Method and apparatus for receiving differential ultra wideband signals

A receiver may receive ultra-wideband, spread-spectrum differential signals. In some embodiments, the receiver may use a variable bandwidth low pass filter. Lowering or raising the bandwidth of the low pass filter may improve performance in particular wideband signal situations.

FIELD OF THE INVENTION

This disclosure relates generally to devices and methods for recovering radio frequency signals that may include ultra wideband signals.

BACKGROUND

With the advent of wireless network systems such as, for example, wireless local area networks, personal area networks and the like, applications that benefit from such wireless technology continue to expand. Typically, in these systems, a digital data stream is modulated and transmitted over a communications channel to one or more receivers. The transmitters and receivers may be in close proximity or spaced apart. The transmitters and receivers that transmit this digital data collectively may comprise a wireless network system.

As applications, and therefore demand, for wireless-networking systems have increased, so has the need to provide for these network systems within the frequency allocations available for their use. As the frequencies available are limited, there is increasing demand for systems that can communicate within the available bandwidth space and yet not interfere overly with other networking systems or be overly sensitive to such interference.

What is needed therefore is a networking communications systems that may have the ability to transmit and receive digital signals within the frequency allocations provided and provide numerous other benefits.

DETAILED DESCRIPTION

Referring now toFIG. 1, an ultra wideband communications system is depicted wherein a first digital device101transmits data to an ultra wideband transmitter103that in turn converts the digital data into radio frequency signals that is transmitted through an antenna105. An ultra wideband signal may, in some embodiments, be a signal that has a bandwidth divided by center frequency greater than approximately 0.25. The frequency spectrum of a transmitted signal may be spread, in some embodiments, by encoding bits with a spectrum-spreading codeword which may be in accordance with regulatory or other standards. In other embodiments, the frequency bandwidth of a transmitted signal is controlled by the width of the transmitted pulse; i.e., the shorter the pulse duration, the wider the bandwidth. An ultra wideband receiver107may be coupled to an antenna109which may receive radio frequency data transmitted by the ultra wideband transmitter103. The ultra wideband receiver107may receive the radio frequency information and converts it into digital data, which may be coupled to a second digital device111.

Digital devices101and111may be any type of digital device that is usefully coupled to an ultra wideband communications system. For example, the digital devices101and111may be desktop computers, printers, network monitors or other digital devices.

Antennas105and109may be any type of antenna that is useful for transmitting and receiving ultra wideband signals. In operation, antenna105may convert voltage signals from ultra wideband transmitter103into radio frequency pulses, while antenna109may receive radio frequency pulses and convert them into a corresponding voltage signals. In some embodiments, the radio frequency pulses may be both positive and negative (“differential”) pulses. Antennas105and109may be constructed as ground plane antennas, dipole antennas, slot antennas or other type of antenna that may be usefully employed. Additionally, while antenna105is shown as a transmitter antenna, in some embodiments, antennas105and109may be useful for both receiving and transmitting.

Referring now toFIG. 2, a block diagram of a receiving device such as may be incorporated into an ultra wideband receiver such as ultra wideband receiver107is illustrated. Antenna109may be coupled to a filter203. Filter203, in some embodiments, may be a band pass filter to reject out of band signals that may interfere with the desired ultra wideband signal. In other embodiments, filter203may be a matched filter, which may be utilized to match the incoming waveform or a notch filter to reject inband interferers.

A low noise amplifier (LNA)205may be coupled to the filter203and to an automatic gain control circuit (AGC)207. The low noise amplifier205may amplify the received ultra wideband signal from the filter. Collectively, the filter203and LNA205may be considered a radio frequency (“RF”) front end. The automatic gain control circuit207may serve to amplify or reduce an ultra wideband signal coupled from the low noise amplifier205to achieve a desired signal level. The AGC207may be utilized to compensate for dynamic fluctuations in the average amplitude of the received ultra wideband signal. In some embodiments, the AGC circuit207may provide an output signal that may be optimized for the dynamic range of an analog to digital converter (ADC)209.

A differential correlator211may be coupled to the output of the AGC circuit207and to the input of a low pass filter or integrator217. The differential correlator211may include a wideband delay element213and a mixer element215. The delay element213may serve to delay an incoming RF waveform from an ultra wideband signal in an amount, in some embodiments, approximately equal to the symbol period. This delayed waveform may be then mixed by mixer215with a current received waveform to provide a mixed signal, which is coupled to low pass filter217.

Low pass filter217may provide a filtering-integration function on the incoming signal from correlator211. This filter may be either a fixed bandwidth filter or, in some embodiments, a variable bandwidth filter. The output of the low pass filter217may be coupled to the analog to digital converter209. The analog to digital converter209may be coupled to a digital demodulator221and a sampling clock219. The ADC209, digital demodulator221, and sampling clock219may be utilized to convert the received ultra wideband signal into digital data.

In some embodiments, receiver201may serve as a differential pulse shift-keying receiver that demodulates sequences of short pulses, each of which represents a single bit. In this case, it should be noted that the ultra-wideband signals are essentially pulses that may be transmitted and detected by transmitters and receivers such as transmitter103and receiver107respectively.

