Power control method of discontinuous transmission

ES 2 271 985 T3

DESCRIPTION

Method for power control of a discontinuous transmission.

Field of the invention

The present invention relates to a system in which the transmission power between the base station and the personal station is controlled during radio communication. Specifically, the invention relates to a method for use in cellular networks whereby power control commands are sent to the personal station and from the personal station to the base station.

Background of the invention

In all cellular systems it must be possible to control, at least, the transmission power of the personal station so that its transmission reaches the base station with a sufficient signal-to-noise ratio, regardless of the distance between the personal station and the base station. . The transmission power will be explained in the following paragraphs using the CDMA (Code Division Multiple Access) system as an example.

WO9107037 and WO9603813 discuss power control in CDMA systems. WO91070937 describes a power control system in which the power transmitted by a mobile unit is inversely adjusted with respect to increases and decreases in the power of the received signal. A power control feedback system can also be used in which a command signal is generated in the cell and transmitted to the mobile unit. WO9603813 describes a power control process in which a single stream of power control bits from a base station controls the transmit power of multiple radios.

Figure 1 shows a CDMA forward traffic channel. This comprises the following code channels: a pilot channel, a synchronization channel, from one to seven radioelectric-search channels and no more than 61 traffic channels. The maximum number is given when there is only one paging channel in addition to the synchronization channel. Each channel code is orthogonally chopped and thus spread using quadrature phase coupling of the random noise sequence. Multiple CDMA forward traffic channels can be utilized at the base station using frequency division multiplexing. The structure shown in Figure 1 is presented in the Proposed DCMA PCS Standard document, Joint Technical Committee (JTC), September 23, 1994. This proposal is also known as IS-95. In the following paragraphs, reference will be made to a CDMA system in accordance with this standard, although the invention is suitable for any type of system.

An unmodulated spread spectrum signal is continuously sent through the pilot channel and is used by PSs (personal stations) for synchronization purposes.

A coded, interleaved, spread and modulated spread spectrum signal is sent through the sync channel. The personal station uses this signal to perform a preliminary time synchronization. The channel bit rate is 1,200 bps and the frame duration is 26,666 ms. Subchannels that send power control commands should not be included in the sync channel.

A coded, interleaved, spread and modulated spread spectrum signal is sent through the paging channel. The bit rate is 9,600 or 4,800 bps and the frame duration is 20 ms. The base station uses the paging channel to transmit header information and information specific to the personal station. The number of these channels can vary on a CDMA forward traffic channel, although the maximum number is 7 channels.

The traffic channel is used to transmit user and signaling information to the PS (personal station). The maximum number of simultaneous forward traffic channels supported by a CDMA traffic channel is 63 minus the number of call and sync channels operating on the same CDMA traffic channel.

The structure of the frame itself is the same in both the forward traffic channel and the reverse traffic channel. The information is transmitted in the form of frames whose length is 20 ms. The base station and the personal station can send information with a variable data transfer rate. The data transfer rates, when using rate set 1, are 9600, 4800, 2400 and 1200 bps respectively, while the number of frame bits corresponding to the different transfer rates is 192, 96, 48 and 24 bits respectively. When rate set 2 is used, the data transfer rates are 14400, 7200, 3600, and 1800 bps respectively, while the corresponding frame bit numbers are 288, 144, 72, and 36 bits. The frame bits consist of information bits, frame quality indicator bits, and encoder tail bits. The bottom line is that in both directions the structure of the traffic frame is identical for different transfer rates, so when identifying the frame structure the data transfer rate will also be known.

Such modulation symbols that are transferred at a lower data rate are also transmitted with a lower energy, but although the data transfer rate varies from frame to frame, the rate of

ES 2 271 985 T3 symbol modulation is kept constant. When E_s or energy per symbol and E_b or energy per information bit, the following Table 1 will be applied according to the standard:

TABLE 1

<td>Data transfer rate</td><td>Energy per modulation symbol</td>

<td> 9600</td><td>Es = Eb / 2</td>

<td> 4800</td><td>Es = Eb / 4</td>

<td> 2400</td><td>Es = Eb / 8</td>

<td> 1200</td><td>Es = Eb / 16</td>

<td> 14400</td><td>Es = Eb / 4</td>

<td> 7200</td><td>Es = Eb / 8</td>

<td> 3600</td><td>Es = Eb / 16</td>

<td> 1800</td><td>Es = Eb / 32</td>

Each forward traffic channel contains a power control subchannel that is used to transmit said power control commands to the personal station during communication, in response to which the personal station will change its transmit power. The power control channel is described in section 3.1.3.1.8 of the proposed specification.

