KR20000066534A - Method for Providing Inter-Frequency Handoff in a Telephone System - Google Patents

Abstract

PURPOSE: A method for providing a hand-off inter-frequencies in a telephone system is provided to maintain a high communication quality by quickly performing the hand-off inter-frequencies when searching a frequency of an other base station in a mobile terminal. CONSTITUTION: A receiving unit(100) of a mobile terminal has a RF receiver(102) for receiving a radio signal selected among radio signals transmitted from a base station transmitting unit(200), a digital demodulator(101) for demodulating the received radio signal to an original signal, and a frequency synthesizer(103) for generating a frequency for selectively receiving one signal among radio signals of the RF receiver(102). The transmitting unit(200) of a base station has a digital modulator(202) for modulating each bit to a modulating symbol, a gain adjuster(203) for controlling an energy level of the modulated transmission symbol, and a RF transmitter(201) for varying and amplifying a frequency of the adjusted radio signal and transmitting the amplified frequency to the outside.

Description

Method for Providing Inter-Frequency Handoff in a Telephone System}

The present invention relates to a telephone system, and more particularly, to a method for controlling inter-frequency handoff in a cellular telephone system based on a code division multiple access scheme.

In general, cellular telephone systems include a service area divided into a plurality of "cells." Each cell provides a wireless communication service to a plurality of mobile terminals through a base station. In order to maintain good communication quality, when a mobile terminal moves between cell boundaries during communication, a handoff for changing a communication link or channel from one base station to another should be established in a timely manner. Therefore, the mobile terminal should be able to measure the quality of the signal provided from other base stations even during communication, and the base station system is provided with the signal quality information in order to properly handoff. Such handoff is called mobile-assisted handoff.

In particular, in the CDMA system, communication can be performed by using different offsets of common pseudo-random noise (PN) codes that spread the bandwidth of a transmission signal while using the same frequency. The CDMA system has a separate primary digital receiver called a "searcher" for searching for other CDMA signals of the same frequency and a secondary digital receiver for data demodulation. In general, CDMA systems have a plurality of secondary receivers called " Rake " receivers to receive signals transmitted over multiple paths. Therefore, the CDMA mobile terminal can measure the quality of another base station signal of the same frequency even during a call.

On the other hand, in the CDMA method, the data rate can be changed in multiple stages, but the same frequency can be used, but the wireless capacity within the same frequency can be increased by reducing the transmission power to reduce interference on other channels instead of lowering the transmission rate. For example, the amount of information of voice data changes with time; In general, about half of voice communication is used for listening, i.e., receiving voice data, and the other half is used for speaking, i.e., receiving voice data. Therefore, using a variable data rate for voice communication in a CDMA system increases the capacity by about 50%.

System capacity in a cellular telephone system can be increased by decreasing the size of the cell and increasing the number of cells in the service area. However, as the size of the cell decreases, handoff occurs too quickly when moving at high speed. Therefore, it becomes difficult for the base station to properly handle the handoff.

Therefore, a hierarchical cell structure in which a macro-cell of a large area for a terminal moving at a high speed overlaps a micro-cell of a small size for a terminal moving at a low speed. structure or layered cell structure) is used. In this cell structure, the handoff between the micro cells and the macro cell, as well as the handoff between the macro cells, must be provided according to the moving speed of the mobile terminal.

In the hierarchical cell structure, since the strength of the base station signal of the macro cell is much larger than the strength of the base station signal of the micro cell, various problems such as a cocktail party effect or an egg phenomenon will occur even in the CDMA scheme. In order to provide a hierarchical cell structure on different frequencies, an inter-frequency handoff should be provided to a code division multiple access system.

In order to perform terminal-assisted handoff between frequencies, the mobile terminal must be able to receive base station signals of different frequencies during communication, and thus additionally have to be equipped with a radio frequency RF receiver, or optionally the mobile terminal can time-division multiple signals of different frequencies. One RF receiver that receives in time-division multiplexing (TDM) mode must be used.

The conventional code division multiple access cellular telephone system described above continuously transmits a forward signal from a base station to a mobile terminal, so that the mobile terminal uses two or more RF receivers to receive base station signals of different frequencies during communication. Should have However, this method is not preferable because the price of the mobile terminal increases and the size increases.