Alternatively, receiver201may serve as a differential pulse shift-keying receiver that demodulates spread spectrum signals where the spreading sequence is illustrated inFIG. 3. As an example, the spreading of the waveform may be accomplished using four chips per symbol.

Referring now toFIG. 3, the operation of the differential correlator211and low pass filter/integrator217is detailed according to embodiments of the present invention. In this illustration, (Ts) represents the symbol period, (St) is the received ultra wideband signal that may be at the input of differential correlator211, S(t-T) is the signal present at the mixer215from the output of the wideband delay element213. When utilized in a spread spectrum differential signal application, the differential correlator211, in some embodiments, may function by mixing signal s(t)301with the delayed signal s(t-T)302to generate a mix signal303. The mix signal303may be processed by a low pass filter/integrator213to produce an integrated waveform305. As can be seen in waveform305in comparison with waveform303, the peaks in waveform303have been averaged out in waveform305. The receiver section201may accomplish a despreading of the spread spectrum ultra wideband signal and may allow the analog to digital converter209to sample at the input bandwidth of the symbol rate Ts. In this example four (4) pulses per time slot may be utilized. However, in an ultra wide band differential signal application, in some embodiments, a single pulse may be utilized per time slot.

As was previously discussed, low pass filter/integrator217may be either a fixed bandwidth filter, or in some embodiments, a variable bandwidth filter. In some embodiments, the band pass characteristics of the low pass filter/integrator217may be adjusted dynamically to achieve the best signal-to-noise ratio of the received ultra wideband signal. Since the pulse repetition frequency (Bp) may be, in some embodiments, much less than that of the actual signal bandwidth (W), the low pass filter/integrator bandwidth (Bf) or integration time (Tp) may vary between Bp<Bf (or 1/Tp)<W. This offers an opportunity to vary the complexity and performance of the ultra wideband receiver.

An optimum bandwidth of the low pass filter/integrator may not necessarily, in some embodiments, be equal to W. Ultra wideband signals may be viewed as waveforms that occupy a much greater bandwidth than the symbol rate. A low pass filter/integrator with high bandwidth may perform better than a low pass filter/integrator with low bandwidth depending on the environment in which the ultra wideband signal is transmitted. For example, in an environment with few signal reflections, a high bandwidth low pass filter/integrator may out perform a low bandwidth low pass filter/integrator. This may be because the low bandwidth low pass filter/integrator may remove much of the wide-band signal energy at the output of the differential correlator211.

However, in an ultra wideband environment with significant multi path reflections, a low bandwidth low pass filter/integrator may out perform a higher bandwidth low pass filter. This may be because the low bandwidth low pass filter/integrator may capture and sum a portion of the energy from the additional paths of the multi path reflections while a high bandwidth low pass filter may only capture the energy from one of the paths. Thus, changing the bandwidth of a low pass filter/integrator either higher or lower may improve performance in particular ultra wideband signal situations.

Referring now toFIG. 4, a receiver section as in201is illustrated with the addition of a bit error detector401. The bit error detector401may be coupled to the digital demodulator221and to the low pass filter/integrator217. In some embodiments, bit error detector401may adjust the bandwidth of the low pass filter/integrator217in response to bit error rates detected. Based in part on the bit error rate, the bit error detector401may either increase or decrease the bandwidth of the low pass filter/integrator217to reduce the detected error rate. The low pass filter/integrator217may have discrete bandwidth settings, which may be selected, in some embodiments, by bit error detector401. In other embodiments, low pass filter/integrator217may be generally variable between an upper and lower bandwidth limit and the bit error detector401controls the bandwidth of the low pass filter/integrator within those parameters. In other embodiments, the bit error detector401may be replaced by a digital or analog symbol error detector.

Acquisition and synchronization of ultra wideband signals may be challenging due to the short impulse nature of the waveform. With a low pass filter bandwidth that is less than half the sample rate (Rs) of the ADC (i.e., Bf<Rs/2), the digital demodulator may perform direct digital matched filtering in order to detect packets with a single preamble sequence. For example, a 100 MHz PRF (Bp) may be detected with a 200 MHz ADC and associated digital demodulator and allow for a packet-by-packet demodulation. A relatively low sample rate may provide a reduction in a power consumed by the receiver107.

In one embodiment, a relatively low sample rate may result in reduced power consumed by receiver107. When receiving an ultra wideband signal with a fixed bandwidth low pass filter/integrator, the bandwidth, in some embodiments, may be between around Bp to considerably higher Bp. However, a variable-bandwidth low pass filter/integrator may allow for improved receiver performance in different signal environments as previously discussed. Additionally, in some embodiments, the wideband delay element213may be adjusted to change the delay length based on the received data rate. In this manner, in some embodiments, an increase or decrease in the data rate may be accommodated again leading, in some embodiments, to improved performance. For example, in environments with little or no multipath signal reflections, a higher data rate may be supported by the receiver107.

Referring now toFIG. 5, an ultra wideband differential pulse transmitter103may include a digital block501that may include a preamble generator, a clocking control and a differential encoder. The digital block may be coupled to a bi-polar modulator and pulse generator503and to a clock circuit505. The modulator503may be coupled to a band pass or high pass filter507that may be coupled to the antenna105. As was described in association withFIG. 1, the transmitter103may encode digital data into ultra wideband differential pulses.