Method and apparatus for despreading spread spectrum signals

• US 5,761,239 A
• Filed: 05/01/1995
• Issued: 06/02/1998
• Est. Priority Date: 11/01/1993
• Status: Expired due to Term

• Current Assignee: Intel Corporation
• Original Assignee: Omnipoint Corp. (Deutsche Telekom AG)
• Inventors: Gold, Robert, Dixon, Robert C.
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): WEBSTER, BRYAN E
• Application Number:
  US08/432,913
• Time in Patent Office: 1,128 Days
• Field of Search: 375/200, 375/206, 375/316, 375/208
• US Class Current: 375/150
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04B 1/70718 with asynchronous demodulation, i.e. not requiring code synchronisation
• CPC Class Codes: H04J 14/005 Optical Code Multiplex
• CPC Class Codes: H04L 7/042 Detectors therefor, e.g. correlators, state machines digital correlators in general G06F17/15

Abstract

A system for despreading and/or demodulating spread spectrum codes is provided. A data signal is encoded using an M-ary transmission technique, wherein predefined symbol codes are associated with a particular series of data bits. A receiver comprises a plurality of delay elements or a shift register with a plurality of taps configured to recognize one or more the predetermined symbol codes. Delayed representations of the received signal are generated and combined to produce a known output sequence, such as a string of consecutive one or consecutive zeroes, when a recognizable spread spectrum code is received. The receiver shift register may be preloaded such that recognition may occur starting with the first received chip. More than one set of taps may be coupled to the same shift register, so that more than one of the M spreading codes may be recognized by the same shift register. A received set of chips may be identified with a code sequence that has a minimal cross-correlation distance or Hamming distance, whereby a received code sequence with imperfect correlation may be recognized as corresponding to a defined symbol. More than one symbol may be simultaneously transmitted using quadrature phase modulation or like techniques, and recognized by the receiver.

Claims

1. A PN generator comprising:

a shift register coupled to a received spread spectrum signal, said shift register comprising, a plurality of stages,

a plurality of taps corresponding to a particular spread spectrum code connected to said shift register, each of said taps providing a delayed version of said received spread spectrum signal,

a second plurality of taps connected to said shift register, said second plurality of taps corresponding to a second spread spectrum code,

a modulo-two adder coupled to each of said taps and to said received spread spectrum signal,

a sequence detector coupled to said modulo-two adder, whereby a sequence indicative of a spread spectrum code in said spread spectrum signal may be identified,

a second modulo-two adder coupled to each of said second plurality of taps and to said spread spectrum code, and a second sequence detector coupled to said second modulo-two adder, whereby a sequence indicative of said second spread spectrum code is identified.

2. The despreader of claim 1 further comprising a symbol selector coupled to said first sequence detector and said second sequence detector, said symbol selector capable of outputting either said first spread spectrum code or said second spread spectrum code.

3. A PN generator comprising:

a shift register coupled to a received spread spectrum signal, said shift register comprising a plurality of stages,

a plurality of taps connected to said shift register, each of said taps providing a delayed version of said received spread spectrum signal,

a modulo-two adder coupled to each of said taps and to said received spread spectrum signal,

a sequence detector coupled to said modulo-two adder, whereby a sequence indicative of a spread spectrum code in said spread spectrum signal may be identified, and

said taps are positioned along said shift register such that said modulo-two adder outputs the same chip value repeatedly for a minimum duration when said spread spectrum code is received.

4. The despreader of claim 3 wherein said minimum duration comprises a number of chips equal to the length of said spectrum code minus the number of stages in said shift register.

5. A PN generator comprising:

a shift register coupled to a received spread spectrum signal, said shift register comprising a plurality of stages,

a plurality of taps connected to said shift register, each of said taps providing a delayed version of said received spread spectrum signal,

a modulo-two adder coupled to each of said taps and to said received spread spectrum signal,

a sequence detector coupled to said modulo-two adder, whereby a sequence indicative of a spread spectrum code in said spread spectrum signal may be identified, and

a reset signal whereby said shift register may periodically be reset to a predetermined sequence.

6. A PN generator comprising:

a shift register coupled to a received spread spectrum signal, said shift register comprising a plurality of stages,

a plurality of tap sets connected to said shift register, each of said tap sets defining a different spreading code,

means connected to each of said tap sets for generating a recognition signal indicating a likelihood of having received one of said spreading codes,

a symbol detector coupled to each of said recognition signals, and outputting a signal indicative of one of said spreading codes in response thereto

wherein said spreading codes are symbol codes, and

wherein each of said recognition signals comprises a sequence of chips, and said symbol detector comprises an accumulator whereby a correlation value corresponding to each of said recognition signals is generated.

7. The despreader of claim 6 wherein said symbol detector comprises error correction means whereby said spreading codes may be recognized by comparing said correlation parameters for each of said recognition signals.