Patent Application
20070279495
A system and method are provided for using diversity processing to process signals in a DOCSIS based wireless network. In one embodiment, the system and method performs diversity processing techniques to overcome the effects of multipath fading. These techniques may include, for example but not limited to, selection diversity, maximal ratio combining, and/or other diversity techniques. Implementing these techniques via a cable network allows the technique to take advantage of the inherent distributed aspect of a cable system. Incorporating diversity processing augments the cable system with wireless access points conveniently and without intrusion into the traditional cable infrastructure design. Path length differences from multiple access points in the cable network are absorbed in the processing.
Cable network headend 110 enables subscriber locations to communicate with external networks, such as external network 170. Headend 110 may include or interface to a cable mode termination system (CMTS), a network management station (NMS), converters, and/or other processing components. A NMS may include one or more servers configured to provide dynamic host configuration protocol (DHCP), time of day (ToD), simple network management program (SNMP), and/or other services needed to allow subscriber locations to communicate with headend 110. The CMTS enables devices located at subscriber locations to exchange digital signals with the network.
Access points 160 may be mounted on existing hardware poles. In accordance with some embodiments, access points 160 enable the cable network to be extended by providing wireless access to the cable headend 100. Access points 160 may be equipped with one or more antennas (not illustrated) for wirelessly transmitting and receiving data to and from one or more subscribing devices (such as subscriber 130) in a designated area. According to an exemplary embodiment, a plurality of access points may be placed at spatially diverse locations such that they are far enough apart to de-correlate multi-path inputs. For example, the access points may be selected such that they are tens of wavelengths apart. Such placement, after processing, may result in an increase in signal-to-noise ratio (SNR) available to the receiver to improve detection performance.
Subscriber location 130 may include an antenna 132 for receiving and transmitting wireless signals to and from access points 160 via wireless communication links 150. Subscriber location 130 also includes customer premise equipment, such as cable modem 134 connected to antenna 132. Cable modem 134 may be configured to convert received data in to a format accessible by user device 136.
According to some embodiments, headend 110 may be configured to perform diversity processing on signals received and transmitted to subscriber locations. As such, headend 110 may include or interface to diversity controller 115. Diversity controller 115 may be integrated into a CMTS, or may be a separate controller device.
Diversity controller 115 may be configured to perform one or more diversity techniques on signals received from the plurality of network antennas located at the cable access points. As described above, the antennas may be spatially separated such that the path to and from each antenna may be different, enabling the antennas to de-correlate multi-path inputs. Diversity controller 115 processes the separately received signals using one or more diversity techniques such as, for example but not limited to, selection diversity, maximal ratio combining, equal gain combining, and/or other spatial combining techniques.
Selection diversity refers to the process of choosing, from among a set of received paths, which path is the cleanest and therefore most likely to be detected successfully. When implementing selection diversity, diversity controller 115 monitors the quality of each incoming signal. This may include, for example, determining the signal-to-noise ratio (SNR) associated with each incoming signal. According to some embodiments, SNR may be measured by the CMTS. In other embodiments, access points may be configured to measure CMTS.
Diversity controller 115 may be configured to select the signal having an optimal SNR. As described above, an administrator may determine which signal is optimal, based on parameters such as cost, performance, and/or other parameters. Diversity controller 115 may continuously monitor the SNR, and if the SNR drops below a predefined threshold, diversity controller 115 may switch to another incoming signal path. For example, diversity controller 115 may switch to the signal path previously calculated to have the next most optimal SNR, or the controller may recalculate the SNR for each signal path, again selecting the path having the optimal SNR. By selecting the best signal path, the likelihood that all of the paths at once are below the predetermined threshold decreases exponentially.
In terms of SNR improvement, the average SNR with diversity selection increases relative to the average SNR of a single channel. More particularly, the average SNR can be shown as follows, with M representing the number of diversity branches available:
Avg SNR (selection diversity)=sum(1/1+1/2+1/3+ . . . 1/M)*Avg SNR (one branch)
A numerical example points out the power of this approach. For M=4, or four diverse branches to select from, and a common Rayleigh fading envelope with an average SNR of 20 dB each, there is roughly a 10% probability that an arbitrary threshold 10 dB lower will be crossed. A 10 dB drop, while arbitrary, represents a significant disruption in terms of supporting modulation profiles and coding gain requirements without adding significant costly single-channel, single receiver, margin to the system. For this same case, but with selection diversity included, the likelihood of the threshold being crossed drops to about 0.01%. This is fully three orders of magnitude of improvement for a very modest number of diverse paths, owing to the exponential relationship.
Maximal ratio combining (MRC) uses the fact that the CMTS is receiving more signal energy than any one path alone offers. In maximal ratio combining, each incoming signal is weighted by a weighting factor in order to optimize the signal noise ratio. Specifically, a weighting factor is chosen based on the SNR of each received signal. After weighting has been applied, the signals are combined and the composite signal may be processed according to typical DOCSIS processing techniques.
Applying maximal rate combining to each signal path results in a favorable increase in SNR. The SNR becomes the sum of the SNR's from each signal path: SNR (MRC)=Σi[SNR(one branch)]i. Most significantly, MRC can produce an acceptable, above threshold, SNR even when no individual SNR is good enough. This means that one network not employing the diversity techniques described herein and exposed to a difficult wireless environment may be unable to support communication and therefore services, while another competing network may perform sufficiently under the same wireless channel conditions.
According to one embodiment, a CMTS located at the cable headend includes a diversity controller and is configured to process the incoming signals. As depicted at 220, the CMTS processes each of the plurality of signals to determine a signal-to-noise (SNR) ratio associated with the signal. The CMTS then obtains an optimal signal based on the calculated SNR, as depicted at 230. As described above, determining the optimal signal involves performing one or more diversity processing techniques such as selection diversity and maximal ratio combining. Finding this optimal signal improves reception of data, as the controller is able to receive a signal having a signal to noise ratio higher than without processing.
The processes described in connection with
Those of skill in the art will appreciated that a computer readable medium may carry instructions for a computer to perform a method of processing data signals in a cable data network, the method comprising at least the steps of: receiving a plurality of data signals from a plurality of antennas located at separate locations in the cable data network; determining a signal-to-noise (SNR) ratio associated with each of the received data signals; and obtaining an optimal signal based on the SNR associated with each of the received data signals. The instructions may further include monitoring the SNR associated with each signal to determine if the SNR has changed and switching to another signal if the SNR of the previously selected signals falls below a predefined threshold.
The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. 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 departing from the spirit or scope of the invention. For example, while the invention has been described herein in terms of a DOCSIS cable network, the system and method may also apply to other cable networks as well. Thus, the present invention is not intended to be limited to the embodiments shown herein, but is to be accorded the full scope consistent with the claims.
This application claims the benefit of U.S. Provisional Application Ser. No. 60/793,693 filed on Apr. 20, 2006, entitled “Robust Wireless High-Speed Data Services Across An HFC Infrastructure Using Wired Diversity Techniques,” which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60793693 | Apr 2006 | US |
