Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1999-07-13
2003-07-08
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S243000, C375S244000, C375S226000, C375S281000, C375S283000, C375S285000, C375S304000, C375S325000, C375S331000, C375S332000, C375S362000, C329S304000
Reexamination Certificate
active
06590945
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a method and apparatus for communication, and more particularly, to a method for the estimation and correction of frequency offset between the local oscillator of a receiver and the carrier frequency of a received signal, and to a radio system having means for frequency offset estimation and correction.
2. Description of the Related Art
A conventional wireless radio system used for telephony (“cellular system”) consists of three basic elements—namely, mobile units, cell sites, and a Mobile Switching Center (“MSC”) In a basic cellular system, a geographic service area, such as a city, is subdivided into a plurality of smaller radio coverage areas, or “cells”. A mobile unit communicates by RF signals to the cell site within its radio coverage area. The cell site's base station converts these radio signals for transfer to the MSC via wire (land line) or wireless (microwave) communication links. The MSC routes the call to another mobile unit in the system or the appropriate landline facility. These three elements are integrated to form a ubiquitous coverage radio system that can connect to the public switched telephone network (PSTN).
A mobile unit contains a radio transceiver, a user interface portion, and an antenna assembly, in one physical package. The radio transceiver converts audio to a radio (RF) signal and converts received RF signals into audio. The user interface portion includes the display and keypad which allow the subscriber to communicate commands to the transceiver. The antenna assembly couples RF energy between the electronics within the mobile unit and the “channel”, which is the outside air, for transmission and reception. Each mobile unit has a Mobile Identification Number (MIN) stored in an internal memory referred to as a Number Assignment Module (NAM).
A cell site links the mobile unit and the cellular system switching center, and contains a base station, transmission tower, and antenna assembly. The base station converts the radio signals to electrical signals for transfer to and from a switching center.
Digital cellular systems and systems combining analog and digital communication techniques are currently more popular than purely analog systems. Presently, there are three basic types of digital cellular technology: Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). Digital cellular systems currently fall within these three categories and many use a combination of these technologies along with analog techniques. There are also variations in the way radio technologies allow duplex operation, called Frequency Division Duplex (FDD) and Time Division Duplex (TDD).
In order to receive a transmitted digital signal, coherent or non-coherent detection is generally used to extract encoded voice or data contained in the transmitted signal. In a typical coherent detector, for example, the modulated waveform is fed to a mixer, wherein the modulated wave is mixed (i.e., multiplied) with a “local oscillator” signal having a frequency that is matched to the frequency of the modulated signal.
In time division multiplexed digital communication systems such as the North American TDMA cellular telephone system, information is transmitted as symbols encoded in the phase of the transmitted signal with respect to its carrier. Proper extraction of the symbols necessitates that the local oscillator frequency used to demodulate the received signal is identical to the carrier frequency of the received signal. As is well known in the art, a frequency difference between the carrier of a modulated signal and the local oscillator used to extract the modulated information causes the apparent phase relationship to “rotate” undesirably.
Certain transmission protocols have the tendency to reduce this effect. For instance, in differential quadrature phaseshift keying (“DQPSK”), the encoded information is contained not in the absolute phase of the modulated signal, but in the difference between the phase of a given symbol and the phase of the previous symbol. In an ideal channel, a frequency offset between the local oscillator of the receiver and the carrier frequency of the transmitted signal does not present a significant problem as long as the symbol frequency is much larger than the frequency offset.
The cellular channel is not ideal, however, and is subject to various types of distortion such as delay spread due to multipath fading, Doppler effect, and the like. A process such as adaptive equalization, which involves adaptive channel distortion characterization, is needed in order to extract symbols accurately from the time-varying channel. To estimate and compensate for channel-induced distortion, cellular systems typically utilize adaptive equalization techniques which predict the channel response based upon the transmission of known data (eg., a so-called pilot signal or training sequence). However, such processes are sensitive to significant uncorrected frequency offsets, which may cause the channel to vary beyond the rate at which the adaptive processes can adapt. Even for DQPSK systems, therefore, accurate frequency offset compensation is necessary.
A conventional approach to frequency tracking is by use of a phase-locked-loop (“PLL”). A PLL circuit is typically formed as a phase detector fed by input and feedback signals, a loop filter and a voltage controlled oscillator for producing a sine wave (i.e., the feedback signal). In a PLL, the phase of the received signal, or a frequency-translated version thereof (i.e., an intermediate frequency (IF) signal), is compared with the local phase reference (i.e., the local oscillator), and the average phase difference over time is used to adjust the frequency of the reference. A basic PLL is characterized by a pull-in range B
p
. However, as B
p
increases, so does the variance of the phase error. AFC (Automatic Frequency Control) units, FLLs (Frequency Locked Loops) or PLL's with phase and frequency detectors are often used to track such signals. These circuits generally produce an estimate of the average input frequency only, and additionally require an elemental PLL if the phase is to be acquired. Unfortunately, phase-locked-loop systems tend to result in a cellular telephone system can be unacceptable. In addition, in cellular systems based on packetized data transfer, control data is often contained in a single packet, which may be lost before phaselock is achieved. An objectionable amount of dead time may also be encountered during handoff from one cell to another. This is true both for conventional, analog phase-locked-loop systems and for digital equivalents. Moreover, in wireless communications AFC design has been constrained by circuit complexity, and system designs have typically made frequency accuracy constraints somewhat loose to avoid prohibitive costs in complexity or processing requirements.
In addition, with the introduction of more optimal modulation schemes such as QPSK, relatively precise frequency estimates are often needed. Frequency errors may arise, for example, from the transmitter/receiver clock not being perfectly locked due to inaccuracies or drift in the crystal oscillator, as well as from large frequency shifts due to the Doppler effect, such as those occurring from vehicles moving at high speeds in open spaces. Many cellular systems allow only a small amount of time for achieving initial signal acquisition and require a minimum tracking error after initial acquisition. However, typical AFC or PLL circuits are not generally able to lock on or track a received signal with wide frequency shifts over a short period of time with a reasonable degree of accuracy.
The widespread demand for increased functionality and capacity in mobile duplex communications equipment has resulted in a rapid advancement in wireless technology. Over the past ten years, for instance, wireless telephony end-user equipment size, weight, and cost have dropped o
Brardjanian Nima
Lee Yong J.
Nabhane Walid E.
Sarraf Mohsen
Tsai Sheng-Jen
Kumar Pankaj
Lucent Technologies - Inc.
Pham Chi
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