From the received personal station signal, the base station calculates the signal strength always at 1.25 ms intervals, said period corresponding to 16 modulation symbols. Judging the signal strength, the base station will command the personal station to increase or decrease the transmit power. In this way, a large power control loop is formed that comprises the personal station, the base station and the bidirectional radio channel located between them. The base station can also control its own transmit power to correspond to the power measurement reports it receives from the personal station. This is because the personal station constantly maintains frame error statistics and sends the power measurement report at regular intervals or when a predetermined threshold is exceeded.

The power control subchannel is formed so that the power control bits are constantly sent between the normal traffic channel bits. The power control bits repeat cyclically at 1.25 ms intervals. In this case, the bit rate of the power control channel is 800 bps. Bit 0 means that the personal station must increase its transmission power and therefore bit 1 means a command to reduce transmission power. The bits are located in the frame in such a way that from the complete traffic frame, which is a convolution-coded and interleaved frame formed from modulation symbols, two successive modulation symbols are eliminated at regular intervals, replacing each other by a power control bit. Thus the duration of a power control bit is 104.166 ms. The procedure is generally known in the art and is called symbol piercing. The perforation figure indicates which symbols have been removed from the frame and replaced by power control bits. The power control bits are transmitted with an energy Eb.

Upon receipt of the power control bit, the personal station will increase or decrease its transmit power in the direction indicated by the bit. The power control bit is considered genuine if it has been received in the 1.25 ms time slot, which is the second time slot from the time slot in which the personal station has transmitted. The power level change is a short step and the standard determines that one bit changes the power level by 1 dB. Therefore, a substantial change in power level will require the transmission of several bits of power control.

As in FDD / TDMA systems, DTX transmission (discontinuous transmission) is also used in CDMA systems. In a broader sense, DTX also includes an asymmetric case in which information is transferred only in one direction, while acknowledgments are transferred in the opposite direction. An example of this situation is an Internet connection. Power control commands are sent to the receiving party at a normal frequency even though the receiving party is sending information only occasionally.

You can check the DTX status in different ways. First, when the personal station discovers that the required data rate is dropping, it will first send information about the data rate to be used in the next radio frame to the base station, and from the next frame, it will use the

ES 2 271 985 T3 indicated rate. Second, the personal station can simply change the data transfer rate during connection. The base station will have knowledge of the transfer rate from the frame structure because, as mentioned above, when the frame structure is identified, the data transfer rate used will also be known because the Frame structure varies based on different data transfer rates.

One of the problems of the presented CDMA system, as well as of other known CDMA systems, is that single or multi-bit power control commands are always sent at the same standard frequency and power. Power control is carried out quickly so that the transmit power adjusts as closely as possible to changes in the radio path. Additionally, the power control commands are sent both in the forward direction and in the reverse direction at the same frequency and, therefore, the power control will not be affected by the data transfer rate used, due to asymmetry of the transmission rate. data transfer or the fact that either party may be in the DTX state. The result is that, in the DTX state and when using a reduced data rate, the power control will use a disproportionately large part of the radio link capacity.

Therefore, one of the objectives of the present invention is to provide a power control method that adapts to the data transfer situation, freeing up radio link capacity for other uses in the DTX state and at a transfer rate reduced data.

This objective is achieved by the method defined in the independent claims.

Brief summary of the invention

The invention is defined by the features of the independent claims.

According to the proposed method, the frequency of the power control commands to be sent through a power control channel varies depending on the traffic. When traffic slows down in at least one direction, due to a DTX state, low data transfer rate, asymmetric data transfer, or any other reason, the frequency of power control commands is reduced. Both the base station and the personal station can reduce the frequency of the commands they are sending. It is also possible to proceed so that the part with the least need to transmit data or which is not transmitting data at all sends power control commands only occasionally, while this same part sends power control commands with a normal frequency when receiving information with a high data transfer rate or with a reduced frequency, if the reception rate (the transmission rate of the other party) is reduced.

An alternative to changing the frequency of the power control commands is to change the energy of the power control bits. If it is desirable to keep the bit error rate of the received power control bits constant, the duration of the power control bit should be extended because the receiver must accumulate energy over a longer period of time to be able to express the bit reliably. If an increase in the bit error rate is allowed in reception, the duration of the power control bit can be kept constant even when its power is reduced. One of the advantages of this case is that it is not necessary to make changes to the receiver.

If the system is by time division and a multi-bit frequency control command is used in this case, the length of the command word may be shortened additionally or alternatively to the frequency change.

Since a lower power control will not adjust as quickly to changing circumstances as power control, it can cause an error in the transmit power of the controlled transmitter. For this reason, any errors caused by slower power control can be compensated for by increasing the dimensions of the power control stages so that they are larger than the dimensions of the fast power control stages.

