HK1072666A - Inter-frequency hho method in a mobile communication system - Google Patents

Description

Inter-frequency hard handoff method in mobile communication system

Technical Field

The present invention relates generally to mobile communication systems, base stations, mobile stations, and inter-frequency (HHO) methods used in these devices, and more particularly to inter-frequency Hard handoff in wideband code division multiple access (W-CDMA) systems.

Background

In the related art inter-frequency HHO method, a Mobile Station (MS) monitors the reception timing of a downlink signal in the frequency of the HHO endpoint using a Common Pilot Channel (CPICH), as shown in fig. 1.

Here, the length and position of a gap (gap) during which no data is transmitted, generated during normal transmission between a base station (BTS) and a mobile station, are irregular. In contrast, the length and position of a gap (transmission gap) generated during the compressed mode during which no data is transmitted therebetween, which gap is used to enable measurement of cells of different frequencies when handover to different frequencies is performed, follow a predetermined pattern and exhibit regularity.

During compressed mode, the mobile station uses these transmission gaps to receive the CPICH being transmitted at the HHO endpoint frequency, whereby the mobile station confirms the maintenance of the reception quality at the HHO endpoint and knows the reception timing of the downlink signal.

In the above-described prior art inter-frequency HHO method, the base station can receive the downlink signal of the HHO destination frequency in advance before performing inter-frequency HHO. But the base station has no mechanism for monitoring the uplink signal in the HHO endpoint frequency. As a result, when the frequency is switched from the HHO start point to the HHO end point, the initial transmission power of the uplink signal at the HHO end point may not be sufficient to ensure appropriate reception quality. Further, since the reception timing in the HHO end frequency is unknown, a considerable time is required to obtain the uplink signal, whereby the communication quality is degraded and the reception is disturbed.

Disclosure of Invention

An object of the present invention is to solve the above-described problems and to provide a mobile communication system, a base station, a mobile station, and an inter-frequency HHO method used in these apparatuses for enabling smooth and stable frequency conversion.

The mobile communication system of the present invention is a mobile communication system that performs CDMA (code division multiple access) communication between a mobile station and a base station, wherein the CDMA communication includes a compressed mode, the compressed mode is an intermittent communication mode including a transmission gap in which no communication is performed; the mobile communication system includes:

a mobile station having means for transmitting a training signal in an uplink direction using the transmission gap; and

and a base station having means for performing training of reception timing and transmission power using the training signal.

The inter-frequency HHO method of the present invention is an inter-frequency HHO (hard handover) method for a mobile communication system in which CDMA (code division multiple access) communication is performed, the CDMA communication including a compressed mode which is an intermittent communication mode having a transmission gap in which communication is not performed; the inter-frequency HHO method comprises the following steps:

transmitting a training signal in an uplink direction from a mobile station using the transmission gap; and

and training the receiving timing and the transmission power in the base station by using the training signal.

In other words, the mobile communication system of the present invention provides a method for realizing stable inter-frequency HHO (hard handover) in a W-CDMA (wideband code division multiple access) system.

More specifically, in the mobile communication system of the present invention, in communication between a base station (BTS) and a Mobile Station (MS), an uplink training signal is transmitted at a frequency of a HHO destination at a transmission gap during a compressed mode.

These training signals enable training prior to inter-frequency HHO for the reception of HHO endpoint frequencies by the base station, whereby uplink reception timing and transmission power can be confirmed prior to inter-frequency HHO. Thereby, smooth and stable inter-frequency HHO, i.e., frequency conversion, can be achieved.

Drawings

Fig. 1 is a timing diagram illustrating an operation of a mobile communication system according to an example of the related art;

fig. 2 is a block diagram showing the structure of a mobile communication system according to an embodiment of the present invention;

fig. 3 is a block diagram showing the structure of the base station of fig. 2;

fig. 4 is a block diagram showing the structure of the base station of fig. 2;

fig. 5 is a block diagram showing the structure of the mobile station of fig. 2;

fig. 6 is a timing diagram illustrating the operation of the mobile communication system according to the embodiment of the present invention;

fig. 7 is a flowchart illustrating an operation of a mobile communication system according to an embodiment of the present invention;

fig. 8 is a flowchart illustrating an operation of a mobile communication system according to an embodiment of the present invention;

fig. 9(a) to 9(d) are diagrams for explaining inter-frequency HHO;

fig. 10(a) and 10(b) show data exchange between different base stations.

