Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2002-04-30
2004-01-27
Tse, Young T. (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S150000, C370S209000
Reexamination Certificate
active
06683907
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The current invention relates to wireless telecommunications. More particularly, the present invention relates to a novel and improved method of compensating for phase and amplitude distortion of multiple signals transmitted through a single channel.
II. Description of the Related Art
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband (ACSSB) are known in the art. Techniques for distinguishing different concurrently-transmitted signals in multiple access communication systems are also known as channelization. The spread spectrum modulation technique of CDMA has significant advantages over other multiple access techniques.
The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, and in U.S. Pat. No. 5,751,761, entitled “SYSTEM AND METHOD FOR ORTHOGONAL SPREAD SPECTRUM SEQUENCE GENERATION IN VARIABLE DATA RATE SYSTEMS”, both assigned to the assignee of the present invention and incorporated by reference herein. Code division multiple access communications systems have been standardized in the United States in Telecommunications Industry Association TIA/EIA/IS-95-A, entitled “MOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE WIDEBAND SPREAD SPECTRUM CELLULAR SYSTEM”, hereafter referred to as IS-95 and incorporated by reference herein.
The International Telecommunications Union recently requested the submission of proposed methods for providing high rate data and high-quality speech services over wireless communication channels. A first of these proposals was issued by the Telecommunications Industry Association, entitled “The cdma2000 ITU-R RTT Candidate Submission”, hereafter referred to as cdma2000 and incorporated by reference herein. A second of these proposals was issued by the European Telecommunications Standards Institute (ETSI), entitled “The ETSI UMTS Terrestrial Radio Access (UTRA) ITU-R RTT Candidate Submission”. And a third proposal was submitted by U.S. TG 8/1 entitled “The UWC-136 Candidate Submission” (referred to herein as EDGE). The contents of these submissions is public record and is well known in the art.
In the CDMA demodulator structure used in some IS-95 systems, the pseudonoise (PN) chip interval defines the minimum separation two paths must have in order to be combined. Before the distinct paths can be demodulated, the relative arrival times (or offsets) of the paths in the received signal must first be determined. The demodulator performs this function by “searching” through a sequence of offsets and measuring the energy received at each offset. If the energy associated with a potential offset exceeds a certain threshold, a demodulation element, or “finger” may be assigned to that offset. The signal present at that path offset can then be summed with the contributions of other fingers at their respective offsets. The use of CDMA searchers is disclosed in U.S. Pat. No. 5,764,687, entitled “MOBILE DEMODULATOR ARCHITECTURE FOR A SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM”, assigned to the assignee of the present invention and incorporated by reference herein.
In the CDMA receiver structure used in some IS-95 systems, data passing from transmitter to receiver is divided into frames which are transmitted at fixed time intervals. Depending on the varying amount of data to be transmitted during each interval, the transmitter places the data into one of several sizes of frame. Since each of these frame sizes corresponds to a different data rate, the frames are often referred to variable-rate frames. The receiver in such a system must determine the rate of each received frame to properly interpret the data carried within the received frame. Such rate determination methods often include the generation of frame quality metrics, which may be used to assess the level of uncertainty associated with the determined frame rate. Methods of performing rate determination and generating frame quality metrics are disclosed in U.S. Pat. No. 5,751,725, entitled “METHOD AND APPARATUS FOR DETERMINING THE RATE OF RECEIVED DATA IN A VARIABLE RATE COMMUNICATION SYSTEM”, assigned to the assignee of the present invention and incorporated by reference herein.
Signals in a CDMA system may be complex PN spread as described in U.S. patent application Ser. No. 08/856,428, entitled “REDUCED PEAK TO AVERAGE TRANSMIT POWER HIGH DATA RATE IN A CDMA WIRELESS COMMUNICATION SYSTEM,” filed Apr. 9, 1996, assigned to the assignee of the present invention and incorporated by reference herein, and in accordance with the following equations:
I=I′PN
I
+Q′PN
Q
(1)
Q=I′PN
Q
−Q′PN
I
. (2)
where PN
I
and PN
Q
are distinct PN spreading codes and I′ and Q′ are two channels being spread at the transmitter.
As described in cdma2000, transmission signals are constructed utilizing orthogonal Walsh coding, with one Walsh code used to transmit a pilot sub-channel signal. The orthogonal Walsh sub-channels used to construct such transmission signals are added together before being transmitted, and travel through the same transmission channels or pathways before being received at the receiver. Each transmission channel, by its inherent nature, alters the phase and amplitude of the signals passing through it, and also adds a component of thermal noise. These channel characteristics change with any movement by transmitter or receiver, but may vary over time even when both receiver and transmitter are stationary. Channel characteristics generally change very slowly compared with the data symbols transmitted through the channel.
Some CDMA receivers employ circuits which estimate the phase and amplitude distortion of the channel. These estimates are then used to compensate for channel distortion, enabling more accurate decoding and demodulation of the received signals. One such circuit for estimating phase and amplitude of a channel, and performing a dot product of that output with the demodulated data signal, is described in detail in U.S. Pat. No. 5,506,865, entitled “PILOT CARRIER DOT PRODUCT CIRCUIT”, assigned to the assignee of the present invention and incorporated by reference herein. In that described implementation, an all-zero pilot channel is received and used to estimate the channel characteristics. The resultant channel estimates are then used to convert demodulated signals to scalar digital values.
All CDMA signals transmitted on orthogonal sub-channels cause mutual interference to each other, as well as acting as jammers for adjacent cell areas. To enable coherent demodulation of orthogonal sub-channel signals, one sub-channel is often dedicated as a pilot carrier. As detailed in aforementioned U.S. Pat. No. 5,506,865, the pilot carrier is used in the receiver to produce estimates of the channel characteristics. The accuracy of these channel estimates is dependent on the strength of the pilot channel signal. Unfortunately, the pilot channel carries no data, so it is desirable to minimize the pilot transmit power. Conventionally the pilot power relative to the data signal power is selected by balancing between these two factors such that the best overall system performance can be achieved. For this reason, a method of producing accurate channel estim
Edwards Christopher
Miller Russell B.
Qualcomm Incorporated
Tse Young T.
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