WO2000065729A1

WO2000065729A1 - Method for rejecting a control channel during a registration process

Info

Publication number: WO2000065729A1
Application number: PCT/US2000/009562
Authority: WO
Inventors: Chin Pan Wong; Charbel Khawand; Jianping Wu Miller
Original assignee: Motorola Inc.
Priority date: 1999-04-26
Filing date: 2000-04-11
Publication date: 2000-11-02
Also published as: AR023764A1; AU4338500A

Abstract

A mobile communication device (102) attempts to locate a valid serving cell (104) upon being powered up. The device begins scanning known control frequencies at which serving cells broadcast control information. The device tunes to a control frequency or control channel (204) and receives a segment or frame of digital samples (206). The segment is evaluated to obtain a first autocorrelation value and a second autocorrelation value at different offsets. A ratio of the autocorrelation values is compared to a predetermined threshold (212). If the received frame does not contain any control information, and the ratio is below the predetermined threshold, the frequency or channel is rejected (216).

Description

METHOD FOR REJECTING A CONTROL CHANNEL DURING A REGISTRATION PROCESS

Technical Field

This invention relates in general to mobile communication systems, and more particularly to registering a mobile communication device for communication service in a frequency re-use communication system.

Background of the Invention

Mobile wireless communication systems are in widespread use and have come to be a common means of facilitating communication. The most popular form of mobile communication systems are frequency re-use, or cellular systems. In these systems a plurality of serving cells are established by placing base stations in a cellular geographic pattern. In conventional cellular communication systems each base station is assigned a control frequency at which it broadcasts control information to facilitate communication service to mobile communication devices in the vicinity. The control frequency is unique within a given region so that there is no interference among base stations. Outside of the region, however, the frequency may be used again, hence the term frequency re-use.

When a mobile communication device is turned on or powered up, it attempts to register for communication service so that it can initiate and receive calls. Generally the process is as follows: The communication device is provided with a list or bandmap of values corresponding to control frequencies or control channels used by the operator of the communication system which the mobile communication device is attempting to register with. The mobile communication device begins scanning the frequencies to determine what frequencies are active in the area, and which is the best one for service. This method involves tuning to the control frequency and attempting to find control information at the frequency. Generally, the device will evaluate the information received at the frequency for a given period of time. If no control information is found after a first attempt, a second attempt is taken to ensure that the control information was not missed because of fading or interference, for example. This "two try" approach is widely used in evaluating control frequencies. The problem that results is that it can take a considerable period of time to locate a suitable control frequency. Some communication systems have hundreds of different control frequencies in use. It often takes on the order of tens of milliseconds to evaluate a frequency before rejecting it. As a result, the user of the communication has to wait a variable period of time for the device to find a suitable serving cell and register. Often this period of time is noticeable, and it would be preferable to reduce the amount of time necessary to register. Therefore there is a need for a method that reduces the amount of time needed to scan control channels and control frequencies to locate serving cells and register the communication device.

Brief Description of the Drawings FIG. 1 shows a system diagram of a communication system including a mobile station in accordance with the invention; and

FIG. 2 shows a flow chart diagram of a method for rejecting a control frequency in accordance with the invention.

Detailed Description of a Preferred Embodiment

While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward. The invention reduces the time needed to register a communication device by quickly determining if a frequency should be rejected or if it should be scanned a second, as is common. Presently, in many communication systems such as those in accordance with the General System for Mobile (GSM) communications specification, a communication device will attempt to find control information at a given control frequency or control channel twice before rejecting it and moving to the next control channel or control frequency. The present invention allows a discrimination to be made after the first time the control channel or control frequency is scanned so that those control channels and control frequencies at which there is no received control information can be rejected after one scan. This is accomplished by the novel use an autocorrelation ratio, and comparing the ratio to a preselected threshold, if the threshold not exceeded, the frequency or channel is rejected and the next frequency or channel is scanned. This substantially reduces the time needed to find a suitable control frequency or channel by eliminating non-productive channels and frequencies. Referring now to FIG. 1, there is shown a system diagram 100 of a communication system including a mobile communication device 102 and a base station 104. The communication system is a digital communication system, as is known in the art. The base station establishes a serving cell in the local vicinity of the base station, as is conventional. The base station comprises transceiver equipment to facilitate communication service. There are a variety of communication schemes that may be used, such as, for example, time division multiple access (TDMA) and code division multiple access (CDMA). It is intended that the base station and mobile communication device establish a radio link 106 according to the preferred air interface. The base station will be operating at a predetermined frequency or frequencies Fl, F2, F3 (108), and so on, shown here in a TDMA format for illustration purposes. Each frequency band is time divisioned into time slots, as is well known in the art. Typically, at least one of these frequency bands 110 is used to broadcast control information.

When the mobile station is within the serving area of the base station and initially not power up, upon being powered up it begins scanning known control frequencies to try and find control information indicating the presence of a serving cell. The device is pre-programmed with a list of control frequencies in use by the particular system operator to which the user of the device has subscribed to for communication service. The list or bandmap contains tuning values that are programmed into a receiver of the communication device so that the communication device can tune to the control frequency. Once so tuned, the device begins evaluating the received signal. Since the communication system is a digital communication system, the communication device begins performing the appropriate digital demodulation to obtain a frame of digital samples. If the frequency to which the receiver is presently tuned is one at which a nearby base station is transmitting, there will likely be control information in the received frame of digital samples. It should be noted that the term "frame" as used here is meant to refer to the digital samples obtained over the time period the frequency was scanned, not a proper frame usually meaning a number of time slots.

