Abstract:
A common dual-mode physical layer architecture for a communications receiver is capable of receiving either frequency hopping spread spectrum signals or direct sequence spread spectrum signals. A delta-sigma modulator is configured as an oversampling A/D converter with predetermined quantization noise shaping characteristics. The respective spread spectrum signal is processed by the delta-sigma modulator and then filtered via a decimation filter such as a (sin(x))/x filter to generate a quadrature signal having sufficient resolution to meet industry standards.

Description:
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates generally to radio frequency (RF) signal receivers and, more particularly, to a radio frequency receiver having a common dual-mode physical layer architecture capable of receiving and converting to baseband, both direct sequence spread spectrum signals and frequency hopping spread spectrum signals. 
     (2) Description of the Prior Art 
     The emergence of the Information Age has resulted in the need to connect virtually all computers into some type of network. Traditionally, groups of computers were networked together through direct, hard-wired connections to form local area networks (LAN), which are often connected to additional networks. Significant expense is associated with running cabling in new structures, as well as replacing, repairing or adding cabling to existing structures. Further, it is difficult to predict the preferred placement for receptacles and location of cabling. In order to eliminate the difficulties associated with cabling and to provide more flexibility in configuring and adding to existing local area networks, wireless local area networks (WLAN) have evolved. These wireless local area networks use radio frequency communications to replace the physical transmission medium of cables. Wireless local area networks are capable of using any number of analog and digital modulation techniques and protocols. In an effort to accommodate multiple modulation techniques, transceivers in wireless local area networks may be configured to deal with multiple modulation techniques. 
     One known direct conversion receiver for accommodating multiple modulation techniques uses a sample and hold circuit for subsampling an input signal. The output of the sample and hold circuit is applied to a sigma-delta loop to provide a high-speed, low-resolution data stream that in turn is applied to a decimator that provides a high-precision, low data rate signal having quadrature outputs. The above sigma-delta loop utilizes a bandwidth selection signal to select one of a preselected set of conversion bandwidths. In this way, only those signals which are of importance for receiving the modulated information are processed through the sigma-delta loop. 
     In view of the above, a receiver that is adaptable to multiple protocols and that uses a fixed A/D conversion rate delta-sigma converter is desirable, since such an architecture offers simplicity of design and will generally occupy less area on an integrated circuit than a similar device that uses a sigma-delta converter having a variable bandwidth. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a common receiver architecture for the conversion of RF signals to baseband for both direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS) signals. The receiver employs a low-pass delta-sigma modulator as an A/D converter, thereby taking advantage of the noise shaping properties of the quantization noise associated with the delta-sigma modulator. By utilizing an oversampling A/D converter with noise shaping, such as a delta-sigma modulator, the reduced bandwidth requirements for FHSS mode signals can be used to increase the resolution in the data path, enabling demodulation of Gaussian shaped Frequency Shift Keying (GFSK) waveforms. Any DC offset problems can be eliminated by incorporating a carrier error that is large relative to any deviation associated with a received waveform. In this way, errors contributed by DC components will be minimized. 
     The quantization noise in a low-pass delta-sigma modulator is shaped with lower frequencies containing less quantization noise than higher frequencies. The bandwidth requirements for FHSS are much less than the requirements for DSSS of a common data rate. This reduction in bandwidth in the FHSS waveform allows for an increased number of effective bits out of the delta-sigma modulator by simply placing a low-pass filter on the output of the delta-sigma modulator. Since the bandwidth is larger in the DSSS mode, the bandwidth of the low-pass filter on the output of the delta-sigma modulator can simply be increased to allow more quantization noise to exist at the output of the low-pass filter, thereby decreasing the effective number of bits, but still allowing for the resolution necessary to perform DSSS demodulation. The aforesaid reduction in bandwidth in the FHSS mode allows for the increased resolution necessary to perform FHSS demodulation. 
     The present invention comprises a low-pass delta-sigma modulator that is implemented as an A/D converter. The A/D converter is followed by a low-pass filter having a variable bandwidth. When demodulating DSSS signals, the bandwidth of the low-pass filter is adjusted to that necessary to capture the DSSS signals. When demodulating FHSS signals, the bandwidth of the low-pass filter is reduced to that necessary to capture the FHSS signals. Due to the noise shaping properties of the A/D converter, the low-pass filter effectively eliminates enough high end quantization noise at the output of the A/D converter to provide the resolution necessary for both DSSS and FHSS demodulation. 
     Accordingly, one feature of the present invention includes provision of a common architecture dual-mode physical layer capable of performing both DSSS and FHSS demodulation. 
     Another feature of the present invention includes provision of a receiver that is integrated on a single integrated circuit chip, and that is useable in a wireless local area network to receive both DSSS and FHSS signals. 
    
    
     These and other features of the present invention will become apparent to those skilled in the art after a reading of the following description of the preferred embodiment when considered with the drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a graphical diagram illustrating the spectral density of quantization noise associated with a delta-sigma modulator for a one-half sampling rate frequency. 
