates to digital demodulators, and measuring the receive signal strength.
Wireless signals in a wireless transmission system are generally affected by the many variables, including the surrounding environment. A wireless receiver may need to take certain actions based on the strength of the received signal. To indicate the strength of a received signal, a receiver signal strength indicator (RSSI) signal is typically generated from a transceiver of the wireless transmission system.
In a certain class of receivers used in digital communications, the receive signal may be limited, or clipped, in order to perform demodulation of the received signal. One such receiver is disclosed in U.S. Pat. No. 5,197,085 entitled “Radio Receiver”, the entire contents of which are hereby incorporated by reference. The demodulator is also described in “A Single-Chip VHF and UHF Receiver for Radio Paging,” Wilson, J. et al., IEEE Journal of Solid-State Circuits, Vol. 26, No. 12, December 1991, also incorporated herein by reference. A block diagram of the receiver 100 is shown in
Furthermore, many commercially available transceiver devices are self-contained in an integrated circuit package, and do not provide access to signals internal to the receiver or demodulator. Thus, in many cases, it is virtually impossible for a circuit designer to add an external RSSI circuit to a transceiver device that does not already provide one on the integrated circuit.
Consequently, an improvement in generating RSSI measurements is desired.
The present invention provides an improved mechanism for generating a receive signal strength indicator (RSSI) in a baseband frequency shift keyed (FSK) demodulator. In the FSK receivers described herein, the down-converted baseband signals on the inphase (I) and quadrature (Q) channels are limited during the demodulation process. The imperfect limiting, or clipping, of the input I and Q signals results in discontinuities, signal leakage and intermodulation components in the clipped signals, represented as high frequency energy within the processed I and Q signals. By high-pass filtering the clipped signals, this energy is extracted in the form of positive and negative pulses or sinusoid fragments occurring at the transition points of the input I and Q signals. Significantly, the degree of clipping affects the nature of the discontinuity, and hence the amount of energy present in the pulses. Therefore, a measure of the magnitude of the pulses may be used as a measure of the receive signal strength.
In one embodiment, the method comprises the steps of (i) receiving inphase and quadrature baseband signals having a relative phase relationship indicative of data symbols; (ii) limiting the amplitude of the inphase and quadrature baseband signals; (iii) generating inphase pulses and quadrature pulses representative of signal amplitude transitions of the inphase and quadrature baseband signals; (iv) generating relative phase pulses representative of the relative phase between the inphase and quadrature baseband signals; (v) generating a data symbol output signal in response to the relative phase pulses; and (vi)generating a receive signal strength indicator signal proportional to the magnitudes of the inphase pulses and quadrature pulses.
There are various signals that may be operated on to form the receive signal strength indicator signal, including the inphase and quadrature pulses, the relative phase pulses, or even the data symbol output signal, which is generally a square wave, but in many demodulators it also contains a voltage ripple signal proportional to the inphase and quadrature pulses. In the embodiments that operate on the data symbol output signal, the data symbol output signal is preferably high-pass filtered to remove the data symbol information, leaving only the voltage ripple.
To obtain the RSSI signal, the pulses are preferably processed by a non-linear circuit such as a rectifier, a squaring amplifier, or mixer. The output of the non-linear circuit is preferably a direct current signal that may then be low pass filtered to determine a measure of the magnitude of the pulses.
In alternative embodiments, an apparatus for generating a receive signal strength measurement is provided. The apparatus preferably includes a frequency shift keyed demodulator that generates a data symbol output signal in response to relative phase pulses representative of the relative phase relationship between inphase and quadrature baseband signals, and a non-linear circuit for generating a receive signal strength indication signal proportional to the magnitude of the relative phase pulses.
The non-linear circuit may be a voltage rectifier, current rectifier, a current or voltage squaring circuit, a mixer, or other circuit that generates a direct current output signal. In addition, the non-linear circuit may include an analog to digital converter, whose output may be collected and processed by a digital signal processor or a digital circuit that performs peak detection or root-mean-square (RMS) detection. The non-linear circuit may operate on inphase and quadrature pulses generated from a clipping operation on the inphase and quadrature channels, relative phase pulses generated from the I and Q phase pulses, or even a square wave data symbol output signal of the demodulator, wherein the square wave data symbol output signal contains a voltage ripple signal proportional to the magnitude of the relative phase pulses.