If control information is not detected, ordinarily the frequency is scanned again, and another frame of digital samples is evaluated for control information. This is because effects like fading and interference can corrupt the control information.

However, according to the present invention, the autocorrelation value of the frame of digital samples is computed twice; once at a first offset and once at a second offset.

The autocorrelation value, when evaluated at zero offset, is indicative of signal energy.

A higher value indicates a higher likelihood of the presence of a control signal being broadcast by a nearby base station. In the preferred embodiment the first offset is one and the second offset is zero. The autocorrelation ratio value R[y], when the second offset is zero (autocorrelation evaluated at the zeroeth position - r[0]) is found from the following equation:

N -l

∑ x(n) » x(n - y)

n = o

Where: N is the total number of samples;

X (n) is the nth sample; and y is the first offset.

In the preferred embodiment, where the first offset is one and the second offset is zero, the ratio is r[l]/r[0], as given in the equation above when y=l . The numerator is the autocorrelation value at the first offset and the denominator is the autocorrelation value at the second offset. The ratio of the is then compared to a threshold value.

It has been found that a ratio of autocorrelation values obtained with different offsets is useful in discriminating between frequencies that have a proper control signal and those that do not. In the preferred embodiment, through routine testing, it was found that when the ratio exceeded a value of 0.5, there was usually a signal present, even though no control information was found. This value was obtained in a TMDA system using quadrature amplitude modulation (QAM). It is expected that different communication systems may result in slightly different preferred threshold values. If this threshold was not exceeded, there was usually no control signal, thus a second scan, as is conventional, would not be an efficient use of time.

Referring now to FIG. 2, there is shown a flow chart diagram 200 of a method for rejecting control frequencies in accordance with the invention. At the start (200) the communication device is powered up. After its initial power up routine it begins the process of locating a serving cell. It first selects a tuning value corresponding to one of a plurality of possible control frequencies, and uses the value in tuning (204) the receiver to the frequency. After the receiver is tuned to the desired frequency, the device begins receiving (206) and processing digital information. The device simply demodulates whatever is received, and performs any decoding as usual to yield a frame of digital samples. The frame length is selected so that a sufficient time period is captured. The device then examines the information to determine if there is any control information present (208). The control information may include frequency offset information, timing/synchronization information, or both. If such information is found, the frequency value is saved and a valid serving cell is declared by the device's software. If no control information is found, the ratio of autocorrelation values is compared to the threshold (212). If the ratio is below the threshold, the frequency is rejected (216). If the ratio exceeds (or meets) the threshold, then if this frequency has been examined twice, it is rejected (214). If not, then the same frequency is scanned again.

Thus the invention solves the problem of repetitive scanning in while attempting to find valid serving cells. By computing the autocorrelation values at first and second offsets and comparing a ratio of the two to a threshold, non-productive frequencies and channels can be rejected. This is because the autocorrelation values can be obtained vary quickly once the segment of digital samples has been received, and in fact can be calculated on a running basis so that as soon as the last sample is received, the ratio may be performed. While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.

What is claimed is:

Claims

1. A method for rejecting a control channel while attempting to register a mobile communication device, comprising: tuning the mobile communication device to a control channel, the control channel selected from a control channel map; processing a signal received on the control channel to obtain a frame of digital samples; evaluating the frame of digital samples for control information; calculating an autocorrelation ratio of the frame of digital samples; rejecting the control channel if no control information is found and the autocorrelation ratio is below a preselected threshold.

2. A method for rejecting a control channel as defined by claim 1, wherein autocorrelation ratio is a ratio of the autocorrelation value of the frame of digital samples evaluated at a first offset and the autocorrelation value of the frame of digital samples evaluated at a second offset.

3. A method for rejecting a control channel as defined by claim 2, wherein the first offset is one and the second offset is zero.

4. A method for rejecting a control channel as defined by claim 1, wherein the preselected threshold is 0.5.

5. A method for a mobile communication device to determine if a control channel is present, comprising: selecting a tuning value from a bandmap of tuning values corresponding to control frequencies; tuning the mobile communication device to the control frequency corresponding to the tuning value; processing a signal received on the control frequency to obtain a frame of digital samples; evaluating the frame of digital samples for control information; calculating an autocorrelation ratio of the frame of digital samples; rejecting the control frequency if no control information is found and the autocorrelation ratio is below a preselected threshold.

6. A method for rejecting a control channel as defined by claim 5, wherein autocorrelation ratio is a ratio of the autocorrelation value of the frame of digital samples evaluated at a first offset and the autocorrelation value of the frame of digital samples evaluated at a second offset.

7. A method for rejecting a control channel as defined by claim 6, wherein the first offset is one and the second offset is zero.

8. A method for rejecting a control channel as defined by claim 1 , wherein the preselected threshold is 0.5.