Therefore, for a selective method in a code division multiple access system, a time interval in which a mobile terminal breaks a part of a received signal and receives a signal having a different frequency provides an inter-frequency handoff using one RF receiver. Should be used. However, such a method would result in a deterioration in the communication quality and a drop in communication would occur due to the failure to receive the required signal data.

Korean Patent Application No. 97-58539 has proposed a method for providing an inter-frequency handoff in a cellular division system of a code division multiple access method that can maintain a good communication quality state in view of the problems of the prior art.

SUMMARY OF THE INVENTION An object of the present invention is to provide an inter-frequency handoff in a telephone system that provides an inter-frequency handoff between a mobile terminal and a corresponding base station nearby when the mobile terminal performs a frequency of another base station in the patent application (97-58539). To provide.

1A to 1D are transmission timing diagrams applied to a handoff control method in a variable rate code division multiple access system.

2 is a block diagram illustrating a cellular telephone system of a code division multiple access method according to the present invention.

3A to 3H are transmission timing diagrams to which a method for providing inter-frequency handoffs according to a code division multiple access cellular telephone system according to the present invention is applied.

4A to 4G are timing diagrams illustrating a continuous frequency search interval in inter-frequency handoff control according to an embodiment of the present invention.

* Description of the symbols for the main parts of the drawings *

100: mobile terminal receiver 101: digital demodulator

102: RF receiver 103: frequency synthesizer

200: base station transmitter 201: RF transmitter

202: digital modulator 203: gain regulator

Hereinafter, a method for providing inter-frequency handoff in a code division multiple access cellular telephone system according to the present invention will be described in detail with reference to the accompanying drawings.

The inter-frequency handoff control method according to the patent application (97-58539) uses a variable rate characteristic of a transmission signal to appropriately control the number of repetitions of a transmission symbol in a frame and the position or shape of the transmission interval. After making the interval, the energy transmission level of the signal is adjusted in inverse proportion to the number of repetitions, and the mobile terminal searches for a base station signal of a different frequency through the interval in which the signal of the transmitted frame is not transmitted. Handoff is performed by sending a signal to a base station.

In addition, as shown in FIGS. 1B to 1D, the patent application does not reduce the number of repetitions of the subframe by (full rate) / (sub rate) and reduces or eliminates the number of repetitions, thereby reducing the power level of the transmission section (full rate). Power level) / (number of sub frame repetitions). That is, in the case of quad rate transmission, the conventional scheme transmits 4 times repeatedly with energy per symbol of x / 4 (FIG. 1B), whereas in the present invention, it repeats twice with energy per symbol of x / 2 (FIG. 1C). , it is transmitted without repetition (Fig. 1D) with energy per symbol of x. As described above, if the number of repetitions of the subframe is reduced or transmitted without repetition, a section in which no signal is transmitted can be created in the frame.

In one embodiment of the present invention, the transmission length of a non-full rate frame is shortened for the mobile terminal to search for another frequency with a single receiver. The modulated symbols of a non-full rate frame are transmitted with a reduced number of repetitions while maintaining the sum of the repeated symbol energies. In general, for 1 / n (n = 2,4,8) transmission, the 1 / m (m ≧ n) rate frame is repeated m / n times and transmitted with n / m symbol energy of the full rate frame. Frames of channels other than the voice channel for the mobile terminal are not transmitted during frames with no transmission interval. The system uses rate limiting to generate a nun-full rate frame.

3A shows full (n = 1) transmission in a half (m = 2) rate frame repeated twice (m / n = 2/1). 3B-3D show 1/2 transmission (n = 2) for 1/2 (m = 2) rate frame, 1/4 (m = 4) rate frame and 1/8 (m = 8) rate frame. . As shown in FIG. 3B, the half rate frame is transmitted without repetition with the symbol energy of the full rate frame (m / n = 1). In FIG. 3C, a quarter rate frame is repeated only two times (m / n = 4/2 = 2) and transmitted at 1/2 times the full rate frame (n / m = 2/4) symbol energy x do. In FIG. 3D, the 1/8 rate frame is repeated only 4 times (m / n = 8/2) and transmitted at 1/4 (n / m = 2/8) symbol energy x. 3E shows 1/4 (n = 4) transmission in a 1/4 (m = 4) rate frame, without repetition (m / n = 4/4) at the symbol energy of a full rate frame.