These as well as other aspects, advantages, and alternatives will become apparent to those of ordinary skill in the art by reading the following detailed description, with reference where appropriate to the accompanying drawings.
An improved mechanism for generating a receive signal strength indicator (RSSI) in a baseband frequency shift keyed (FSK) demodulator is provided.
The I and Q channel baseband signals are then provided to the demodulator 102, which is depicted in
The inphase phase pulses are mixed, or multiplied by the clipped quadrature channel signal, and the quadrature phase pulse is mixed or multiplied by the clipped inphase channel signal, by multipliers 112, 114, respectively. The multiplication of the phase pulses by the opposite channel generates relative phase pulses. The relative phase pulses at the outputs 102E, 102F, of the mixers 112, 114, respectively, are shown in
It was discovered that the degree of clipping by the limiting amplifiers 104, 106 results in signal artifacts that may be exploited to form a receive signal strength indication signal. In particular, the degree of clipping affects the nature of the discontinuity of the square wave signal at outputs 102A, 102B, and hence the amount of energy present in the phase pulses. Therefore, a measure of the power or magnitude of the phase pulses may be used as a measure of the receive signal strength. Because the peak amplitude of the pulse signal is proportional to the receive signal strength, the peak pulse amplitude may be used as a measure of the magnitude. Similarly, a low pass version of the signal amplitude may be used. In this regard, the “magnitude” of the pulses is meant to describe any voltage, current or power characteristic of the phase pulses that is proportional to the received signal strength.
In one embodiment, the method comprises the steps of (i) receiving inphase and quadrature baseband signals having a relative phase relationship indicative of data symbols; (ii) limiting the amplitude of the inphase and quadrature baseband signals; (iii) generating inphase pulses and quadrature pulses representative of signal amplitude transitions of the inphase and quadrature baseband signals; (iv) generating relative phase pulses representative of the relative phase between the inphase and quadrature baseband signals; (v) generating a data symbol output signal in response to the relative phase pulses; and (vi)generating a receive signal strength indicator (RSSI) signal proportional to the magnitudes of the inphase pulses and quadrature pulses.
There are various signals that may be operated on to form the RSSI signal. The RSSI may be generated from the inphase and quadrature pulses generated by the high-pass filters 108 or 110, or both. In addition, the relative phase pulses generated by mixers 110, 114, or the combined output 102G of the summer 116 may be used. As a further alternative, the data symbol output signal at output 102H may be used. Theoretically, the signal at output 102H is generally a square wave as shown in
One embodiment that operates on the data symbol output signal is shown in
To obtain the RSSI signal, the voltage ripple signal is preferably processed by a non-linear circuit 414 such as a rectifier, a squaring amplifier, or mixer. Either a full-wave or half-wave rectifier may be used. In particular, a Gilbert cell or four quadrant multiplier is preferably used for the non-linear circuit 414. The output of the non-linear circuit 414 is preferably a direct current signal that may then be low pass filtered by low-pass filter 416 to determine a measure of the magnitude of the pulses. In an alternative embodiment, the non-linear circuit may include an analog to digital converter, whose output may be collected and processed by a digital signal processor that may perform squaring, or by a digital circuit such as an accumulator or arithmetic logic unit that performs peak detection and/or numerical averaging.
Two data symbol output signals from demodulator 402 are shown in
In alternative embodiments, an apparatus for generating a receive signal strength measurement is provided. The apparatus preferably includes a frequency shift keyed demodulator that generates a data symbol output signal in response to relative phase pulses representative of the relative phase relationship between inphase and quadrature baseband signals, and a non-linear circuit for generating a receive signal strength indication signal proportional to the magnitude of the relative phase pulses.
The non-linear circuit may be a voltage rectifier, current rectifier, a current or voltage squaring circuit, a mixer, or other circuit that generates a direct current output signal. The non-linear circuit may operate directly on the inphase and quadrature pulses generated from a clipping operation (performed by e.g., limiting amplifiers 104, 106) on the inphase and quadrature channels. Alternatively, the relative phase pulses generated by the multipliers 112, 114 may be used.
An exemplary embodiment of the invention has been described above. Those skilled in the art will appreciate that changes may be made to the embodiment described without departing from the true spirit and scope of the invention as defined by the claims.