Pulse or digital communications – Synchronizers – Frequency or phase control using synchronizing signal
Reexamination Certificate
1999-12-28
2003-08-12
Corrielus, Jean (Department: 2631)
Pulse or digital communications
Synchronizers
Frequency or phase control using synchronizing signal
C370S500000
Reexamination Certificate
active
06606363
ABSTRACT:
BACKGROUND
This invention relates generally to a method and system for controlling the reference frequency in a radio receiver. More particularly, this invention relates to a method and system for estimating a frequency offset between a carrier frequency of a transmitter and a local reference frequency of a receiver in a communication system.
Modern communication systems, such as cellular radiotelephone systems and satellite radio systems, employ various modes of operation (analog, digital, dual mode, etc.) and access techniques such as frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and hybrids of these techniques.
In North America, a digital cellular radiotelephone system using TDMA is called the Digital Advanced Mobile Phone System (D-AMPS), some of the characteristics of which are specified in the TIA/EIA/IS-136 standard published by the Telecommunications Industry Association and Electronic Industries Association (TIA/EIA). Another digital communication system using direct sequence CDMA is specified by the TIA/EIA/IS-95 standard. There are also frequency hopping TDMA and CDMA communication systems, one of which is specified by the EIA SP 3389 standard (PCS 1900). The PCS 1900 standard is an implementation of the GSM system, which is common outside North America, that has been introduced for personal communication services (PCS) systems.
Several proposals for the next generation of digital cellular communication systems are currently under discussion in various standards setting organizations, which include the International Telecommunications Union (ITU), the European Telecommunications Standards Institute (ETSI), and Japan's Association of Radio Industries and Businesses (ARIB).
FIG. 1
illustrates an exemplary communication system. The system includes at least one transmitter
100
and at least one receiver
150
. Although the transmitter
100
and the receiver
150
are depicted in
FIG. 1
as a base station (BS) and a mobile station (MS), respectively, it will be appreciated that the transmitter can be implemented in many ways, e.g., as a terrestrial or satellite repeater, and the receiver can be implemented in many ways, e.g., as a fixed cellular terminal (wireless local loop). A BS and a MS are depicted in FIG.
1
and described in the following for illustrative purposes only.
The BS
100
and the MS
150
communicate via a radio air interface
125
. Each neighboring BS
100
is assigned a particular carrier frequency, and each BS
100
allocates specific time slots for each MS
150
.
In systems such as the proposed IMT2000 system for the next generation mobile telephony which is based on DS-CDMA, data may be transmitted as symbols in a control channel. The downlink data from the BS is segmented into superframes, each having a duration of 720 ms. Each superframe is divided into 72 frames, a frame having a duration of 10 ms. Each frame is divided into 15 slots, and each slot divided into 2560 chips. Depending on the communication channel, 2560 chips are grouped into a number of symbols. For example, in the Broad Cast Control Channel (BCCH) there are 10 symbols of 256 chips each. A certain number of these symbols are already known and transmitted as pilot symbols from the BS to MSs.
FIG. 2
shows the segmentation of the data in the Broad Cast Channel (BCCH).
To communicate with a BS
100
, a MS
150
must be time and frequency synchronized to the BS
100
. In other words, the local frequency reference and time reference of the MS
150
must be synchronized with the carrier frequency assigned to the BS
100
and the time slot(s) allocated by the BS, respectively. In a CDMA system, the MS
150
must be synchronized with the BS's carrier frequency and the code words transmitted.
To synchronize the MS
150
, the BS
100
transmits a frequency synchronization signal, e.g., pilot symbols or pilot groups. The MS
150
receives and demodulates the transmitted frequency synchronization signal in any suitable manner.
In cellular systems, a frequency offset or deviation may exist between the transmitter carrier frequency and the local oscillator of the receiver. The frequency offset results from different factors, including temperature variation, aging, and manufacturing tolerances. To address this offset, a phase ramp can be estimated and compensated for in an Automatic Frequency Control (AFC) control loop. Estimation can be based on the received frequency synchronization signal, e.g., pilot symbols or pilot groups. The frequency offset can be estimated by studying the phases of the pilot symbols in consecutive slots. This is described, e.g., in commonly assigned U.S. Pat. No. 6,104,767, and herein incorporated by references
In the mobile radio channel, multi-path is created by reflection of the transmitted signal from obstacles in the environment, e.g., buildings, trees, cars, etc. In general, the mobile radio channel is a time varying multi-path channel due to the relative motion of the structures that create the multi-path.
A characteristic of the multi-path channel is that each path through the channel may have a different phase. For example, if an ideal impulse is transmitted over a multi-path channel, each pulse of the received stream of pulses generally has a different phase from the other received pulses. This can result in signal fading.
The phases of pilot symbols include the frequency offset between the MS and the BS plus the Doppler spread. Typically, the channel is modeled as discrete rays. In a Rayleigh fading radio channel, the frequencies of the path-rays are distributed in a frequency zone of twice the Doppler spread around the frequency offset or deviation between the transmitter carrier frequency and the receiver local reference frequency. This is shown in
FIG. 3
which illustrates a distribution of frequencies in a zone f
off
−f
Dopp
and f
off
+f
Dopp
including the Doppler spread f
Dopp
around a frequency offset f
off
.
A large Doppler spread may complicate estimation of the frequency offset. For example, the quantity of the Doppler spread in combination with the frequency offset can exceed the rate of receipt of the pilot symbols. Hence, the frequency offset cannot reliably be estimated based on consecutive pilot symbols or pilot groups. Even if the total frequency offset is slightly less than the pilot receipt rate, it has been observed that the quality of the estimate is not satisfactory. To prevent aliasing, the rate of receipt should ideally be at least twice the frequency offset.
Estimation of the frequency offset in time limited applications, such as idle MS mode when the MS has a short time to estimate the frequency offset, can also be a problem. In this time-limited scenario, the frequencies of the received path-rays may not be distributed around the frequency offset as in FIG.
3
. They may be distributed as in
FIG. 4
in which the dotted curve shows the frequency zone of the received path-rays in the limited time. This will result in a biased frequency offset estimate indicated in
FIG. 4
by the denotation “&Lgr;”.
A further concern is a low Signal-to-Noise Ratio (SNR), which affects the detected phases of the pilot symbols and thereby affects the estimated frequency offset.
Whether due to the Doppler spread, limited frequency offset estimation time, low SNR, or some other factor the received pilot symbols may not be adequate for determining the frequency offset. There is thus a need for a technique for reliably estimating the frequency offset in situations where the pilot symbols are not adequate.
SUMMARY
It is therefore an object of the present invention to provide a technique for synchronizing a remote station to a communication network even where the synchronization signals from the communication network are not adequate.
According to exemplary embodiments, this and other objects are met by a method and apparatus for estimating an offset between a carrier frequency of a transmitter and a local reference frequency of a receiver. A received signal
Atarius Roozbeh
Ramesh Rajaram
Burns Doane Swecker & Mathis L.L.P.
Corrielus Jean
Telefonaktiebolaget LM Ericsson (Publ)
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