Detailed Description

The following description refers to the accompanying drawings, which relate to embodiments of the present invention. Fig. 2 is a block diagram showing a system configuration of a mobile communication system according to an embodiment of the present invention. In fig. 2, a mobile communication system according to an embodiment of the present invention is composed of: base stations (BTS)1 and 2, a Mobile Station (MS)3, and a base station controller (RNC: radio network controller).

The mobile station 3, after completing monitoring of the downlink signal, reports completion of monitoring of the downlink signal to the base station 1 using a HHO (hard handover) start point frequency, and reports the time (frame and slot (TS)) of a transmission gap during which transmission of the uplink training signal will start. The transmission gap is reported from the mobile station 3 to the base station 2 by the base station 1 and the base station controller 4 ((1) in fig. 2).

When the specified time arrives, the mobile station 3 transmits an uplink training signal to the base station 2 at the HHO end frequency at the transmission gap during the compressed mode in the communication between the base station 1 and its own station ((2) in fig. 2).

If the base station 2 does not receive the uplink training signal from the mobile station 3 at a specified time, the base station 2 reports a NACK (negative acknowledgement) signal to the mobile station 3 through the base station control station 4 and the base station 1, and if the uplink training signal is received, transmits an ACK (acknowledgement) signal to the mobile station 3 ((3) in fig. 2).

Thus, if the base station 2 can receive the uplink training signal from the mobile station 3, the base station 2 can implement training regarding HHO end-point frequency reception before performing inter-frequency HHO, can check uplink reception timing and transmission power before performing inter-frequency HHO, and can implement smooth and stable inter-frequency HHO, that is, frequency conversion. Subsequently, the mobile station 3 and the base station 2 start communicating with each other ((4) in fig. 2).

Fig. 3 is a block diagram showing the configuration of the base station 1 in fig. 2. In fig. 3, the base station 1 is composed of the following parts: receiving section 11, search/decoding section 12, HHO control section 13, ACK/NACK transmitting section 14, Local Oscillator (LO) 15, and transmitting section 16.

Fig. 4 is a block diagram showing the structure of the base station 2. In fig. 4, the base station 2 is composed of the following parts: reception section 21, search/decoding section 22, uplink training signal reception section 23, training signal information storage section 24, HHO control section 25, LO 26, and transmission section 27. Although not shown in the drawings, the base stations 1 and 2 also have each part shown in fig. 3 and 4. However, a redundant part such as a transceiving unit may be defined as one unit.

Fig. 5 is a block diagram showing the structure of the mobile station 3 of fig. 2. In fig. 5, the mobile station 3 is composed of the following parts: a receiving unit 31, a search/decoding unit 32, a downlink signal monitoring unit 33, a HHO control unit 34, an uplink training signal transmitting unit 35, an LO 36, and a transmitting unit 37.

Fig. 6 is a timing diagram illustrating the operation of the mobile communication system according to the embodiment of the present invention; fig. 7 and 8 are flowcharts illustrating the operation of the mobile communication system according to the embodiment of the present invention; fig. 9(a) to 9(d) are diagrams for explaining inter-frequency HHO; fig. 10(a) and 10(b) are diagrams for explaining data exchange between different base stations. A mobile communication system according to an embodiment of the present invention will be described with reference to fig. 2 to 10.

First, a description is given of procedures for inter-frequency HHO in W-CDMA (wideband code division multiple access) communication. W-CDMA communication is a third generation mobile communication method discussed in 3GPP (third generation partnership project).

In general, the base station 1 occupies a plurality of frequencies, and communicates with the mobile station 3 using any one of the frequencies. However, as shown in fig. 9(a), when the mobile station 3 communicating in cell a at frequency f1 moves to cell B, which is the communication area of base station 1 and cell B is the communication area of base station 2 only at frequency f2, the mobile station 3 has to change the reception frequency from frequency f1 to frequency f 2. This operation is called "inter-frequency HHO".

