A method and apparatus for locating a digital control channel in a radiocommunication system

Positioning method and device of digital control channel in wireless communication system

The specification of US Patent Application No. 08/147,254 entitled "A Method for Communicating in a Wireless Communicating System" filed on November 1, 1993 for the related application is used as reference material in this application.

Background technique

The present invention mainly relates to a wireless communication system with a control channel, and particularly relates to the positioning of a digital control channel in such a system.

Traditional wireless communication systems have always been analog. However, the rapid development of wireless communication systems forces system designers to find a way to increase the capacity of the system under the condition that the communication quality does not drop below the user tolerance. One way to increase capacity is to change from analog communication to digital communication technology. In North America, the transition from the analog AMPS system to the digital system (D-AMPS) has achieved this change, and its new standard is IS-54B.

Before the introduction of digital technology, a large number of existing user infrastructure equipment could only work in the analog domain. Therefore, the dual-mode (analog and digital) standard was adopted in IS-54B to provide analog compatibility while realizing digital communication capabilities. . For example, under the IS-54B standard, there are both analog and digital service channels. The operator of the system can replace the analog service channel with a digital service channel (and vice versa), and dynamically adjust the changing service between analog and digital users. mode.

In addition to the traffic channel, a control channel is also provided in the wireless communication system to transmit call setup data messages between the base station and the mobile station. According to IS-54B, for example, there are 21 dedicated analog control channels, and fixed frequencies are assigned to the A and B carrier frequencies of these channels. They are called "dedicated" analog control channels because they always use the same frequency. Therefore, when the mobile station needs to monitor the data transmitted on the channel, it is easy to locate it.

For example, in the idle state (that is, powered on but not used), a mobile station in the IS-54B system is tuned to the strongest control channel on its known frequency (usually a control channel in the cell where the mobile station is located at the time). Channel) and continuously monitor it, and can receive or initiate a telephone call through the corresponding base station. When an idle mobile station moves between cells, it will eventually "lost" the radio connection on the control channel of the "old" cell and tune to the control channel of the "new" cell. The initial tuning and conversion of the control channel is done automatically. This is done by scanning all the control channels on the known operating frequency in the cellular system to find the "best" control channel. When a control channel with good reception quality is found, the mobile station remains tuned to this channel until quality degradation occurs again. In this way, all mobile stations "keep in touch" with the system almost all the time.

For this hybrid system solution, it can be expected that the number of analog users will gradually decrease and the number of digital users will increase. In the end, it is possible to replace all analog traffic channels with digital traffic channels. At this point, the current dual-mode device can be replaced with a cheaper single digital mobile device. However, this single digital mobile device cannot scan the analog control channels currently available in the IS-54B system.

Therefore, it is necessary to provide a digital control channel in a wireless communication system for supporting digital technologies such as the above-mentioned IS-54B. In addition to compatibility, digital control channels are also required in consideration of other factors in the above-mentioned applications, for example, in order to enhance the sleep mode of the mobile device, so as to extend the life of the battery. Since IS-54B uses a dedicated control channel, it is also hoped that there will be greater flexibility in the number of control channels allocated and the frequency of the control channels in order to increase the system capacity and support the layering of microcells, picocells, etc. Mesh structure. However, the problem is that if the digital control channel is not located on a known frequency, how can the remote device locate these control channels that need to be monitored? A conventional wireless communication system called GSM that has been used in Europe at present is already an all-digital system. In this system, the mobile device directly scans all available channels until a digital control channel is identified. However, for systems with a large number of channels, this positioning technique is too slow. Not only that, due to the "handoffs that occur when the mobile device moves from one cell to another, the digital control channel positioning problem after the end of the call will become more serious. This is because the mobile device is here. The time can no longer use the control channel positioning information that it has been monitoring before the call.

SUMMARY OF THE INVENTION The present invention overcomes the above and other defects and limitations in the existing systems and methods. According to the present invention, the positioning of the digital control channel can be simplified. The method can be based on finding one on one or a group of specific channels. The relative possibilities of the digital control channel define a search pattern and provide digital control channel sounding information on other channels.