Brief description of the figures

The invention will be described in greater detail with reference to the attached figures in which:

Figure 1 shows the radio channels in a CDMA system.

Figure 2 shows a known power control.

Figure 3 shows a power control with the reverse channel link in a DTX state.

Figure 4 shows a power control with the forward channel link in a DTX state.

Figure 5 shows the power control in an asymmetric transfer.

Figures 6a to 6b show the transmission energy as a function of time in various cases.

ES 2 271 985 T3

Figure 7 is a block diagram of a possible embodiment, and

Figure 8 is a block diagram of the embodiment.

Detailed description of the invention

Figure 2 shows a traffic connection between a personal station PS and a transmitting / receiving base station BTS in a CDMA system. As far as power control commands are concerned, the data transfer is carried out here according to known technology, whereby in the forward channel the base station sends power control commands between a stream of bits of information with a standard frequency. For clarification purposes, information is displayed here by large arrows, while power control commands are displayed by small arrows. Similarly, on the reverse channel, the personal station PS sends power control commands between a stream of information bits with a standard frequency. As explained above, in known systems, the power control commands are sent through the reverse channel and the forward channel at a standard frequency, regardless of the information data transfer rate or that it is not sent in absolute any information. But in the proposed method, the frequency of these power control commands is reduced by transmitting to the part that needs the least transmission or is not transmitting at all.

Fig. 3 shows a case where the transmitting / receiving base station BTS sends information to a personal station, but the personal station does not send any information to the transmitting / receiving base station. In this case, the reverse channel is in a DTX state. Its information transfer rate is in this case low and the transmission power requirements of the channel and, similarly, its reception power are low. This situation is very common when the personal station is an Internet connection where the main flow of information is from the network to the personal station. Because only little information is sent on the reverse channel, since the personal station sends acknowledgments from the upper layer, etc., only occasionally, no quick control of the transmit power of the personal station is necessary. For this reason, the frequency of the power control commands to be sent to the personal station is reduced according to the invention. This can be seen in the figures skipping every second power control command, whereby the small arrows formed by dotted lines show the commands that have been skipped. On the other hand, power control commands must be sent frequently through the reverse channel, for example at the normal system frequency because the personal station must control the transmission of the base station that sends a large amount of information.

Fig. 4 shows a case where the personal station sends information to the transmitting / receiving base station BTS but the base station does not send information to the personal station. In this case, the forward channel is in a DTX state. Such a situation is very common when the personal station sends a fax or files to the network. In accordance with the invention, the frequency is now reduced as regards the power control commands to be sent through the reverse channel to the base station. This is shown in the figure skipping every second power control command whereby the small arrows formed by dotted lines show the commands that have been skipped. On the other hand, power control commands must be frequently sent through the forward channel, for example at the normal system frequency because the base station must control the transmission of the personal station sending a large amount of information.

The examples shown above affect a case where the reverse channel or the forward channel is in a DTX state. The method can also be used in a case where the data transfer is asymmetric, such that a transfer occurs in both directions, but the rate is higher in one direction than in the other direction. The transmission frequency of the power control commands is in this case higher for the link on which less information is sent. Figure 5 shows such a case. In this case, less information is sent through the reverse channel than through the forward channel, so power control commands are sent at a higher frequency through the reverse channel than through the forward channel.

The method is also perfectly suited for use for data transfer, where the transfer rate will vary during communication in one direction or in both directions. The transmission frequency of power control commands sent in one direction is controlled in proportion to the change in data transfer rate in the opposite direction.

These cases have been described above when the radio channel resources have been freed up by reducing the transmission frequency of the power control commands. This same result is also achieved in alternative ways.

An alternative is to agree on the length of the command word in systems where the command word consists of several bits. Said systems are time division and / or frequency division systems.

Another alternative is to control the energy of the individual power control bit. When for example a DTX state is changed in one direction, the energy of the power control bits to be sent in the opposite direction is reduced. If it is desirable to keep the bit error rate of the received power control bits constant, the duration of the power control bits should be extended, since the receiver must accumulate energy throughout

ES 2 271 985 T3 a longer period to be able to express the bit reliably. The duration of the bit is extended by sending it in several parts. This alternative is especially advantageous in a system according to the Proposed CDMA PCS Standard, Joint Technical Committee (JTC) because it is not necessary to make changes to the transmitter, but the necessary changes are limited to the power control algorithm. If an increase in the bit error rate is allowed in reception, the duration of the power control bits can be kept constant even when their power is reduced. This case has the advantage that it is not necessary to make changes to the receiver.