     FIG. 1B is a graphical diagram illustrating a comparison between the bandwidth requirement for DSSS demodulation and the bandwidth requirement for FHSS demodulation. 
     FIG. 2 is a simplified schematic diagram illustrating a receiver architecture. 
     FIG. 3A is a simplified schematic diagram illustrating one embodiment of the present invention configured to accommodate demodulation of FHSS mode signals using a narrow bandwidth low-pass filter. 
     FIG. 3B is a graphical diagram illustrating the relationships between the quantization noise and the bandwidth of the low-pass filter for the embodiment shown in FIG.  3 A. 
     FIG. 4A is a simplified schematic diagram illustrating the embodiment of the present invention depicted in FIG. 3A configured to accommodate demodulation of DSSS mode signals using a wide bandwidth low-pass filter. 
     FIG. 4B is a graphical diagram illustrating the relationships between the quantization noise and the bandwidth of the low-pass filter for the embodiment shown in FIG.  4 A. 
     FIG. 5 is a simplified schematic diagram illustrating one embodiment of the present invention suitable for accommodating demodulation of both FHSS and DSSS mode signals. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following descriptions, like reference characters designate like or corresponding parts throughout the several views. Referring now to the drawings in general and to FIG. 1A in particular, it will be understood that the illustrations are for the purpose of describing a preferred embodiment of the invention and are not intended to limit the invention thereto. As shown in FIG. 1A, a graphical diagram illustrates the typical spectral density of quantization noise  10  associated with a low-pass delta-sigma modulator for a particular sampling rate frequency. The quantization noise  10  can be seen to be small or nonexistent at low frequencies and increasingly larger at higher frequencies. The foregoing quantization noise  10  characteristics of low-pass delta-sigma modulators can be advantageous such that a common A/D architecture can be used to demodulate both direct sequence spread spectrum signals and frequency hopping spread spectrum signals. 
     FIG. 1B is a graphical diagram illustrating a comparison between the bandwidth requirement for DSSS demodulation and the bandwidth requirement for FHSS demodulation. A DSSS or direct sequence spread spectrum mode signal  12  has a single sided bandwidth of 11 MHz. During transmission of the DSSS signal  12 , the total bandwidth is therefore 22 MHz. An FHSS or frequency hopping spread spectrum mode signal  14  has a single sided bandwidth of 1 MHz for a given transmission packet and frequency line. The foregoing DSSS signal  12  is always allowed to occupy the same transmission channel whereas the FHSS signal, although still retaining a 1 MHz single sided bandwidth, must change its transmission channel when a new packet is transmitted. With continued reference to FIGS. 1A and 1B, it can be seen that more quantization noise  10  will show up at the output of a delta-sigma modulator when a DSSS signal  12  is passed therethrough than when an FHSS signal  14  is passed therethrough. The wider signal bandwidth of the DSSS signal  12  allows more time for the inherent quantization noise  10  to reach a higher noise level. Thus, the signal to noise ratio can be seen to be smaller for a DSSS signal  12  passing through a low pass delta-sigma modulator than for an FHSS signal  14  passing through the same delta-sigma modulator. 
     In FIG. 2, the radio frequency and intermediate frequency stages of a typical super-heterodyne receiver are shown. Transmitted signals are received at an antenna  1  and passed to a low-noise amplifier  2  into a mixer  3  driven by a local oscillator  4  operating substantially at the carrier frequency. The resultant signal is filtered using a filter  5  and passed through an intermediate frequency amplifier  6  to a complex, quadrature mixer  7  driven by complex local oscillators  8  operating substantially at the intermediate frequency. The complex signals are filtered using filter  9 . The filters  5  and  9  may be surface acoustic wave (SAW) filters. The complex baseband signals are sampled by an analog-to-digital Converter  24  to provide a digitized, complex, baseband signal. 
     FIG. 3A is a simplified schematic diagram illustrating a demodulation system  20  according to one embodiment of the present invention configured to accommodate demodulation of FHSS mode signals  14  using a variable bandwidth low-pass filter  22  such as a (sin(x))/x filter that is well known to those skilled in the art and that is adjusted to have a narrow bandwidth. A description of filters suitable for use with the present invention and employing simpler add and delay operations is provided in reference to Hogenauer, “An Economical Class of Digital Filters for Decimation and Interpolation,”  IEEE Transactions on Acoustics, Speech and Signal Processing , Vol. ASSP-29, No. 2, Apr., 1981 (herein incorporated by reference). The present invention is not so limited, however, and it shall be understood that other types of low-pass filters or decimation filters can also be used as long as the requisite low pass filtering functionality is retained to yield the desired signal-to-noise ratios. The demodulation system  20  has a pair of delta-sigma modulators  24  that each functions as an A/D converter. Each delta-sigma modulator  24  is followed by a variable bandwidth low-pass filter  22  adjusted to have a narrow bandwidth, As described above. The low-pass filter  22  has a bandwidth of 1 MHz to pass the single-sided FHSS mode signal  4  having a 1 MHz single-sided bandwidth. As also stated above, since quantization noise associated with a delta-sigma modulator is shaped with lower frequencies containing less quantization noise than higher frequencies, the effectiveness of the delta-sigma modulator  24  as an A/D can be controlled with a low-pass filter at the output of the delta-sigma modulator  24 . The demodulation system  20  includes use of a dual delta-sigma modulator/dual variable low-pass filter scheme having both in-phase (I)  26  and quadrature (Q)  28  inputs and outputs. When converting the single-sided FHSS mode signal  14  to baseband, the bandwidth of the low-pass filter  22  is decreased to allow accurate sampling of the input signal by significantly eluminating quantization noise associated with the delta-sigma modulator  24 . The demodulator system  20  yields an output signal having a resolution (at least 7-bit) that easily exceeds the signal quality standards necessary to comply with known industry standards. 
