Multiple access coding for radio communications

• US 5,742,678 A
• Filed: 05/09/1996
• Issued: 04/21/1998
• Est. Priority Date: 04/10/1992
• Status: Expired due to Term

• Current Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Original Assignee: Ericsson, Inc. (Telefonaktiebolaget LM Ericsson)
• Inventors: Bottomley, Gregory E., Dent, Paul W.
• Primary Examiner(s): CANGIALOSI, SALVATORE A
• Application Number:
  US08/647,187
• Time in Patent Office: 712 Days
• Field of Search: 380/6, 380/29, 380/28, 380/33, 380/34, 380/37, 380/42, 375/205, 342/352
• US Class Current: 380/270
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04J 13/00 Code division multiplex systems for frequency hopping H04B1/713
• CPC Class Codes: H04J 13/004 Orthogonal
• CPC Class Codes: H04J 13/0048 Walsh
• CPC Class Codes: H04J 13/12 Generation of orthogonal codes
• CPC Class Codes: H04J 13/16 Code allocation
• CPC Class Codes: H04L 1/0057 Block codes H04L1/0061, H04L1/0064 take precedence
• CPC Class Codes: H04L 2209/04 Masking or blinding
• CPC Class Codes: H04L 2209/34 Encoding or coding, e.g. Huffman coding or error correction

Abstract

Individual information signals encoded with a common block error-correction code are assigned a unique scrambling mask, or signature sequence, taken from a set of scrambling masks having selected correlation properties. The set of scrambling masks is selected such that the correlation between the modulo-2 sum of two masks with any codeword in the block code is a constant magnitude, independent of the mask set and the individual masks being compared. In one embodiment, when any two masks are summed using modulo-2 arithmetic, the Walsh transformation of that sum results in a maximally flat Walsh spectrum. For cellular radio telephone systems using subtractive CDMA demodulation techniques, a two-tier ciphering system ensures security at the cellular system level by using a pseudorandomly generated code key to select one of the scrambling masks common to all of the mobile stations in a particular cell. Also, privacy at the individual mobile subscriber level is ensured by using a pseudorandomly generated ciphering key to encipher individual information signals before the scrambling operation.

Claims

1. A mobile station in a spread spectrum mobile radio communication system having at least one base station and a plurality of mobile stations, the system providing secure communications between the base station and each mobile station by using a cipher key unique to each mobile station, the mobile station comprising:

a source coder for converting speech information into digital information and for assembling the digital information into a block including M bits of the digital information;

a transmitter sequence generator for producing a unique enciphering bit sequence according to a function of the cipher key unique to the mobile station and a code key, wherein the cipher key is associated with an identification code of the mobile station that is known to both the mobile station and the at least one base station;

an adder for enciphering the block by modulo-adding the block and the unique enciphering bit sequence, whereby the adder produces an enciphered information signal;

an orthogonal block coder for converting the enciphered information signal to an encoded signal block including N bits, wherein N is greater than M;

a bit-wise XOR circuit for combining the encoded signal block with a spread spectrum mask code unique to the mobile station, whereby the bit-wise XOR circuit generates a spread-spectrum chip stream for transmission; and

a modulator for modulating a radio carrier signal with the spread-spectrum chip stream.

2. The mobile station of claim 1, wherein the unique enciphering bit sequence is produced according to a function of secret information, and the spread spectrum mask code is a function only of non-secret information.

3. In a mobile communication system for exchanging information between a communication network and a mobile terminal using direct sequence spread spectrum multiple access, an apparatus in the mobile terminal for providing private communication with the communication network, comprising:

means for generating a spread spectrum sequence and a cipher sequence;

means for storing a first bit pattern for determining the cipher sequence and a second bit pattern for selecting the spread spectrum sequence, the first bit pattern being known only to the apparatus and the communication network;

means for clocking the sequence generating means through a number of internal states to produce a first output sequence that depends on the first bit pattern and to produce a second output sequence that depends on the second bit pattern and is independent of the first bit pattern;

coding means for converting speech for transmission into a sequence of digital words having a first number of bits;

ciphering means for combining the digital words with the first output sequence to produce ciphered words, wherein the ciphered words have the first number of bits;

Walsh-Hadamard coding means for converting the ciphered words into corresponding Walsh-Hadamard codewords having a second number of bits, wherein the second number is equal to two raised to the power of the first number;

spread spectrum coding means for combining the Walsh-Hadamard codewords with the second output sequence to produce a digital spread spectrum signal; and

means for modulating a carrier signal with the digital spread spectrum signal for transmission to the communication network.

4. In a mobile communication system for exchanging information between a communication network and a plurality of mobile terminals using direct sequence spread spectrum multiple access, a receiver in the communication network for receiving and decoding enciphered signals transmitted by a mobile terminal, comprising:

means for receiving a radio signal from the mobile terminal and for converting the received radio signal to a stream of numerical samples;

means for storing a first bit pattern for determining a cipher sequence and for storing a second bit pattern for selecting a spread spectrum sequence, wherein the first bit pattern is known only to the communication network and the mobile terminal;

a sequence generator for sequentially generating a spread spectrum sequence that depends only on the second bit pattern and a cipher sequence that depends on the first bit pattern;

means for clocking the sequence generator, wherein the sequence generator sequentially generates the spread spectrum sequence and the cipher sequence in response to the clocking means;

spread spectrum decoding means for combining the stream of numerical samples with the spread spectrum sequence to produce sample blocks containing a first number of decoded samples;

Walsh-Hadamard decoding means for transforming the sample blocks into digital words having a second number of bits, wherein the first number is equal to two raised to the power of the second number;

deciphering means for combining the digital words with the cipher sequence to produce deciphered words having the second number of bits; and

decoding means for converting the deciphered words into a speech signal for transmission over a public telephone network.