In general, in order to make a k / n length non-transmission interval in a frame where the number of symbol repetitions is "m + partial"-some symbols are m and the remaining symbols are m + 1, "[m * (1 minimum integer less than or equal to -k / n)] + partial "and transmitted at an energy greater than n / k times the symbol energy of the original frame. On the other hand, if the original frame is not repeated (when m is 1), if some of the symbols of the frame are not transmitted by applying this method, it can be recovered by an error correction encoding technique of the receiver. In addition, the system may use a rate limiting technique for a frame requiring a non-transmission interval to prevent such a case, so that all symbols may be transmitted even if the number of repetitions of the symbols is reduced.

3F is an embodiment according to the present invention for explaining that the total number of repetitions of a transmission frame is not an integer. As shown, the original transmission frame is repeated twice, in part three times in total, which may be referred to as (k partial) repetition (when k = 2). If half of the entire transmission frame is transmitted, the half transmission frame has a repetition number of (1+ partial). The energy of the transmitted symbol should be increased in proportion to the original signal. In this case, the signal strength is increased in inverse proportion to the reduced transmission interval compared to the entire transmission frame, or each symbol having a different number of repetitions is individually increased in inverse proportion to the reduced number of repetitions.

3G shows one partial repetition time of the original transmission frame. In this case, the transmission interval is limited to 1/2 of the total interval, and the number of repetitions is 0 + partial repetition allowing a part of the entire frame to be transmitted.

3h shows that the original transmission frame has one repetition number. The transmission of this original frame is similar to the embodiment described above. Such an embodiment may transmit a part of the entire frame to be transmitted even if the transmission interval is limited.

2 is a block diagram illustrating a transmission and reception apparatus applied to a method for controlling inter-frequency handoff in a code division multiple access cellular telephone system according to the present invention. Referring to FIG. 2, in a code division multiple access cellular telephone system, an inter-frequency handoff transmitter / receiver includes a receiver 100 of a mobile terminal and a transmitter 200 of a base station.

The receiver 100 of the mobile terminal includes an RF receiver 102 for receiving a radio signal selected from radio signals transmitted from the base station transmitter 200, a digital demodulator 101 for restoring a radio signal to an original signal, and The RF receiver 102 consists of a frequency synthesizer 103 which generates a frequency for selectively receiving any one of the wireless signals.

The transmitter 200 of the base station comprises a digital modulator 202 for configuring a transmission frame of data in units of frames and modulating each bit into modulation symbols, and a gain adjuster for controlling energy levels of the modulated transmission symbols. 203 and an RF transmitter 201 for varying and amplifying the frequency of the gain-adjusted radio signal and transmitting it out through the antenna.

The gain adjuster 203 reconstructs a modulated frame as a transmission frame by inserting a non-transmission section including a mobile terminal search section in the modulated frame. The gain adjuster may be located after the digital modulator as shown in FIG. 2, or may be located within the digital modulator, ie, prior to baseband filtering of the digital modulator.

If the transmission signal processed in the above-described manner is transmitted by the RF transmitter 201, the RF receiver 102 of the mobile terminal receiver 100 receives the radio signal. Then, the digital demodulator 101 demodulates the radio signal into a signal as shown in FIGS. 3C and 3D. The mobile terminal varies the frequency of the frequency synthesizer 103 during a period in which no signal is transmitted within the frame (hereinafter, referred to as a "frequency search interval"), and then searches for a base station signal of a different frequency, thereby eventually performing an inter-frequency terminal auxiliary handoff. Can be done.

However, the mobile terminal does not know the number and interval of frequency search sections. This problem can be solved by sending information indicating the time when the frequency search interval occurred from the base station to the mobile terminal. The frequency search interval may occur aperiodically in a single form, and / or may occur periodically or continuously for a given interval.

On the other hand, in Fig. 3A, since the frequency search interval cannot be made for the full rate, the frequency search interval can be inserted by the limitation of the encoding rate of a device such as a vocoder if necessary. The point is that the cellular division telephone system of the code division multiple access system applies a rate limit that appropriately limits the encoding rate of the vocoder which inserts the signaling data into the voice frame during communication, and the deterioration of voice quality due to this can be ignored.