     FIG. 3B is a graphical diagram illustrating the relationships between the quantization noise  10  and the bandwidth of the low-pass filter  22  for the FHSS signal demodulator system  20  shown in FIG.  3 A. An 88 MHz sampling rate is seen to produce a filtered response having very little quantization noise  10 . The signal-to-noise ratio generated by the demodulator system  20  is thus sufficient to accommodate FHSS signal processing. 
     FIG. 4A is a simplified schematic diagram illustrating the embodiment of the present invention depicted in FIG. 3A, but that is now configured to accommodate demodulation of DSSS mode signals  12  using the variable low-pass filter  22  adjusted to have a bandwidth sufficiently wide to capture a DSSS mode signal  12  while effectively eliminating substantially that portion of the quantization noise  10  that resides beyond 11 MHz. The variable bandwidth low-pass filter  22  of the DSSS demodulator system  20  is now adjusted such that the low-pass filter  22  has a bandwidth of 11 MHz. Since more quantization noise  10  is passed through the low-pass filter  22  when the bandwidth is adjusted from 1 MHz to 11 MHz, the signal-to-noise ratio between the DSSS mode signal  12  and the quantization noise  10  is reduced from that obtained between the FHSS mode signal  14  and the quantization noise  10  when the low-pass filter  22  is adjusted to have a bandwidth of 1 MHz. The signal-to-noise ratio obtained during processing of DSSS mode signals  12  using the present invention, however, has been found by the present inventors to easily meet industry signal quality standards for DSSS signal processing. The DSSS output signal generated by the demodulator system  20  with a wide bandwidth low-pass filter  22  and using an 88 MHz sampling rate was found to have greatly improved (at least 4-bit) accuracy. 
     FIG. 4B is a graphical diagram illustrating the relationships between the quantization noise  10  and the bandwidth of the variable bandwidth low-pass filter  22  for the embodiment shown in FIG.  4 A. The signal-to-noise ratio between the DSSS mode signal  12  and the quantization noise  10  is now less than that obtained when demodulating an FHSS mode signal  14 . The DSSS mode signal  12  amplitude, however, is sufficiently high that a high resolution output signal having 4-bit resolution is generated via the demodulation system  20 . 
     FIG. 5 is a simplified schematic diagram illustrating one embodiment of the present invention suitable for accommodating demodulation of both DSSS and FHSS mode signals  12 ,  14 . In-phase (I)  26  and quadrature (Q)  28  input signals, either DSSS mode signals  12  or FHSS mode signals  14 , are passed through delta-sigma modulators  24 . The delta-sigma modulators  24 , as discussed above, produce noise during quantization of the input signals  12 ,  14  (quantization noise  10 ). This quantization noise  10  can be characterized as the low frequency response side of a pass-band where the quantization noise is non-existent at DC and increases with increasing frequency to a maximum level at higher frequencies. Because the foregoing FHSS mode signal  14  has a narrow single-sided bandwidth of only 1 MHz, the resultant output signal processed through the variable low-pass filter  22  will see only negligible quantization noise effects. Because the foregoing DSSS mode signal  12  has a wider single-sided bandwidth of 11 MHz, the resultant output signal processed through the variable low-pass filter  22  will see a greater amount of quantization noise  10 ; but due to the effects of low-pass filtering, the signal will still have enough resolution to meet industry quality standards for DSSS mode signals. 
     Certain modifications and improvements will occur to those skilled in the art upon a reading of the foregoing description. By way of example, just as the inventive embodiments disclosed herein describe specific dual-mode physical layer architectures, different architectures are possible with superior performance over classic single protocol architectures known in the art. For example, the present invention could be implemented with higher order delta-sigma modulators as well as multi-bit delta-sigma modulators. Further, the present invention can be constructed using various combinations of the aforesaid elements, as long as the requisite signal processing is present to tailor the quantization noise appearing at the output of the variable bandwidth low-pass filter(s). It should be understood that all such modifications and improvements have been deleted herein for the sake of conciseness and readability but are properly within the scope of the following claims.