There are two ways to change the frequency f1 to the frequency f 2: one method is that a change from the frequency f1 to the frequency f2 occurs within the range of the base station 1 (see fig. 9 (b)); one method is that a change from the frequency f1 of the base station 1 to the frequency f2 of the base station 2 occurs within the overlapping area of the areas of the base station 1 and the base station 2 (see fig. 9 (c)). Both methods may be employed.

Further, the present embodiment can also be applied to change the frequency from the frequency f1 to the frequency f2 in the following cases: for example, when the mobile station 3 communicates at the frequency f1 in the cell a as the communication area of the base station 1 and the base station 1 cannot accommodate the mobile station 3, or when processing at the frequency f1 becomes impossible due to a failure or maintenance (see fig. 9 (d)). In these cases, the above-described method may be used, in which the change from the frequency f1 to the frequency f2 occurs in the above-described area of the base station 1.

Before the mobile station 3 achieves inter-frequency HHO, a mode of performing intermittent communication, which is referred to as "compressed mode", is started. Within a transmission gap, which is a time interval in which communication is not performed in such a compressed mode, the frequency is changed from the HHO start frequency (f1) to the HHO end frequency (f2), and the mobile station 3 receives the CPICH (common pilot channel), which is a reference signal continuously transmitted by the base station 2 on all frequencies. In this way, despite the shift to the frequency of the HHO endpoint, it can be confirmed that the same reception quality as before the shift is obtained, i.e., the power output enables the same reception quality to be obtained; and the signal reception timing at the HHO termination is checked.

The compressed mode is an intermittent communication mode having a time interval during which no communication is performed, as compared with the normal communication mode, as shown in fig. 1.

Normally, the mobile station 3 has only one LO 36 and therefore cannot receive the CPICH of the HHO destination transmitted on the HHO destination frequency f2 while communicating at the HHO start frequency f 1.

However, as shown in fig. 1, by providing transmission gaps, which are time intervals during which no communication is conducted, and by switching from the HHO start frequency f1 to the HHO end frequency f2 within these time intervals, monitoring of the downlink signal in the HHO end frequency f2 becomes possible.

Details regarding compressed mode are described in the 3GPP (third generation partnership project) standard TS25.212V3.5.04.4 "compressed mode" and TS25.215 V3.5.06.1.1 "compressed mode".

Even in the normal mode, intermittent communication occurs in which communication in a portion lacking data for transmission is suspended. However, in intermittent communication in the normal mode, the position and length of an interval during which transmission is suspended depend on the condition of transmitted data; the compressed mode differs from this in that the transmission of data is explicitly suspended in compliance with set rules which follow a predetermined pattern independent of the data.

For downlink transmission by this method, a signal of appropriate reception quality can be received at the mobile station 3 despite the change from the HHO starting frequency f1 to the HHO ending frequency f 2.

However, for uplink transmission, the base station 2 lacks a means for monitoring the signal of the HHO endpoint frequency f2 sent by the mobile station 3, and as a result, there arise problems in that the initial uplink transmission power at the HHO endpoint may not be able to ensure appropriate reception quality, and since the signal reception timing at the HHO endpoint is unknown, time is required to acquire the uplink signal, as compared with downlink transmission.

These problems increase the likelihood that interference or reception quality of uplink communications will degrade when transitioning from the HHO starting frequency f1 to the HHO ending frequency f2, or further, the likelihood that the extended time required to acquire the received signal will result in a loss of communications.

Fig. 6 shows a state when HHO between the mobile station 3 and the base station 2 is trained using the HHO end frequency (f2) at the position of the transmission gap in the compressed mode in the W-CDMA communication method.

However, as shown in fig. 6, after the monitoring of the downlink signal is completed, an uplink signal for training (hereinafter referred to as an uplink training signal) may be transmitted at the HHO end frequency in the transmission gap that is normally used by the mobile station 3 to monitor the downlink signal in the HHO end frequency, thereby enabling training at the base station 2.

The mobile station 3, after having completed monitoring of the downlink signal by means of the downlink signal monitoring unit 33, reports to the base station 1, using the HHO starting point frequency, the completion of monitoring of the downlink signal and the time (frame and Time Slot (TS)) of the transmission gap in which transmission of the uplink training signal from the uplink training signal transmitting unit 35 is to be started. The content of the report is reported from the base station 1 to the base station control station 4, and from the base station control station 4 to the base station 2.

