One of the main problems in digital wireless communication system is the dispersive fading cause by multi-path propagation in transmission channels. Therefore high performance adaptive equalizers play a very important role in modern digital wireless communication systems to combat multi-path fading. Inter symbol interference is one effect of multi-path fading.
In a digital modem, the gain control signal can be derived directly from the received I F signal. This approach however lacks precision, especially in a selective fading environment. To improve the accuracy, Automatic Gain Control circuits are built based on decision-directed algorithms. Thus in order to have a valid and stable control for the Automatic Gain Control AGC circuit, it is required to wait for the synchronization loops (carrier/clock) and also the equalizer to stabilize. This process takes several symbol intervals before all the loops converge and increases the re-synch time.
Standard implementations of Adaptive Time Domain Equalization (ATDE), requires in order to eliminate inter symbol interference (ISI) only to adjust the side coefficients of the ATDE relative to the center tap coefficients. The center coefficients for the through- and cross-sections of the ATDE are normally set to 1 and 0, respectively. These coefficients are fixed and are not dynamic.
By using the present invention, an intermediate frequency (IF) automatic gain control can be used alone with the incorporated AGC in the ATDE circuit. This implementation has the advantage of reducing the re-synch time for the modem as well as capability of correcting I/Q amplitude and phase imbalance.
An object of the invention is an improvement for a method and apparatus for gain control. The method of gain control for a QAM signal in a communication system includes at least one digital modem. The improvement involving using a non-decision aided algorithm at the intermediate frequency level for coarse tuning and using a modified Adaptive Time Domain Equalizer (ATDE) to fine tune the level of the I/Q baseband signals and perform I/Q amplitude and phase imbalance correction along with a channel equalization.
Another object of the invention is a method of reducing re-synch time and I/Q amplitude and phase imbalance correction in a digital modem for a QAM signal. The method involving controlling the signal gain with a non-decision aided automatic gain controller to produce a baseband signal, and tuning the baseband signal in an Adaptive time domain equalizer utilizing non-fixed center tap coefficients.
Yet another object of the invention is an improvement for a method, in a communication system including at least one digital modem, for gain control for a QAM signal. The improvement comprising using a modified Adaptive Time Domain Equalizer (ATDE) to tune the level of the I/Q baseband signals and perform I/Q amplitude and phase imbalance correction along with channel equalization, wherein center tap coefficients are not fixed.
Per one embodiment, the present invention provides, in a communication system including at least one digital modem, a method of processing a quadrature amplitude modulation (QAM) signal, the method comprising coarse tuning inphase/quadrature baseband signals using a non-decision aided algorithm at the intermediate frequency Ievel; and using a modified adaptive time domain equalizer (ATDE) to fine tune the inphase/quadrature baseband signals based on dynamic center tap coefficients having a range and to perform inphase/quadrature amplitude and phase imbalance correction. The center tap coefficients may be updated at least each symbol period. The ATDE may be internal to the digital modem. The coarse tuning may use peak detection. The communication system may be a cable network system, a point-to-point communication system, or a point-to-multipoint communication system. The communication system may operate in the millimeter wavelength spectrum. The communication system may be a time division multiple access system for communicating data in a frame format. The data density within each frame may be dynamically variable. The communication system may be a time-division duplex system. The communication system may be an adaptive time division duplex system. A forward/reverse ratio may be dynamically configurable.
Per one embodiment, the present invention provides a communication system with at least one digital modem for processing quadrature amplitude modulation (QAM) signals, wherein the at least one digital modem comprises an automatic gain controller for coarse tuning a baseband signal; and an adaptive time domain equalizer (ATDE) for fine tuning the baseband signal; wherein the ATDE modifies a dynamic center tap coefficient within a predetermined range. The ATDE may include a lowpass filter/accumulator and a limiter. The automatic gain controller may use peak detection. The communication system may be a cable network system, a point-to-point communication system, or a point-to-multipoint communication system. The communication system may operate in the millimeter wavelength spectrum. The communication system may be a time division multiple access system for communicating data in a frame format. The data density within each frame may be dynamically variable. The communication system may be a time-division duplex system. The communication system may be an adaptive time division duplex system. A forward/reverse ratio may be dynamically configurable.