According to an embodiment of the present invention, the channels are organized into probability groups arranged according to the relative probability of finding the digital control channel in each group. The mobile device can search for a digital control channel in the probability group at the highest level, and then search in the probability group at the second highest level, and so on, until a digital control channel is found.

According to other embodiments of the present invention, information can be provided on other channels, such as traffic channels or analog control channels, and the information can direct the mobile station to a specific channel where the digital control channel can be found, or to direct it to a specific channel in which a digital control channel can be found. A group of digital control channels. In this way, the positioning process can be simplified compared with the way of searching for channels sequentially.

According to another embodiment of the present invention, the mobile device can receive information about the location of the digital control channel during the termination of the call. In this way, the mobile device does not need to repeat the process of trying to determine the position of the digital control channel immediately after the call ends. This method is particularly effective when the mobile station moves into a new cell during the call.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other objectives, features and advantages of the present invention can be further understood with reference to the following detailed description in conjunction with the accompanying drawings. Among them: Figures 1(a)-1(d) are examples of probability group schemes according to the present invention. Figure 2 (a) shows an example of the downlink digital traffic channel according to the present invention; Figure 2 (b) shows the conventional uplink digital traffic channel; Figure 2 (c) shows the channel according to the present invention An example of the time slot format of the uplink digital control channel; Figure 2 (d) shows an example of the time slot format of the downlink digital control channel according to the present invention; Figure 3 is a flow chart, which shows that according to the present invention An exemplary digital control channel detection method of the invention; Fig. 4(a) is a table for explaining a message format according to an example of the present invention; Fig. 4(b) is a table for more detailed explanation Fig. 4(a) shows the items of the table; and Fig. 5 is a block diagram of a wireless communication system according to the present invention.

Detailed Disclosure of the Invention According to the illustrative embodiments of the present invention, multiple technologies can be used simultaneously or individually to accelerate the positioning of the digital control channel by the mobile station.

One method that can help mobile stations search for digital control channels is to group effective frequencies into groups, and these groups are assigned with different probabilities to reflect the relative probability of finding a digital control channel in each group. In this way, the time spent by the mobile station for detection can be significantly shortened. The two tables in Figures 1(a) and 1(b) respectively represent how channels in the A-band and B-band are allocated according to different relative probabilities in order to support the detection of digital control channels. Similarly, Figures 1(c) and 1(d) show another such example. For example, before the mobile station receives any digital control channel locator (digital control channel locator) information, the mobile station can use this scheme as a starting point for digital control channel locating (see below). Once the mobile station receives the digital control channel locator information, it can replace the channel group probability chart described here with the received information.

Another solution to help the mobile station search for the digital control channel (DCC) is to place the digital control channel positioning information on other channels than the digital control channel. Therefore, when the mobile station is searching for DCC, it reads this channel Time, you can speed up the search. For example, the digital control channel locator (DL) is a parameter that can be placed on the digital traffic channel, and it provides information that helps the mobile station to find the digital control channel. The DL indicates to the mobile station the RF channel that carries the digital control channel. According to the number of effective bits used to represent the DL and the number of channels in the system, the DL can uniquely identify the channel belonging to the digital control channel, or can limit the search to a certain subset of possible channels. For example, if there is a 7-bit DL, the DL values 1, 2, 3...127 can be transformed into channel numbers 1-8, 9-16, 17-24,...1009-1016, respectively. In this way, for example, if the channel number of a digital control channel is 10, then the DL value 2 is transmitted on each digital service channel of the same cell. The DL value 0 is not used as any digital control channel positioning information, but used to indicate that the system does not provide DL information.