At the time designated to the base station 2, the mobile station 3 transmits an uplink training signal with a transmission power (txshow) designated in advance from the uplink training signal transmitting unit 35, and the base station 2 attempts to receive the signal.

If the base station 2 is able to receive the uplink training signal at the uplink training signal receiving unit 23, the reception timing (Pos _ rv) and the reception SIR (signal to interference ratio) (SIR _ rv) at that time are measured by the uplink training signal receiving unit 23, and the result is held in the uplink training signal information holding unit 24.

The uplink training signal reception unit 23 next calculates the difference { Δ SIR ═ (SIR _ trgt) - (SIR _ rv) } between the reception SIR and the target SIR (SIR _ trgt) necessary to obtain a predetermined appropriate reception quality, and reports an ACK signal to the base station 1 through the base station control station 4 using the HHO origin frequency, which is then reported from the ACK/NACK transmission unit 14 of the base station 1 to the mobile station 3. The ACK signal contains the difference between the target SIR and the received SIR (Δ SIR).

On the other hand, if the uplink training signal reception unit 23 of the base station 2 fails to receive the uplink training signal during the transmission gap during which the uplink training signal is reported, the base station 2 reports a NACK signal to the base station 1 through the base station control station 4 using the HHO starting point frequency, and then the ACK signal is reported from the ACK/NACK transmission unit 14 of the base station 1 to the mobile station 3.

The mobile station 3 adds a predetermined offset power (═ Δ txshow) to the transmission power of the preceding uplink training signal after having received the NACK signal, and transmits the uplink training signal at this power. This retransmission of the uplink training signal is continued until an ACK signal can be received.

In other words, when the NACK signal is received N times, the transmission power of the (N + 1) th uplink training signal can be expressed by the following equation:

transmission power of TxPoint + DeltaTxPoint N

However, a large number of retransmissions (retx) of the uplink training signal and the resulting excessive transmission power may result in an increase in the interference level to other users and degrade the reception characteristics, and thus when the number of retransmissions reaches a predetermined value (Nmax), the retransmission is performed at the transmission power of the uplink training signal returned to txshow.

After having received the ACK signal, the mobile station 3 changes the frequency to the HHO destination frequency in accordance with the inter-frequency HHO, and starts transmission and reception. At this time, the value of the uplink transmission power may be calculated by the following equation:

TxPoint + Delta SIR

In this formula, txshow is the transmission power of the uplink training signal when the ACK signal can be received.

If uplink transmission is performed at this transmission power by means of training by the above-described uplink training signal, reception at the base station 2 at a target SIR (SIR _ trgt) is possible, wherein the transmission power at which sufficient reception quality can be obtained.

Further, the reception timing is found to be Pos _ rv by the previous training, so that the concentrated search by the search/decode unit 22 in the vicinity of the timing enables the uplink signal to be quickly received at the base station 2 with good reception quality.

Next, with reference to fig. 7 and 8, the operation of the mobile communication system of the embodiment of the present invention is explained. As shown in fig. 1, the mobile station 3 performing inter-frequency HHO first obtains the reception timing of the downlink signal at the HHO destination frequency by receiving the CPICH (common pilot channel), which is a reference signal transmitted by the base station 1 at all times on all frequencies within the transmission gap interval in the compressed mode, and the mobile station 3 completes the monitoring of the downlink signal by the downlink signal monitoring unit 33 (step S1 in fig. 7).

The mobile station 3 next reports completion of monitoring of the downlink signal and the position of the transmission gap (frame number, slot number) during which the uplink training signal is transmitted to the base station 2 through the base station 1 and the base station control station 4 at the HHO starting frequency (step S2 of fig. 7).

As shown in fig. 6, the mobile station 3 then sets the transmission power txplow to an initial value txplow _ ini at the reported transmission gap position (step S3 in fig. 7), and transmits an uplink training signal (step S5 in fig. 7). At this time, the mobile station 3 initializes the retransmission number retx to 0 (step S4 in fig. 7).