The above-described transmission method may be applied to the reverse link transmitted from the mobile terminal to the base station, and the mobile terminal is used as an empty section for transmitting synchronization signals of different frequencies. Thus, base stations of different frequencies may be pre-synchronized with the mobile terminal for handoff.

The start position of the transmitted symbol may be randomized by a user long code in a specially set random code, that is, an IS-95 type code division multiple access system to minimize interference between different mobile terminals. In intensive frequency retrieval and synchronization signal transmission, non-transmission intervals may be allocated for consecutive frames at any interval. For 1 / n length continuous transmission, the starting positions of the transmitted symbols are staggered by the number of 1 / n frames per frame, for example 20 ms frames. If no part of the frame is staggered, the starting position is the beginning of the frame. Such an implementation can minimize guard time overhead for frequency exchange and code resynchronization.

4A to 4H illustrate the positional allocation of consecutive frequency search sections. The frequency search section is extended with the frame length by increasing the starting point of the frequency search section by the length of the transmission section in successive frames. For example, in the case of a quarter rate without repetition (FIGS. 4E to 4F), the start point of the frequency search interval of the current frame is the first of four transmittable intervals, and the second transmission interval is allocated in the next frame. In addition, since the starting point of the frequency search section is randomly assigned to each mobile terminal, it is possible to distribute the interference between the mobile terminals probabilistically.

As shown in FIGS. 4A-4G, indicating assigning different locations. Each start position of a transmission symbol during successive frequency search periods may be allocated to the front part and / or the rear part of the frame alternately every two frames in order to maximize the frequency search period. This allocation is appropriate for a code division multiple access system using various offsets for the frame boundaries of each mobile terminal.

As described in detail above, in the code division multiple access cellular telephone system of the present invention, the method for providing an inter-frequency handoff provides a high communication quality by quickly establishing an inter-frequency handoff when the mobile terminal searches for a frequency of another base station. It has a sustainable effect.

The method according to an embodiment of the present invention greatly improves the inter-frequency handoff requirements of TIA / EIA / IS-95 with minimal changes in the physical layer that do not require a change in the channel structure. Thus, there is no drop in voice quality that can be felt even in the worst case environments where strong frequency scanning is required. Furthermore. Power control and timing tracking are well maintained compared to the discarding of the entire frame for inter-frequency handoff.

Therefore, the present invention can be modified in various ways by those skilled in the art because a change in the frame configuration is expected according to the radio frequency signal change, etc. The present invention is not limited only to the above-described configuration and operation and the present invention It includes all modifications and equivalents falling within the scope of the technical spirit of the.

Claims (11)

Modulating data at an energy transmission level in units of frames by the number of control iterations of the symbol including a non-integer multiple; Reconstructing a transmission frame by inserting a search period into a part of the modulated frame; And performing handoff by retrieving frequency information of a base station using the transmission frame. 2. The method of claim 1, wherein the step of modulating further comprises controlling energy per bit of information of the symbol. 2. The method of claim 1, wherein the step of modulating further comprises controlling the position of the symbol. The method of claim 1, further comprising the step of controlling the formation of a transmission interval having a variable rate characteristic of the transmission frame. 2. The method of claim 1, wherein the number of repetitions is added to or subtracted from an integral multiple. The method of claim 1, wherein when the search frame is inserted into a partial section of the modulated frame, the modulated frame is inserted alternately in front of and behind the modulated frame. 7. The method of claim 6, wherein the receiving system alternately uses the front and rear portions of the modulated frame as a search section regardless of the transmission form. Forming a non-transmission interval in the transmission frame by controlling any one of the number of repetitions of the transmission symbol and the position or shape of the transmission interval in the frame using the variable rate characteristic of the transmission frame; Controlling the energy transmission level of the transmitted symbol to be inversely proportional to the number of iterations; And the mobile terminal searches for a base station of a different frequency or transmits a signal to a base station of a different frequency during the non-transmission period. 9. The method of claim 8, wherein the non-transmission interval of the transmission frame is a frequency search interval. 10. The method of claim 8, wherein the energy transmission level of the transmission symbol is determined by the distribution of the full rate energy level by the number of iterations. 9. The method of claim 8, wherein the number of repetitions is selected from integer multiples or non integer multiples.