Pulse or digital communications – Spread spectrum
Reexamination Certificate
1998-12-21
2003-09-16
Liu, Shuwang (Department: 2734)
Pulse or digital communications
Spread spectrum
C375S142000, C375S147000, C370S209000, C370S320000
Reexamination Certificate
active
06621850
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to spread-spectrum communications systems. More specifically, the present invention relates to a method and apparatus for receiving orthogonal signals in a spread-spectrum communications system.
BACKGROUND OF THE INVENTION
In a typical wireless communication system, a plurality of mobile stations is accommodated. Typically, more than one mobile station utilizes the wireless communications system at any given time. Such a communications system is sometimes called a multiple access communications system.
Radio frequency (RF) signals are used in multiple access communication systems to carry traffic between the mobile stations and base stations. With the enormous and ever increasing popularity of multiple access communications systems (e.g. cellular phone communications systems), the RF spectrum has become a very scarce resource. As a result, it is more and more important for service providers of multiple access communication systems to efficiently use the RF spectrum allocated to them and to maximize the capacity of the multiple access communications systems to carry traffic.
Many different techniques which allow multiple mobile stations to access a multiple access communications system simultaneously have been utilized such as time division multiple access (TDMA), frequency division multiple access (FDMA) and code division multiple access (CDMA). CDMA utilizes spread-spectrum modulation techniques which have certain advantages over TDMA and FDMA. Many new communications systems employed today utilize CDMA. The use of CDMA in a multiple access communications system is disclosed in U.S. Pat. No. 4,901,307 entitled “Spread-Spectrum Multiple Access Communications System Using Satellite or Terrestrial Repeaters” and issued to Qualcomm Incorporated on February 13, 1993. This patent is incorporated by reference herein in its entirety.
A typical multiple access communications system which utilizes CDMA (a “CDMA Communications System”) has not only a plurality of mobile stations but also a plurality of base stations with which the mobile stations communicate. In addition, a typical CDMA communications system has at least one forward CDMA channel and at least one reverse CDMA channel. Each forward CDMA channel and each reverse CDMA channel is assigned a unique frequency band, none of which overlap with each other. Typically, there is a guard frequency band between the frequency band(s) used by the forward channel(s) and the frequency band(s) used by the reverse channel(s).
Communications from the base stations to the mobile stations are carried in the forward CDMA channel(s). Each forward CDMA channel is composed of a plurality of code channels. The code channels share the frequency band assigned to the respective forward CDMA channel using spread-spectrum modulation techniques. Each mobile station is associated with a unique code channel. (i.e. communications from the base stations to the mobile stations are carried in the respective code channels).
Similarly, communications from the mobile stations to the base stations are carried in the reverse CDMA channel(s). Each reverse CDMA channel is composed of a plurality of code channels, typically called access channels and reverse traffic channels. The code channels share the frequency band assigned to the respective reverse CDMA channel using spread spectrum modulation techniques. Each mobile station is associated with a unique reverse traffic channel. (i.e. communications from the mobile stations to the base stations are carried in the respective reverse traffic channels). The access channels are typically used to page or notify the base stations of outgoing calls.
In CDMA communications systems defined by ANSI Standard J-STD-008or TIA/EIA Standard IS-95A, which are incorporated by reference herein, each frequency band utilized by the forward CDMA channel(s) and the reverse CDMA channel(s) is 1.23 MHZ wide. In addition, each forward CDMA channel and each reverse CDMA channel is further divided into 64 code channels.
In a CDMA communications system, the base stations may be satellites circulating the globe or stations located on the ground (“terrestrial base stations”) or both. At the UHF or higher frequency bands commonly used by CDMA communications systems, a signal from a mobile station commonly arrives at a base station via a plurality of different paths. (i.e. a plurality of signals are commonly received at the base station for each signal sent from the mobile station). Similarly, a signal from a base station directed to a mobile station commonly arrives at the mobile station via a plurality of different paths.
The time it takes (typically called a path delay) for a signal to arrive at its intended destination is commonly different for each path. Moreover, significant phase differences between the signals travelling on different paths may occur especially at the UHF or higher frequency bands. In other words, signals may arrive at a base station from a mobile station (or a mobile station from a base station) from many different directions or paths, each with a different path delay and phase. When signals are received at each base station and each mobile station, destructive summation of the signals may occur with on occasion deep fading. Such multipath fading is common at the UHF or higher frequency bands.
Multipath fading on the signals between the satellites and the mobile stations is not usually as severe as the multipath fading on the signals between the terrestrial base stations and the mobile stations. Since the satellites are normally located in the geosynchronous earth orbit, the distances between mobile stations and any given satellite are relatively the same. In addition, if a mobile station changes location, the distance between the mobile station and a satellite does not change significantly. In contrast, the distances between the mobile stations and the terrestrial base stations can vary quite significantly. One mobile station may be a few hundred feet away from a terrestrial base station and another mobile station may be miles away from the same base station. In addition, if a mobile station changes position, the distance between the mobile station and the terrestrial base station may change significantly. Consequently, the change in the position of the mobile station may change the path delays and phases of all of the signals carried on the respective paths between the mobile station and the terrestrial base station.
In light of the above, signals between the satellites and the mobile stations typically experience fading that is characterized as Rician Fading. In contrast, signals between the terrestrial base stations and the mobile stations typically experience more severe fading that is characterized as Rayleigh Fading. The Rayleigh fading is due, in part, by the signals being reflected from a plurality of objects (e.g. buildings) between the mobile stations and the base stations.
Since a CDMA communications system utilizes a wide band signal in each forward CDMA channel and in each reverse CDMA channel, multipath fading typically only affects a small part of each wide band signal. In other words, CDMA by inherent nature uses a form of frequency diversity to mitigate the deleterious affects of multipath fading.
In addition to frequency diversity, CDMA communications systems commonly use time diversity and space (or path) diversity to mitigate the deleterious affects of multipath fading. Time diversity is commonly employed through the use of repetition, time interleaving and error detection and correction decoding schemes. Space diversity is commonly employed in the reverse CDMA channel(s) through the use simultaneous communication links from each mobile station to a base station employing a plurality of antennas. Each antenna services one of the simultaneous communication links. Space diversity is also commonly employed in the forward CDMA channel(s) and in the reverse CDMA channel(s) by exploiting the unique characteristics of the spread-spectrum signals used by CD
Li Bin
Tong Wen
Liu Shuwang
Nortel Networks Limited
LandOfFree
Block detection receiver does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Block detection receiver, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Block detection receiver will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3045478