Figures 6a to 6c show ways of carrying out the proposed method. These show the transmission of energy as a function of time. Figure 6a shows an advanced technique in which power control commands are sent with a standard frequency in an information stream and with the same energy E_b with which the information symbols are being transmitted.

Figure 6b shows an embodiment of the method according to the invention in which the transmission frequency of the power control commands has been reduced, but its transmission energy Eb remains the same as with the information.

Fig. 6c shows an embodiment in which the energy of the power control bits has been reduced so that it is lower than the transmission energy Eb of the information symbols. In this case the duration of the power control is extended by sending the bit, for example bit a, in two parts. A power control bit is received for a longer period so that the receiver is able to reliably express the power control bit.

Figure 7 shows a block diagram of some possible events of the personal station PS and the transmitting / receiving base station BTS. PS and BTS are supposed to establish communication using normal, ie fast power control. When the personal station PS observes the decrease in the required data rate in step 711, it places information on the required lower data rate in the traffic frame and sends the frame to the base station BTS through the radio interface in step 712. The information can for example affect only the transmission of the personal station, that is the reverse channel, while its reception, that is the forward channel, or the information can affect both directions. In this way, the information can say that at the moment nothing is going to be sent through the reverse channel, so that the personal station will be in a DTX state on this channel.

The base station separates the information about the new transfer rate from the frame that it has received in step 713, and changes its power control process in accordance with the modified transfer rate in step 714. In the case of a state DTX on the reverse channel would scarcely transmit power control commands with better energy or reduce the length of the command word. Prior to this, the personal station has adapted its own power control to match the changed data transfer rate so that it can correctly pick up the power control commands from the frames received by it. Likewise, it can also transmit power control commands to the base station in the manner according to the invention, which is adapted to the transmission rate of the base station.

The data transfer rate can remain unchanged until the end of the communication or it can change again as seen in step 716. This last case means returning to step 711 from which progress is made in the way that has been done. described. Therefore, it is possible to start over using a normal data transfer rate or any reduced rate, or even disconnect. A dotted line shows the signaling between the PS and the BTS that might be required upon disconnection.

Figure 8 shows the same main characteristics of figure 7, but adapted to a system in accordance with the proposed IS-95 standard. The difference is that because according to this standard the structure of the traffic frame is different for different data transfer rates, the personal station does not need to separately notify the change in data transfer rate. Without having to do anything else, it starts using a frame structure according to the new transfer rate in step 812. Starting from the received frame structure, the base station identifies the new transfer rate, in step 813, and change its power control to conform to the invention, in step 714. The personal station has also changed its own power control in step 714, so that operation can continue using the method according to the invention. During communication, the power control can be changed again or this modified power control can continue to be used until disconnection, as described in connection with Figure 7.

In relation to Figures 7 and 8, it has been proposed that the personal station originates the change, but it can also originate from the base station, being able to inform the personal station about the new transfer rates, thereby both will change their power control algorithms according to the new situation. It is also possible for the personal station and the network to negotiate before starting the operation or to agree on the transfer rates during the operation and, after reaching an agreement, to fix their power control algorithms accordingly.

When the frequency or energy of the power control commands are calculated according to the proposed method, the requirement of the E link will increase._b/ N_or (received signal energy / noise energy) compared to a case where power control commands are sent at a high frequency. This is because a slower power control is not able to follow all changes in the signal. However, the increase in requirement E_b/ N_or it is

ES 2 271 985 T3 is very small and the system can be sized such that a full profit is obtained despite such an increase. It should also be noted that, because the reception power required by the DTX connection is significantly lower than the reception power of the active users, the connection is significantly lower than the reception power of the active users without significant errors. minor caused by slower power control. On the other hand, errors can be compensated for by increasing the transmission power stage change caused by the power control command.

The proposed method can be carried out in many ways, while staying within the definitions of the claims. For example, it is possible that only the base station changes the power control frequency or the energy of the power control bits that it sends when the personal station is always operating in the same way. The invention can then be easily integrated into an IS-95 system. Some personal stations may be equipped with features in accordance with the invention and those that enjoy fast power control, in accordance with the specification, will operate normally even when the power control commands arrive at a lower frequency. Personal stations simply observe that, for example, one out of every two power control commands does not arrive.

In some cases, the power control frequency or the energy of the power control bits may change in inverse proportion to the load of the opposite transfer direction. For example, if at any given time, most of the operation takes place on the forward channel, that is from the base station to the personal station, and there is very little significant traffic on the reverse channel, the situation is such that, for For example, five personal stations receive information from the network at the same time while only one transmits in the network direction.