Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1999-07-30
2003-05-06
Phu, Phuong (Department: 2631)
Pulse or digital communications
Receivers
Interference or noise reduction
C375S232000, C375S233000
Reexamination Certificate
active
06560299
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to diversity receivers.
BACKGROUND OF THE INVENTION
Conventional television signals transmit video information in analog form by modulating the amplitude and frequency of a carrier signal. Digital television systems convert an analog video signal into digital information, which is transmitted by pulse modulating the amplitude and phase of a carrier signal. For example, in broadcasting high definition television (HDTV), digital information is transmitted by an 8 level amplitude modulation technique, known as 8 VSB (vestigial sideband). In 8 VSB, modulation of a carrier to one of 8 levels (i.e., one of 8 symbols) defines 3 bits of digital information for each symbol clock interval. While analog television signals degrade gracefully in the presence of interference, digital broadcast systems can fail completely when the bit error rate overcomes the error tolerance of the system. Bit errors result from weak signals, noisy signals or signals subject to fading and distortion.
In an ideal radio wave propagation environment, there exists an unobstructed line-of-sight path between the transmitting and receiving antennas. Additionally, no other objects exist which may reflect the transmitted wave along another path to the receiving antenna. As is more often the case, however, there is no direct line of sight between the antennas. In an outdoor environment, natural or artificial obstructions, such as buildings, hills, and trees block the direct line of sight. Furthermore, these obstructions reflect the transmitted signal such that multiple versions with varying amplitudes, phases and time delays are simultaneously received.
The indoor environment is even more complicated since there is rarely an unobstructed path between the transmitter and receiver antennas. Furthermore, objects causing signal reflection and absorbtion are numerous and are often in motion.
Reception of the transmitted signal along multiple paths from the transmitter to the receiver causes signal distortion which manifests itself in a variety of ways. The different paths have different delays that cause replicas of the same signal to arrive at different times (like an echo) and sum at the receiver antenna, causing inter-symbol interference. The phases of these multipath signals may combine constructively or destructively resulting in large range of possible signal strengths. Additionally, this signal strength may vary with time or antenna location, and is known as signal fading. Signal fading can range from frequency selective fades to a flat fade over the entire frequency spectrum of interest. Indoor signals typically have severe multi-path distortions and are changing rapidly in time, ranging from flat fades to deep in-band nulls to relatively unimpaired signals. Conventional indoor TV antennas and outdoor antennas used with existing 8 VSB receivers often do not produce reliable and uninterrupted reception of digital broadcast HDTV signals.
To mitigate the adverse effects of multiple path (multipath) distortion and signal fading, it is known to use a diversity receiver. In a diversity receiver, two (or more) independent antennas are used to receive two (or more) separate versions of the same signal. Each independent antenna provides a signal with different (ideally, uncorrelated) noise, fading and multi-path factors. These different signals may be obtained through various forms of diversity including spatial, temporal, polarization and direction-of-arrival diversity.
A diversity receiver has a plurality of receiver channels to process the plurality of antenna signals. By appropriate combining of the information extracted from each signal by each channel of the diversity receiver, a diversity receiver provides equal or superior performance compared to a non-diversity receiver operating on the “best” of the received signals alone.
In the prior art, the received signals in each of the individual receiver channels of a diversity receiver are processed separately and then combined. That is, each of the multiple receiver channels in a prior art diversity receiver is an independent receiver. Each of the respective diversity antenna signals is processed in one of the independent receiver channels of the diversity receiver.
Each receiver channel is independent in the sense that each includes a respective separate tuner, front-end function (such as for baud clock recovery and carrier recovery) and separate equalizer filter. The separate receiver channels of the prior art provide separate signal outputs, which are then combined in some manner.
In one prior art approach, the output of the separate diversity receiver channel having the stronger input signal (i.e., the higher signal to noise ratio) is selected over the output of the diversity receiver channel having the weaker input signal. In a second prior art approach, the outputs of the two diversity receiver channels are combined equally, regardless of input signal strength. In a third prior art approach, the outputs of the two receiver channels are combined in a maximal ratio combiner in accordance with the respective signal to noise ratio of each signal. In a maximal ratio combiner, the receiver channel with the highest signal to noise ratio provides the greatest contribution to the final output. In general, a prior art diversity receiver processes the received diversity signals in separate receiver channels with regard to receiver functions such as tuning, automatic gain control (AGC), baud clock recovery, RF carrier recovery, and forward equalization.
SUMMARY OF THE INVENTION
In one embodiment of a diversity antenna system for use in conjunction with the present invention, first and second identical antennas are separated, but oriented identically within a plane. The first and second antennas are separated by several wavelengths to provide respective first and second RF signals; The first and second RF signals are said to be reception by the use of spatial diversity since it is known that the multipath propagation channels (in a parameterized sense) from the transmitter to the first antenna and from the transmitter to the second antenna are nearly uncorrelated.
The present invention is embodied in a multiple channel diversity receiver with joint signal processing. In particular, the first and second RF signals are processed jointly in a multiple channel diversity receiver with respect to tuning, automatic gain control (AGC), baud clock recovery, RF carrier recovery and forward equalization. Joint processing, as compared to independent processing of the prior art, means that the multiple channels of the diversity receiver are linked or cross coupled to each other through various joint processing circuitry.
TUNING
In accordance with the present invention, the first and second RF signals are processed jointly in the multiple channel diversity receiver with respect to tuning. In particular, the first and second tuners in the first and second channels of the multiple channel diversity receiver share at least one joint local oscillator. In the case of a dual conversion tuner, first and second joint local oscillators are shared. A first joint local oscillator is shared in the RF stage of the first and second tuners, and a second joint local oscillator is shared in the IF stage of the first and second tuners. By sharing one or more joint local oscillators in separate tuners, the first and second channels of the multiple channel diversity receivers will therefore be coherent in frequency and phase and thus have common phase noise characteristics.
AUTOMATIC GAIN CONTROL (AGC)
In accordance with the present invention, the first and second RF signals are processed jointly in the multiple channel diversity receiver with respect to AGC. In particular, the respective tuners in the first and second channels of the multiple channel diversity receiver share at least one joint AGC loop. The maximum difference between the AGC feedback signal in the control loop for the first channel and the AGC feedback control signal in the control loop for
Endres Thomas J
Hulyalkar Samir N
Schaffer Troy A
Shah Anand M
Strolle Christopher H
Jacobson Allan
Phu Phuong
LandOfFree
Diversity receiver with joint signal processing does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Diversity receiver with joint signal processing, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Diversity receiver with joint signal processing will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3015456