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Sequential detection system and method with adaptive bias
| Details |
Inventors: Somayazulu, V. Srinivasa;
Assignee: Sharp Laboratories of America, Inc. (Camas, WA)
Primary Examiner: Chin; Stephen
Assistant Examiner: Jiang; Lenny
Attorney, Agent or Firm: Ripma; David C., Rabdau; Matthew D., Krieger; Scott C.
An adaptive CDMA sequential detection method is provided. The method permits the use of the sequential detection process to determine an unknown spreading code in an environment where the signal to noise ratio of the received signal varies. A communication system, such as W-CDMA, provides a periodic reference signal that the receiver can demodulate without the spreading code. The reference signal is used to estimate and constantly update the signal to noise calculation. From this estimate a bias is updated for use in the sequential detection of the spreading code. A system to update the CDMA sequential detector bias value is also provided. |
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DETAILED DESCRIPTION This invention relates generally to cellular telephone communications, and more particularly to a system and method for CDMA receivers to adaptively change the bias in the sequential detection of a code, to account for the effects of fading.
Spread spectrum communication techniques allow communicating users to operate in noisy radio frequency (RF) spectrums, and are especially effective against narrow-band interferers.
Spread spectrum communications can be effected at relatively low power spectral densities, and multiple users can share the same frequency spectrum.
Further, receivers can be designed to protect against multipath.
These system characteristics encouraged early development of the technology by the military.
Common forms of spread spectrum systems include chirp, frequency hopping, and direct sequence or pseudonoise (PN).
The chirp system transmits an impulse signal in the time domain that is spread monotonically in the frequency domain.
A receiver converts the spread frequency signal back into an impulse signal.
These frequency-spread impulse signals have applications in radar, for the pulse position modulation of information, or both, such as the R.
sup.
3 transponder developed by General Dynamics, Electronics Division in the 1970s.
Frequency hopping systems communicate by synchronizing users to simultaneously change the communication frequency.
Direct Sequence systems spread a digital stream of information, typically in a quadriphase modulation format, with a PN code generator, to phase shift key modulate a carrier signal.
The pseudonoise sequence of the PN code generator is periodic, and the spread signal can be despread in a receiver with a matching PN code.
Direct Sequence systems have excellent immunity to noise.
The PN codes used typically permit a large number of users to share the spectrum, with a minimum of correlation between the user's PN codes.
However, Direct Sequence system require large RF bandwidths and long acquisition times.
The IS-95 standard defines key features of the so-called second generation code division multiple access (CDMA) communication system, a type of Direct Sequence spread spectrum modulation
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Telecommunications 