Once the DL value is determined, it is coded to form a CDL, which is transmitted on the digital traffic channel with bit positions 314 to 324 in the TDMA time slot, for example. This situation is shown in Figure 2(a), which illustrates an exemplary digital traffic channel base-to-mobile slot format. The number at the bottom of the data field represents the number of bits in the field. The fields other than the CDL are the same as the traditional IS-54B base station-to-mobile service channel time slot, which can be used for reference by interested readers. Examples of the time slot formats of the uplink digital traffic channel, uplink digital control channel, and downlink digital control channel are shown in Figures 2(b), 2(c) and 2(d) respectively. It can be used for reference in the discussion. Those familiar with the art can see that other bit positions can also be used as the CDL field in the time slot. However, the advantage of this specific position is that it corresponds to the IS-54B downlink digital traffic channel time slot RSVD field that has not been used before. In this way, the change of IS-54B air interface can be minimized. In the IS-54B specification, these RSVD bits are reserved as 0, and they are used to indicate that no positioning information is provided. Another possibility is to set the DL in the layer 2 frame of the DTC.

According to the exemplary embodiment of the present invention, all channel numbers are valid candidates for digital control channel allocation. Considering that DL does not have to uniquely identify a specific channel number, it is hoped that a priority scheme can be established, which can be used to search for digital control channels in each channel group identified by DL. A mobile station that receives a DL value related to a particular channel group will not automatically search all channels, but will search for digital control channels in the group according to this priority scheme. In this way, if the DL value is 1, the mobile station can start from channel 8, followed by channel 7, and so on, to check channels 8 to 1 in order to find the digital control channel.

The above describes examples that can be used to accelerate the positioning of the digital control channel, and other embodiments of the present invention will be described below, and the technical solutions therein are applicable to various occasions. For example, referring to the flowchart in Figure 3, suppose a mobile station is looking for a digital control channel on the A-band carrier frequency of the IS-54B system. As shown in box 10 in the figure, assuming that there is no other available information in the mobile station, the mobile station first checks in the highest probability group, for example, check group 1 with channel number 1-26 in Fig. 1(a). In this channel group, the mobile station selects the channel to be read first according to certain predetermined criteria. For example, as shown in FIG. 20, such a standard may be the measured channel signal strength within the probability group. Another way is to read the channels in the group in numerical order. In this way, the mobile station measures the signal strength (RSSI) of channels 1-26 and arranges these channels in order from strong to weak. Here, the channel "X" is used to indicate the channel with the highest signal strength, and this channel is selected to be read in box 30. If it is recognized in block 40 that the selected channel "X" is an analog channel, that is, an analog control channel or an analog traffic channel, the flow returns to block 30 to select the next-highest channel to read. On the other hand, if the channel "X" is a digital channel, the flow proceeds to a decision block 50 to identify whether the digital channel is a control channel or a traffic channel. This recognition can be done in a variety of ways.

In order to distinguish between the digital service channel and the digital control channel, it is also necessary to refer to the IS-54B standard for explanation. As shown in Figure 2(a) and 2(c), although the downlink time slot format of IS-54B digital traffic channel and digital control channel have structural common points, there are still some differences that can be used to distinguish Digital control channel and digital service channel. First of all, due to the difference in channel coding between the Digital Check Color Code (DVCC) and Super Frame (SFP) fields, there are always 4 bits different in the 12 bits of each pair of CDVCC and CSFP codewords, regardless of the CDVCC or CSFP code. Which one of the words is sent by the base station (however, the error caused by the damage of the wireless channel will cause the code word received by the mobile station to be different from the code word sent). Specifically, the 4 check bits of CSFP are opposite to the check bits of CDVCC. Secondly, the CDVCC content on the digital traffic channel is fixed from one time slot to the next time slot, while the CSFP content on the digital control channel changes from time slot to time slot in a predictable manner.

Another available identification method is that the channel coding and interleave used on the digital service channel are different from those used on the digital control channel and have nothing to do with the DTC service (voice or FACCH). For example, the digital traffic channel may use 1/2 rate coding, while the digital control channel uses 1/4 rate coding. In addition, the IS-54B SACCH and RESERVED fields on the digital control channel have different functions. The actual functions of the fields shown in Figures 2(a) to 2(d) have nothing to do with the content of this application. However, the functions of these fields are explained in more detail in the above-mentioned related applications.

