Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
1999-01-25
2003-09-23
Chin, Stephen (Department: 2734)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S145000, C375S149000, C375S150000, C375S343000, C375S367000, C370S335000, C370S342000, C370S441000, C370S479000
Reexamination Certificate
active
06625200
ABSTRACT:
BACKGROUND
The present invention relates to mobile phones or radio apparatus for receiving spread spectrum signals, such as code division multiple access (CDMA) signals, with which time synchronization is achieved.
Radiocommunication systems involve the transmission of information over an air interface, for example by modulating a carrier frequency with that information. Upon reception, a receiver attempts to accurately extract the information from the received signal by performing an appropriate demodulation technique. However, in order to demodulate a received signal, it is first necessary to synchronize timing between the transmitter and the receiver. Different levels of synchronization may be required depending upon the design of the radiocommunication system. For example, in most systems clocking differences between the transmitter and the receiver create differences in timing at a bit level. Moreover, in some radiocommunication systems, information is transmitted in bursts, sometimes referred to as “frames”, which represent chunks of information that are independently detected and decoded. In these types of systems it is also desirable to locate the beginning of a frame, so that information relevant to a particular receiver is isolated and demodulated. Likewise, some systems (e.g., time division multiple access or TDMA systems) further subdivide frames into timeslots to create channels that are time multiplexed with one another. In these systems it is further desirable to synchronize the receiver to the beginning of each timeslot.
Some systems provide channelization using a spread spectrum technique known as code division multiple access (CDMA). In CDMA systems, the information data stream to be transmitted is first coded or spread using a unique spreading code and then combined with a long PN-sequence or a shorter scrambling-sequence. In the latter case, the scrambling-sequences are planned from cell to cell so that neighboring cells use different scrambling-sequences or scrambling-masks. The information data stream and the PN-sequence or the scrambling sequence can have the same or different bit rates. The multiplication of the information data stream with the unique spreading code and long PN-sequence results in an output stream of chips. Thus, in CDMA systems, it is also desirable to synchronize the receiver to the chip boundaries.
To further understand the synchronization tasks associated with signal processing in a CDMA radiocommunication system, consider the following example.
FIG. 1
illustrates the use of base stations to transmit radio waves to mobile users (mobile stations) in a cellular system. In a CDMA system, base station
10
can transmit signals to mobile stations
14
and
15
as a single (composite) signal. The signal directed to mobile station
14
is typically coded with a short code that is orthogonal or mostly orthogonal to a short code that is used to code the signal directed to mobile station
15
. These signals are then spread with a second code that is sometimes referred to as a long code, associated with base station
10
. The sum of the two coded and spread signals is then transmitted by base station
10
.
When mobile station
14
receives the composite signal, mobile station
14
multiplies the spread signal with the long code and the short code to recreate the signal directed to mobile station
14
and the signal directed to mobile station
15
is suppressed as interference noise. Similarly, mobile station
15
multiplies the spread signal with the long code and the short code assigned to mobile station
15
to recreate the signal directed to mobile station
15
and the signal directed to mobile station
14
is suppressed as interference noise. The receivers associated with mobile stations
14
and
15
must have acquired the various levels of synchronization to the received signal which were described above, in addition to learning or knowing the applicable long and short codes, in order to implement despreading, demodulation and decoding of the information residing in that signal. Many different techniques have been developed in order to acquire synchronization at each of the various levels. For frame synchronization these techniques typically depend, in large degree, on the frame structure and the manner in which overhead or control information is conveyed to the mobile station. Overhead information is usually provided on one or more broadcast control channels which are transmitted by base stations using known channels to which mobile stations can quickly lock onto and receive the overhead information including, among other things, information used to acquire frame synchronization with that base station. Those skilled in the art will appreciate that many radiocommunication systems have unsynchronized base stations, i.e., base stations which do not share a common timing reference signal. Accordingly, frame synchronization is a process which needs to be performed, for example, at start-up (i.e., when a mobile is powered on), as a mobile station moves from cell to cell and when measuring on channels associated with neighboring cells as part of cell reselection procedures (e.g., to confirm that a mobile station is listening to a “best” serving base station).
Of course, as with most signal processing tasks performed by the receiver, reducing the delay associated with synchronization is important in improving the receiver's performance. Many types of communication services, in particular speech communication, are relatively delay intolerant. Thus, system designers are continuously seeking for ways in which to reduce the amount of time that it takes to perform any given signal processing task, including time synchronization.
SUMMARY
A radio receiver employing a synchronization method according to the present invention is able more rapidly to find and acquire synchronisation with a CDMA signal by means of at least a two step process in which, at a first step, a number of candidate synchronisation frequencies or timings are identified, followed by confirming one of the candidates as a correct synchronisation state at a second or final step. According to the present invention, the confirmation step may be performed at the same time as the step of identifying further candidates by processing the same received signal samples in different ways.
In an exemplary implementation, a first correlation means correlates shifts of a stream of received signal samples using a correlation length over which the received signal does not drift significantly in phase, amplitude or timing, thus allowing coherent correlation. Coherent correlations are not in general expected to reach a sufficient signal-to-noise ratio to unambiguously identify with adequate certainty that correct synchronisation has been achieved. Consequently a number of coherent correlations corresponding to like timing postulates are further accumulated non-coherently in a number of bins, each bin corresponding to a timing postulate. Non-coherent accumulation involves adding the magnitudes or square magnitudes of the coherent correlations, where the squared magnitude is equal to the sum of the squares of the real and imaginary parts of the coherent correlation value.
When non-coherent or magnitude accumulation must take place for a prolonged time period in order to identify a likely candidate timing, and the time period is so long that a drift of timing may occur that is of the order of plus or minus one timing bin width or more, the present invention may employ a drift compensation type of accumulation described in U.S. patent application Ser. No. 08/768,975 to Paul W. Dent, filed Dec. 18, 1996, the disclosure of which is hereby incorporated by reference herein.
An exemplary CDMA system according to the present invention, using a 4 megachip per second modulation, searches time bins that are one chip wide, i.e. 0.25US. The receiver time and frequency reference has an initial error of +/−10 parts per million, which results in a drift of one bin per 25 mS. This exemplary CDMA system
Chin Stephen
Coats & Bennett P.L.L.C.
Ericsson Inc.
Ha Dac V.
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