Per one embodiment, a method of processing a quadrature amplitude modulation (QAM) signal comprises controlling the signal gain with a non-decision aided automatic gain controller to produce a baseband signal; and tuning the baseband signal in an adaptive time domain equalizer utilizing non-fixed center tap coefficients having boundaries.
Per one embodiment, the present invention provides, in a communication system including at least one digital modem, a method of processing a quadrature amplitude modulation (QAM) signal, the method comprising tuning inphase/quadrature baseband signals using a modified adaptive time domain equalizer (ATDE); and performing inphase/quadrature amplitude and phase imbalance correction, wherein center tap coefficients of said ATDE are not fixed.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description.
The modified ATDE system unlike prior solutions uses the center tap coefficients of the equalizer to fine tune the level of the baseboard signal before delivering it to the decision stage. The center tap coefficients are not fixed, rather they are dynamic and are updated as the equalization algorithm updates other coefficients. In order to protect the ATDE from divergence, an expected dynamic range for the fine tuning, the center coefficients are limited to a maximum and minimum value. These maximum and minimum values are predetermined and adjustable by the user. These values can be percentage deviations from the expected value, for example + or −5%. Because of the course tuning by the non-decision directed AGC, the range of center tap coefficients can be limited.
In
The respective output of the tuned I/Q signals are cross added as is known in the art at adders 205, resulting in I′ and Q′ base band signals which proceed to the decision stage.
In
The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. The various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. The present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Number | Name | Date | Kind |
---|---|---|---|
4683578 | Betts et al. | Jul 1987 | A |
4775988 | Chevillat et al. | Oct 1988 | A |
4796279 | Betts et al. | Jan 1989 | A |
4803438 | Mizoguchi | Feb 1989 | A |
4868850 | Kaku et al. | Sep 1989 | A |
4870370 | Hedberg et al. | Sep 1989 | A |
5018166 | Tjahjadi et al. | May 1991 | A |
5083304 | Cahill | Jan 1992 | A |
5119401 | Tsujimoto | Jun 1992 | A |
5241688 | Arora | Aug 1993 | A |
5274670 | Serizawa et al. | Dec 1993 | A |
5483552 | Shimazaki et al. | Jan 1996 | A |
5509030 | Mortensen | Apr 1996 | A |
5838744 | Zheng | Nov 1998 | A |
5870438 | Olafsson | Feb 1999 | A |
5896423 | Okamoto | Apr 1999 | A |
5949821 | Emami et al. | Sep 1999 | A |
5978415 | Kobayashi et al. | Nov 1999 | A |
5999578 | Ha | Dec 1999 | A |
6009132 | Scholtz | Dec 1999 | A |
6049361 | Kim | Apr 2000 | A |
6148046 | Hussein et al. | Nov 2000 | A |
6208288 | Shoucri et al. | Mar 2001 | B1 |
6243577 | Elrefaie et al. | Jun 2001 | B1 |
6442217 | Cochran | Aug 2002 | B1 |
6459687 | Bourlas et al. | Oct 2002 | B1 |
6510150 | Ngo | Jan 2003 | B1 |
6510188 | Isaksen et al. | Jan 2003 | B1 |
6597733 | Pollmann et al. | Jul 2003 | B2 |
6603818 | Dress et al. | Aug 2003 | B1 |
6836184 | Daughtry et al. | Dec 2004 | B1 |
6856655 | Garcia | Feb 2005 | B1 |
6870880 | Tokunaga et al. | Mar 2005 | B2 |
6907048 | Treadaway et al. | Jun 2005 | B1 |
6985523 | Sims et al. | Jan 2006 | B2 |
20010024475 | Kumar | Sep 2001 | A1 |
20020118741 | Tokunaga et al. | Aug 2002 | A1 |
20020137510 | Sims et al. | Sep 2002 | A1 |
Number | Date | Country | |
---|---|---|---|
20040001541 A1 | Jan 2004 | US |