Pulse or digital communications – Spread spectrum
Reexamination Certificate
2001-01-05
2004-05-04
Corrielus, Jean B. (Department: 2631)
Pulse or digital communications
Spread spectrum
C370S209000, C375S267000, C375S299000
Reexamination Certificate
active
06731668
ABSTRACT:
BACKGROUND
1. Field of the Invention
The present invention generally relates to the field of wireless communication systems. More specifically, the invention relates to transmission for wideband code division multiple access communication systems using multiple input multiple output channels.
2. Related Art
In wireless communication systems several users share a common communication channel. To avoid conflicts arising from several users transmitting information over the communication channel at the same time, some regulation on allocating the available channel capacity to the users is required. Regulation of user access to the communication channel is achieved by various forms of multiple access protocols. One form of protocol is known as code division multiple access (CDMA). In addition to providing multiple access allocation to a channel of limited capacity, a protocol can serve other functions, for example, providing isolation of users from each other, i.e. limiting interference between users, and providing security by making interception and decoding difficult for a non-intended receiver, also referred to as low probability of intercept.
In CDMA systems each signal is separated from those of other users by coding the signal. Each user uniquely encodes its information signal into a transmission signal. The intended receiver, knowing the code sequences of the user, can decode the transmission signal to receive the information. The encoding of the information signal spreads its spectrum so that the bandwidth of the encoded transmission signal is much greater than the original bandwidth of the information signal. For this reason CDMA is also referred to as “spread spectrum” modulation or coding. The energy of each user's signal is spread across the channel bandwidth so that each user's signal appears as noise to the other users. So long as the decoding process can achieve an adequate signal to noise ratio, i.e. separation of the desired user's signal from the “noise” of the other users' signals, the information in the signal can be recovered. Other factors which affect information recovery of the user's signal are different conditions in the environment for each subscriber, such as fading due to shadowing and multipath. Briefly, shadowing is interference caused by a physical object interrupting the signal transmission path between the transmitter and receiver, for example, a large building. Multipath is a signal distortion which occurs as a result of the signal traversing multiple paths of different lengths and arriving at the receiver at different times. Multipath is also referred to as “time dispersion” of the communication channel. Multipath fading may also vary with time. For example, in a communication unit being carried in a moving car, the amount of multipath fading can vary rapidly.
A number of methods have been implemented to provide effective coding and decoding of spread spectrum signals. The methods include error detection and correction codes, and convolutional codes. In wireless communications, especially in voice communications, it is desirable to provide communication between two users in both directions simultaneously, referred to as duplexing or full-duplexing. One method used to provide duplexing with CDMA is frequency division duplexing. In frequency division duplexing, one frequency band is used for communication from a base station to a mobile user, called the forward channel, and another frequency band is used for communication from the mobile user to the base station, called the reverse channel. A forward channel may also be referred to as a downlink channel, and a reverse channel may also be referred to as an uplink channel. Specific implementation of coding and modulation may differ between forward and reverse channels.
The information in the user's signal in the form of digital data is coded to protect it from errors. Errors may arise, for example, as a result of the effects of time-varying multipath fading, as discussed above. The coding protects the digital data from errors by introducing redundancy into the information signal. Codes used to detect errors are called error detection codes, and codes which are capable of detecting and correcting errors are called error correction codes. Two basic types of error detection and correction codes are block codes and convolutional codes.
Convolutional codes operate by mapping a continuous information sequence of bits from the digital information of the user's signal into a continuous encoded sequence of bits for transmission. By way of contrast, convolutional codes are different from block codes in that information sequences are not first grouped into distinct blocks and encoded. A convolutional code is generated by passing the information sequence through a shift register. The shift register contains, in general, N stages with k bits in each stage and n function generators. The information sequence is shifted through the N stages k bits at a time, and for each k bits of the information sequence the n function generators produce n bits of the encoded sequence. The rate of the code is defined as R=k
, and is equal to the input rate of user information being coded divided by the output rate of coded information being transmitted. The number N is called the constraint length of the code; complexity—or computing cost—of the code increases exponentially with the constraint length. A convolutional code of constraint length 9 and code rate 3/4, for example, is used in some CDMA systems.
The highly structured nature of the mapping of the continuous information sequence of bits into continuous encoded sequence of bits enables the use of decoding algorithms for convolutional codes which are considerably different from those used for block codes. The coding performed by a particular convolutional code can be represented in various ways. For example, the coding may be represented by generator polynomials, logic tables, state diagrams, or trellis diagrams. If the coding is represented by a trellis diagram, for example, the particular trellis diagram representation will depend on the particular convolutional code being represented. The trellis diagram representation depends on the convolutional code in such a way that decoding of the encoded sequence can be performed if the trellis diagram representation is known.
For signal transmission, convolutional coding may be combined with modulation in a technique referred to as “trellis coded modulation”. Trellis coded modulation integrates the convolutional coding with signal modulation in such a way that the increased benefit of coding more than offsets the additional cost of modulating the more complex signal. One way to compare different methods of signal transmission is to compare the bandwidth efficiency. Bandwidth efficiency is typically measured by comparing the amount of information transmitted for a given bandwidth, referred to as the “normalized data rate”, to the SNR per bit. The maximum normalized data rate that can possibly be achieved for a given SNR per bit is the theoretical maximum capacity of the channel, referred to as the “Shannon capacity” of the channel. The more bandwidth efficient a method of signal transmission is, the more nearly it is able to use the full Shannon capacity of the channel. A channel with multiple transmit antennas and multiple receive antennas which uses all possible signal paths between each pair of transmit and receive antennas, referred to as a multiple input multiple output (“MIMO”) channel, is known to have a higher Shannon capacity under certain channel conditions than a similar channel which uses only one transmit-receive antenna pair.
For signal reception, the signal must be demodulated and decoded. There are many methods of decoding convolutional codes, also referred to as “detection.” One method of decoding convolutional codes that uses the trellis diagram representation is Viterbi decoding. In the trellis diagram, each path through the trellis corresponds to a possible encoded sequ
Corrielus Jean B.
Nguyen Thien T.
Qualcomm Incorporated
Rouse Thomas R.
Wadsworth Philip
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