Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1999-02-10
2002-11-19
Vincent, David (Department: 2732)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S441000, C370S474000
Reexamination Certificate
active
06483828
ABSTRACT:
DESCRIPTION
1. Technical Field of the Invention
The invention relates to the field of telecommunications and more particularly, to a system and method for providing variable user information rates and for improving performance and bandwidth utilization in telecommunications systems using orthogonal codes for error control.
2. Description of Related Art
The cellular telephone industry has made phenomenal strides in commercial operations throughout the world. Growth in major metropolitan areas has far exceeded expectations and is outstripping system capacity. If this trend continues, the effects of rapid growth will soon reach even the smallest markets. The predominant problem with respect to continued growth is that the customer base is expanding while the amount of electromagnetic spectrum allocated to cellular service providers for use in carrying radio frequency communications remains limited. Innovative solutions are required to meet these increasing capacity needs in the limited available spectrum as well as to maintain high quality service and avoid rising prices.
Currently, channel access is primarily achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. In FDMA systems, a physical communication channel comprises a single radio frequency band into which the transmission power of a signal is concentrated. In TDMA systems, a physical communications channel comprises a time slot in a periodic train of time intervals transmitted over the same radio frequency. Usual methods of implementing a TDMA system incorporate FDMA as well.
Spread spectrum comprises a communications technique that is finding commercial application as a new channel access method in wireless communications. Rudimentary spread spectrum systems have been around since the days of World War II. Early applications were predominantly military-oriented (relating to smart jamming and radar). However, there is an increasing interest today in using spread spectrum systems in communications applications, including digital cellular radio, land mobile radio, and indoor/outdoor personal communication networks.
Spread spectrum operates quite differently from conventional TDMA and FDMA communications systems. In a direct sequence code division multiple access (DS-CDMA) spread spectrum transmitter, for example, a digital symbol stream for a given dedicated or common channel at a basic symbol rate is spread to a “chip” rate. This spreading operation involves applying a spreading code that is channel-unique (sometimes referred to as a “signature sequence”) to the symbol stream that increases its transmission rate (as well as the bandwidth requirement) while adding redundancy. Typically, the digital symbol stream is multiplied by the unique digital code during spreading. The intermediate signal comprising the resulting data sequences (chips) is then summed with other similarly processed (i.e., spread) intermediate signals relating to other (dedicated or common) channels.
A scrambling code that is unique to a base station (often referred to as a “long code” since in most cases it is longer than the spreading code) is then applied to the summed intermediate signals to generate an output signal for multi-channel transmission over a communications medium. The intermediate signals derived from the various dedicated or common channels thus advantageously share one transmission communications frequency band, with the multiple signals appearing to be located on top of each other in both the frequency domain as well as the time domain. Because the applied spreading codes are unique to each channel, however, each intermediate signal that is transmitted over the shared communications frequency is similarly unique, and may be distinguished from others through the application of proper processing techniques at the receiver end.
In the DS-CDMA spread spectrum mobile station (receiver), the received signals are recovered by applying (i.e., multiplying, or matching) the appropriate scrambling and spreading codes to despread, or remove the coding from the desired transmitted signal and return to the basic symbol rate. Where the spreading code is applied to other transmitted and received intermediate signals, however, only noise is produced. The despreading operation thus effectively comprises a correlation process comparing the received signal with the appropriate digital code to recover the desired information from the channel.
Orthogonal codes or near-orthogonal codes (i.e., codes having relative high relative auto-correlation and low relative cross-correlation values) are used for error control in telecommunications systems. Walsh codes are one example of an orthogonal code. In a coding scheme using Walsh codes, a k-bit information word (“infoword”) is converted into a 2
k
-bit sequence using the Hadamard transform. Such a conversion will be referred to in this patent application as a (2
k
,k) orthogonal code. A Walsh code for encoding a 2-bit information sequence to a 4-bit orthogonal codeword is shown below:
H
4
=
&LeftBracketingBar;
1
1
1
1
-
1
1
-
1
1
1
1
-
1
-
1
1
-
1
-
1
1
&RightBracketingBar;
Here, the four rows of the matrix H
4
form the code words for the four information symbol sequences whose binary values are equal to the numeric values of the four row indices. For example, the information sequences (0,0), (0,1), (1,0) and (1,1) would be mapped to the codewords (1,1,1,1), (1,−1,1,−1), (1,1,−1,−1) and (1,−1,−1,1) respectively. The Fast Hadamard Transform is then used to demodulate the incoming signal non-coherently. The Fast Hadamard Transform acts as a correlator, and those code-word(s) (or component(s) thereof) that have the highest correlation values can then be identified as the transmitted code-word(s). The uniqueness of the mapping between the 2-bit information sequences and the 4-symbol code words (in the example above) allows the unambiguous detection and decoding of the transmitted symbols.
The use of the Walsh-Hadamard codes described above result in a bandwidth expansion of 2
k
/k (i.e., a k-bit information word expands to a 2
k
-bit code word) which is quite high. The inverse of the bandwidth expansion is referred to as the coding rate. Consequently, these Walsh-Hadamard codes are normally used in very low signal-to-noise ratio environments. For example, the IS-95 CDMA system compensates the low signal-to-interference ratios in the system through the use of such codes. Another application for such schemes is in random access protocols for cellular and mobile satellite systems, where users may be trying to initially synchronize to the system, or attempting to originate a call or answer an incoming call.
It has been found desirable to find new techniques to improve the coding rate in a telecommunications system employing orthogonal codes. It has further been found desirable to find techniques for reducing the bandwidth expansion. An ideal telecommunications system employing orthogonal codes would thus have only that bandwidth expansion that is necessary to separate the multiple transmissions from various users without reducing the efficiency of spectrum utilization by too large a factor.
SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to provide improved performance in telecommunications systems using orthogonal codes for error control. It is a further object of the present invention to reduce bandwidth expansion in such telecommunications systems. It is also an object of the present invention to improve the coding rate in a telecommunications system employing orthogonal codes.
Orthogonal codes or near-orthogonal codes (i.e., codes having high relative auto-correlation and low relative cross-correlation values) are used for error control in communication systems. For example, the IS-95 CDMA cellular standard uses a (64,6) orthogonal Walsh code on the reverse link (i.e., the link from the mobile station to the base station) for error control. As noted earlier, such a set of Walsh codes converts
Balachandran Kumar
Chennakeshu Sandeep
Hassan Amer
Koorapaty Havish
Ramesh Rajaram
Ericsson Inc.
Moore & Van Allen PLLC
Stephens Gregory
Vincent David
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