The base station 2 waits for the uplink training signals at the transmission gap positions reported from the mobile station 3, and if it fails to receive these signals (steps S11 to S13 in fig. 8), reports a NACK signal to the mobile station 3 at the HHO starting frequency (step S14 in fig. 8).

The mobile station 3 compares the retransmission number retx with the predetermined maximum retransmission number Nmax after having received the NACK signal (steps S6 and S7 in fig. 7). If the number of retransmissions retx is smaller than the maximum number of retransmissions Nmax, the mobile station 3 adds the transmission power Δ txshow to the previous transmission power txshow and adds 1 to the number of retransmissions retx (step S8 in fig. 7).

On the other hand, if the retransmission number retx is equal to or greater than the maximum retransmission number Nmax, the mobile station 3 sets the transmission power txshow to the initial value txshow _ ini and initializes the retransmission number retx to 0 (step S9 in fig. 8). This is done to prevent an adverse effect on the reception characteristics of the further mobile station 3 that would occur when the transmission power txshow is increased indefinitely.

After the mobile station 3 updates the transmission power txnow and the retransmission number retx in this way, the uplink training signal is transmitted again at the transmission gap position at the transmission power txnow (step S5 in fig. 7).

If the base station 2 receives the uplink training signal (step S13 in fig. 8), the base station 2 calculates two values Pos _ rv (reception timing of the uplink training signal) and SIR _ rv (reception SIR of the uplink training signal), and stores these values in the training signal information holding unit 24 (step S15 in fig. 8).

The base station 2 compares the target SIR (SIR _ trgt) and the reception SIR (SIR _ rv) that can obtain a predetermined appropriate uplink reception characteristic, and calculates the difference (Δ SIR) between the target SIR and the reception SIR by the following formula (step S16 in fig. 8):

ΔSIR＝(SIR_trgt)-(SIR_rv)

the base station 2 transmits an ACK signal to the mobile station 3 at the HHO starting frequency (step S17 in fig. 8). Here, information on the difference (Δ SIR) between the target SIR and the reception SIR is included in the ACK signal.

The mobile station 3 performs inter-frequency HHO, switches the frequency, and then starts receiving the downlink signal after having received the ACK signal, and transmits the uplink signal at the transmission power shown by the following formula (step S10 in fig. 7):

uplink transmission power TxPoint + Δ SIR

Base station 2 performs inter-frequency HHO and searches for an uplink signal from mobile station 3 in the vicinity of the timing (Pos _ rv) at which the uplink training signal is received on the above-described HHO endpoint frequency (step S18 in fig. 8), whereby base station 20 can quickly receive the uplink signal. The control regarding the above-described inter-frequency HHO (including the training control prior to the inter-frequency HHO) is realized by the HHO control units 13, 25, and 34 of the base stations 1 and 2 and the mobile station 3.

As for the method shown in fig. 9c, when the base station 1 that transmits the HHO starting frequency is different from the base station 2 that transmits the HHO ending frequency, information such as success (ACK) or failure (NACK) of receiving the uplink timing signal and Δ SIR is exchanged between the base station 1 and the base station 2 by the base station control station 4 that is at a higher level than the base station 1, as shown in fig. 10(a) and 10 (b).

Thus, when inter-frequency HHO is performed in the mobile communication system of the W-CDMA method, an uplink training signal is transmitted at the HHO end frequency at the transmission gap position in the compressed mode, and by means of this uplink training signal, the base station 2 trains under the HHO end frequency before performing inter-frequency HHO to enable smooth and stable frequency switching, with respect to reception of the HHO end frequency.

As described in the above description, according to the present embodiment, in a mobile communication system including a compressed mode which is an intermittent communication mode having transmission gaps during which communication is not performed in communication between a mobile station and a base station, the transmission gaps are used for transmitting training signals in the uplink direction from the mobile station, and these training signals are used for training of reception timing and transmission power, whereby the present embodiment obtains an effect of enabling smooth and stable frequency conversion.

Claims (20)

1. A mobile communication system in which code division multiple access communication is performed between a mobile station and a base station, the code division multiple access communication including a compressed mode which is an intermittent communication mode having a transmission gap during which communication is not performed; the mobile communication system includes:

a mobile station having means for transmitting a training signal in an uplink direction using the transmission gap; and

a base station having means for performing training of reception timing and transmission power using the training signal.

