Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
1999-08-10
2003-05-20
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Angle modulation
C375S231000, C375S233000, C375S324000, C375S329000, C375S330000, C375S350000, C329S315000
Reexamination Certificate
active
06567480
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 sample timing adjustment and frequency offset estimation and compensation, and to a radio system having sample timing adjustment means and frequency offset estimation and compensation means.
2. Description of the Related Art
The present invention relates generally to signal recovery in communication systems and is particularly applicable to, and is described below in the context of, a digital cellular communication system such as the North American TDMA (Time Division Multiple Access) cellular communication system compatible with EIA/TIA documents IS-54 (Revs. A and B) and IS-136.
A conventional wireless radio system used for telephony 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 radio frequency (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 (landline) 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 frequency signal for transmission to a cell site 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 technology, in which information consisting of voice and data is digitally encoded onto an RF carrier signal, or systems which are compatible with digital and analog cellular communication standards are currently more popular than analog systems. Presently, there are three basic types of digital cellular technology; Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA) and Code Division Multiple Access (CDMA). Digital cellular systems currently fall within these three categories and many use a combination of one or more of these technologies along with analog techniques.
In order to satisfy a demand for a tenfold increase in system capacity over conventional analog cellular systems, the telephone industry group (TIA) of the Electronics Industry Association (EIA/TIA) promulgated an Interim Standard for time division multiplexed (TDM) wireless digital telephony in the late 1980's, known as IS-54. IS-54 (revs. A and B), and the more current Interim Standard for time division multiplexed wireless telephony, IS-136, use Time Division Multiple Access (TDMA) digital technology to effectively increase the limited bandwidth available for cellular communications. The EIA/TIA IS-54 (revs. A and B) and IS-136 standards are well known in the art and are incorporated herein by reference.
In a TDMA system under IS-54, for instance, data is communicated in symbol bursts arranged in time slots each comprising 162 symbols which include a sync (synchronization) word of 14 symbols followed by an information sequence. Communication from a cell site to mobile units is performed on a time division multiplexed basis whereby each cellular channel is used within each cell to facilitate simultaneous communication with 3 to 6 mobile units. Typically 3 to 6 users (data channels) share a common 30 kHz channel in TDMA operation. Each user transmits data in an assigned time slot that is part of a larger frame. The sync word is used to facilitate timing recovery, i.e., to determine an optimum time for sampling the received signal for further processing to recover the communicated information. It is well known that timing recovery and the necessary processing of the samples are made more difficult by a low signal-to-noise ratio (SNR) and that a low SNR can often be present in cellular communication systems.
In order to receive a transmitted digital signal, the communicating units must determine the beginning and end of signals intended for them, known as frame/slot synchronization. The complexity and accuracy of frame/slot synchronization depend upon the number of points at which the signal is sampled and the ability of the system to compensate for signal distortion. Increasing the number of samples per transmitted symbol results in an increase in the accuracy of the receiver at the expense of a higher complexity. Another crucial function required in TDMA communication is the need to determine the optimum time within a symbol interval to sample the signal and determine the relative phase angle of the symbol. Once the optimum point for symbol timing is determined, all the symbols within a burst can be demodulated using carrier recovery circuitry and the burst decoded and converted into an analog speech signal or other data.
Generally, a received signal is oversampled at N times the symbol rate, wherein N is an integer. Thus N sets of samples of a received signal are stored in a decision block, and various known techniques for determining the optimum set of samples may be applied to extract the information contained in the received signal.
Various methods for determining optimum sampling timing are known, and some such methods determine, on a burst-by-burst basis, the optimum sampling timing by selecting a set of samples which exhibit the best correlation with the previously-known sync word. After determination of an initial optimum sampling timing through timing recovery using the sync word, however, it is necessary to maintain an optimum sampling timing throughout the entire information sequence. This is typically referred to as timing tracking or sample timing adjustment, and serves to avoid cumulative errors of the sampling times during the information sequence, which, if not corrected, can detract from the recovery of the communicated information. Determination of an optimum sampling timing using the sync word does not compensate for signal distortion occurring in a portion of a signal burst separate from the sync word. There is thus a need for a sampling timing adjustment method which is capable of determining optimum sampling timing in a high-loss environment, such as a cellular communication channel, which is sufficiently rapid to effect frequent, high-speed sampling timing adjustments without knowledge of an information sequence in multiplexed or burst communications, such as TDMA cellular communications, and which has minimal processing overhead and hardware requirements.
In a TDMA cellular system, channel-induced signal distortion often appears as a phase shift induced between encoded symbols in a received signal. Since phase modulation of a carrier is used to encode information in TDMA cellular systems, unwanted phase-shifts in a modulated signal may render a signal undetectable. By itself, sampling timing adjustment may not adequately compensate for unwanted phase shift. In addition to determining an optimum sampling timing, therefore, techniques for reducing or eliminating unwanted phase shifts are required in a TDMA cellular system.
In time division multiplexed digital
Brardjanian Nima
Lee Yong J.
Matusevich Alex
Sarraf Mohsen
Tsai Sheng-Jen
Ahn Sam
Chin Stephen
Lucent Technologies - Inc.
Troutman Sanders Mays & Valentine
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