Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-08-30
2004-05-11
Chin, Stephen (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S222000, C370S515000
Reexamination Certificate
active
06735242
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to timing alignment for demodulation of a digital signal and, more particularly, to signal timing alignment using a delay lock loop (DLL) in Direct Sequence Spread Spectrum (DSSS) communications.
BACKGROUND OF THE INVENTION
Spread spectrum communication schemes were originally developed for the military to solve the problem of signal jamming by an enemy. The basic idea behind spread spectrum is that by broadcasting a signal over a wide range of frequencies, the enemy is prevented from jamming the signal. There are three main types of spread spectrum communications: Frequency-Hopping; Direct Sequence; and Hybrid. The Frequency-Hopping scheme avoids jamming by hopping from frequency to frequency in a pattern known to the transmitter and receiver but not to the jammer. In Direct Sequence Spread Spectrum (DSSS) communications, a transmitted signal is broadcast over a very wide band of frequencies to avoid narrow-band interference. Because a transmitted signal is spread over a broad frequency band, many transmitters may be broadcasting in the same bands. DSSS systems use a unique identifier code assigned to each transmitter to enable a receiver to separate a signal of a particular transmitter from that of all the other signals in the same bandwidth. Hybrid spread spectrum schemes are a combination of the Direct Sequence and Frequency-Hopping schemes.
One example of a DSSS-type system is a Code Division Multiple Access (CDMA) type system. In a CDMA type system, multiple users simultaneously communicate while sharing the same wideband frequency spectrum. Each of the users is assigned a unique digital pseudonoise (PN) code sequence. During transmission, the PN code sequence is used to spread the transmitted signal over the CDMA frequency spectrum. Spreading is sometimes referred to as “chipping” because each bit of the signal is chipped into “chips” by the spreading code. In other words, a bit to be transmitted is broken down into chips by the spreading code and, after transmission, the chips are reassembled into the original bit at the receiver. The spread (or “chipped”) signal is then RF modulated and transmitted on a carrier frequency. The same carrier frequency may also carry other users' transmissions that have been spread by their unique PN sequences. Transmissions by all other transmitters appear only as additional noise to a receiver listening to a particular transmitter. Assuming that all the transmissions are received at approximately the same power level, carefully choosing the PN code sequences to be orthogonal allows the noise from the other transmitters to be filtered out by the receiver.
Synchronization of PN code sequences between the receiver and transmitter is a serious problem in DSSS systems. To accurately decode a DSSS transmission, a receiver must synchronize its PN code generator to the transmitter PN code generator. Synchronization is usually accomplished in two steps. The first step, called acquisition or detection, includes bringing the PN code sequences generated in the transmitter and receiver into coarse alignment, typically within one code chip interval (in other words, within one chip period). The second step, called the time tracking loop, involves fine synchronization to the received signal and continuous tracking to maintain the best possible waveform alignment during reception by means of a feedback loop. The time tracking loop corrects for the Doppler effect as a mobile station moves toward or away from a base station.
Due to the importance of synchronization in DSSS systems, many synchronization schemes have been proposed that utilize various types of detectors and decision strategies. A common feature of most synchronization schemes (such as the maximum likelihood acquisition method discussed below) is that the received signal and the locally generated PN code sequence(s) are first correlated to determine the measure of similarity between the sequences. Next, the measure of similarity is compared to a predetermined threshold to decide if the signals are in synchronization. If there is no synchronization, the acquisition procedure provides a change in the phase of the locally generated PN code sequence and another correlation is attempted as a part of the signal search through the receiver's phase space.
Initial acquisition or detection of a DSSS signal may be accomplished using the maximum likelihood acquisition method. In maximum likelihood acquisition, the received signal and the locally generated PN code sequence(s) are first correlated in the receiver to determine the measure of similarity between the signals. Next, the measure of similarity indicated by the correlated results is compared to a threshold to decide if the two signals are in coarse synchronization. The threshold may be determined a priori or may be an adaptive threshold, set according to the results of correlations with previous PN code phases. In the adaptive threshold method, the entire PN code space is searched and the PN code phase resulting in the maximum threshold is used to receive further communications.
Most American CDMA cellular (mobile) systems operate according to the Telecommunications Industry Association/Electronic Industry Association (TIA/EIA) IS-95 cellular system standard. IS-95 is also commonly known as CDMAone.
In an IS-95 system, the downlink transmission consists of at least one common channel and a number of radio channels. A permanent signal is transmitted on a common pilot channel. It may be used by the mobile for coherent communication to estimate the path loss, so as to set power control initially, and to acquire synchronization to the network. Additional channels are set aside for paging and other downlink information.
In an IS-95 system, a mobile station must quickly search, acquire, and synchronize to many different signals while maintaining communications with the system. The mobile station must initially acquire a pilot channel from a base station of the system upon power-up or entry into the system. As the mobile station moves through the system, it must continually search for stronger pilot channels of base stations located near the base station with which the mobile station is communicating. The mobile station searches for pilot channels based on PN pilot channel phase information received from the system. The pilot channels in IS-95 are transmitted by each base station using the same system PN code but with different phase offsets. The code phase offsets allow the pilots to be distinguished from one another and thus uniquely identify each base station to the mobile station. All pilot channels in the IS-95 system use Walsh code 0 (a sequence of all 1's).
In an IS-95 system, when the mobile station has detected acquisition of a pilot channel of a certain base station (or system) PN code phase, the mobile station attempts to decode a synchronization (SYNC) channel at the same PN code phase. This SYNC channel is spread by the base station PN code phase and a unique Walsh PN code sequence that identifies the SYNC channel transmissions. The SYNC channel frames transmitted on each SYNC channel from each base station are aligned with the pilot PN sequence of that base station, so correct detection and acquisition of the pilot channel allows the SYNC channel frame to be received and decoded. The SYNC channel frame includes a SYNC Channel Message that provides system parameters to the mobile station. The system parameters in the SYNC channel frame include the timing of the base station's pilot sequence with respect to the system timing and the base station's paging channel data rate. Once the mobile station has obtained information from the SYNC Channel Message, the mobile station adjusts its timing to correspond to the system's timing and begins monitoring the paging channel.
If the process of synchronizing to the system, which includes acquiring the pilot channel and synchronizing to the SYNC channel, involves false detections of the pilot channel, significant penalti
Kenney Thomas J.
Xu Weiping
Chang Edith
Nokia Corporation
Patel Milan
Weber Tom
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