2. The mobile communication system of claim 1, wherein the mobile station further comprises:

means for monitoring downlink signals in the transmission gaps; and

means for reporting, after completion of the monitoring of the downlink signal, a location of a transmission gap during which transmission of the training signal will begin to the base station.

3. The mobile communication system as claimed in claim 2, wherein the means for monitoring downlink signals receives a common pilot channel which is a reference signal transmitted by the base station at all times on all frequencies and monitors downlink signals.

4. The mobile communication system of claim 1, wherein the mobile station further comprises:

means for retransmitting the training signal at a power obtained by adding a predetermined offset power to a transmission power used when a previous training signal is transmitted, after receiving a notification from the base station that the training signal was not successfully received.

5. The mobile communication system according to claim 4, wherein said means for performing training measures and saves a reception timing and a received signal-to-interference ratio at that time after successfully receiving said training signal.

6. A base station for code division multiple access communication with a mobile station, wherein said code division multiple access communication includes a compressed mode, said compressed mode being an intermittent communication mode having a transmission gap during which no communication is performed; the base station includes:

means for performing training of reception timing and transmission power by means of a training signal in an uplink direction, wherein the training signal is transmitted from the mobile station using the transmission gap.

7. The base station of claim 6, further comprising means for receiving the training signal based on a reported position of the transmitted gap from the mobile station.

8. The base station of claim 6, further comprising means for reporting to a mobile station a failure to receive the training signal.

9. The base station of claim 7, further comprising means for reporting to a mobile station a failure to receive the training signal.

10. The base station of claim 6, wherein said means for performing training measures and stores a reception timing and a received signal-to-interference ratio at that time after successfully receiving said training signal.

11. A mobile station that performs code division multiple access communication with a base station, wherein the code division multiple access communication includes a compressed mode, which is an intermittent communication mode having a transmission gap during which communication is not performed; the mobile station includes:

means for utilizing the transmission gap to transmit a training signal in an uplink direction.

12. The mobile station of claim 11, further comprising:

means for monitoring downlink signals in the transmission gaps; and

means for reporting to the base station, after completion of monitoring of the downlink signal, a location of the transmission gap during which transmission of the training signal will begin.

13. The mobile station of claim 12, wherein the means for monitoring downlink signals receives a common pilot channel, which is a reference signal that the base station transmits at all times on all frequencies, and performs monitoring.

14. The mobile station of claim 11, further comprising:

means for retransmitting the training signal at a power obtained by adding a predetermined offset power above a transmission voltage used when transmitting a previous training signal, after receiving a notification from the base station that the training signal was not successfully received.

15. The mobile station of claim 11, wherein the base station measures and stores a reception timing and a received signal-to-interference ratio at that time after successfully receiving the training signal.

16. A method of performing an inter-frequency hard handover in a mobile communication system in which code division multiple access communication between a mobile station and a base station is performed, the code division multiple access communication comprising a compressed mode, the compressed mode being an intermittent communication mode having a transmission gap during which communication is not performed; the inter-frequency hard handoff method comprises the following steps:

transmitting a training signal in an uplink direction from the mobile station using the transmission gap; and

training reception timing and transmission power in the base station by the training signal.

17. The inter-frequency hard handoff method of claim 16 further comprising the steps of:

monitoring, in the mobile station, a downlink signal within the transmission gap; and

reporting, from the mobile station to the base station, a location of the transmission gap during which transmission of the training signal will begin, after the monitoring of the downlink signal is completed.

18. The inter-frequency hard handoff method of claim 17, wherein the step of monitoring a downlink signal is a step of receiving a common pilot channel, which is a reference signal transmitted by the base station at all times, and performing monitoring.

19. The inter-frequency hard handoff method of claim 16 further comprising the steps of:

in response to a notification from the base station that the training signal reception was unsuccessful, transmitting the training signal from the mobile station at a power obtained by adding a predetermined offset power above its transmission power.

20. The inter-frequency hard handover method of claim 16, wherein the training step is a step of measuring and storing a reception timing and a received signal-to-interference ratio at that time after the training signal is successfully received.