If the channel "X" is a digital control channel, the positioning procedure has reached its goal and the flow goes to the END box. On the other hand, if the channel "X" is a digital traffic channel, the procedure moves to block 60, where it is determined whether the digital traffic channel includes digital control channel positioning information, such as the aforementioned DL field. If not, the mobile station reads another channel, and the flow returns to block 30. If so, this information is used in block 70 to find the digital control channel.

As an alternative to the above-mentioned probability group scheme, in hybrid systems such as IS-54B that still have analog control channels, digital positioning information can be placed on these channels. For example, digital control channel information can be placed on each of the 21 dedicated analog control channels on the two carrier frequencies of IS-54B. The mobile station can first tune to the strongest effective analog control channel, determine the position of the digital control channel with respect to the cell, and then directly tune to this digital control channel.

According to another embodiment of the present invention, the information on the digital control channel can also be provided to the mobile station when the mobile station experiences the termination of a call. Generally, one of the messages associated with the termination of the call transmitted from the base station to the mobile station is a RELEASE message, which informs the mobile station to search for DCC on a designated frequency. If the digital control channel positioning information related to the cell where the mobile station is located is placed on the RELEASE message at the end of the call, the mobile station does not need to go through any steps to locate a new digital control channel. In this way, no matter whether the mobile station is "switched" during the original connection, it can obtain the location information of the digital control channel.

Figures 4(a) and 4(b) show example message formats that can be used to provide information in a RELEASE message to the mobile station for searching for a digital control channel. Figure 4(a) shows an overview of an exemplary RELEASE message format, which includes an O-type (optional) DCC information field, which has 29 bits. Figure 4(b) shows an example of this 29-bit format. The "Parameter Type" (Parameter Type) field indicates that the field is a DCC information field. The "Number of Values" field indicates how many information units are in the message. The "Channel" field indicates the frequency on which a control channel can be found, and the "DVCC" field provides digital check color code information. Those familiar with the art should know that this signal format is just an example, it can illustrate a method of providing digital control channel positioning information, and other formats can also be used.

Fig. 5 is a block diagram of a cellular mobile radio telephone system according to an embodiment of the present invention, which can be used to implement the above-mentioned method. The system shown in this example has a base station 110 and a mobile station 120. The base station includes a control and processing device 130 connected to the MSC 140, which is connected to a public switched telephone network (not shown).

The base station 110 for one cell includes a plurality of voice channels managed by a voice channel transceiver 150, and the transceiver 150 is controlled by the control and processing device 130. Each base station also includes a control channel transceiver 160 capable of managing more than one control channel. The control channel transceiver 160 is controlled by the control and processing device 130. The control channel transceiver 160 broadcasts control information to the mobile station locked on the control channel through the control channel of the base station or cell. The voice channel transceiver manages the service or voice channel. As described above, the digital control channel positioning information may be included on the service or voice channel.

When the mobile station first enters the idle mode, it periodically scans the base station control channel like base station 110 to determine which cell it should lock on or reside in. The mobile station 120 receives the absolute and relative information broadcast on the control channel in its voice and control channel transceiver 170. Then, the processing device 180 calculates the received control channel information including the characteristics of the candidate cell, and determines which cell the mobile station should be locked to. The received control channel information includes not only absolute information related to the relevant cell, but also relative information of other cells adjacent to the cell related to the control channel. While monitoring the main control channel, these adjacent cells are periodically scanned to determine whether there is a more suitable candidate. For additional information about the implementation of mobile stations and base stations, please refer to P. Dent and B. Ekelunds pending US Patent Application No. 07/967,027 filed on October 27, 1992 called "Multi-Mode Signal Processing". The document is used as a reference in this application.

The above-mentioned embodiments are only intended to fully illustrate the present invention, but not to limit the present invention. Although the above embodiments are for base stations and mobile stations, the present invention can also be applied to any wireless communication system. For example, satellites can send and receive data while communicating with remote devices such as portable devices, PCS devices, personal digital assistants, and so on.

Therefore, those skilled in the art can implement many changes in implementation details according to the specification of the present invention. All these changes and modifications are considered to fall within the scope and spirit of the present invention, and the claims define the scope of the present invention. .