The invention relates to a transmission power control in a connection of a radio system.
It is vital to have power control of signals in a radio system. This is of particular importance in a CDMA (Code Division Multiple Access) radio system, which is interference-limited. The main task of the power control in a CDMA radio system is to set signal powers to the desired level, and hence increase capacity by decreasing interference.
For example, in a WCDMA (Wide band CDMA) radio system the power control mechanism comprises an inner loop power control and an outer loop power control.
The purpose of the inner loop power control is to eliminate rapid variations in the strength of a received signal caused by the radio channel and propagation.
In the uplink inner loop power control, a base station compares the measured SIR (Signal Interference Ratio) of the received signal to a target SIR. If the measured SIR of the received signal is below the target SIR, the base station transmits a signal commanding the user terminal to increase its transmission power. Correspondingly, if the SIR of the received signal is above the target SIR, the base station transmits a signal commanding the user terminal to decrease its transmission power.
In the uplink outer loop control, a radio network controller (RNC) compares the quality of service to a target quality. The quality can be measured as, for instance, BER (Bit Error Rate), BLER (Block Error Rate), FER (Frame Error Rate), CRC (Cyclic Redundancy Check), soft information from the decoder, ratio of received bit energy and noise, etc. If the quality of service is below the target quality, the RNC commands the base station to increase its target SIR. Similarly, if the quality of service is above the target quality, the RNC commands the base station to decrease its target SIR.
In radio systems utilizing a packet-switched connection, the packets are usually protected against noise, fading and interference by channel coding, such as FEC (Forward Error correction Coding). In spite of protection, failure may occur in the reception of a packet, which can be compensated for by retransmission. The retransmission takes place when the receiving transceiver of packets requests the faulty packet to be repeated. This can be performed by an ARQ (Automatic Repeat Request) mechanism. In a receiver utilizing HARQ (Hybrid ARQ), the faulty packet and the retransmitted packet can be combined. The combining can be especially effective if different transmissions of the same packet are utilized in decoding.
There are, however, problems related to the use of retransmission with power control, especially in the case of HARQ. When a packet is communicated unsuccessfully, the outer loop power control increases the target SIR, which unnecessarily leads to a higher transmission power during retransmission of the packet. The increased transmission power increases interference and decreases the capacity and service quality of the radio system.
The outer loop control can also set the retransmission target SIR a predetermined amount lower than the target SIR of the first transmission of a packet. The transmission power of a retransmission may become too low and unnecessary many retransmissions may be performed which impairs the throughput and increase interference. Additionally, these problems tend to increase as the number of users per one cell grows.
An object of the invention is to provide an improved method, a network infrastructure, a radio system, a base station, a radio network controller, a computer program product.
According to an aspect of the invention, there is provided a method of controlling transmission power in a radio system using a transmission power control, the method comprising: communicating between at least two transceivers of the radio system using a connection over a radio interface; transmitting, from a transceiver receiving packets during communication, a request to retransmit at least one packet having failure in reception; retransmitting, from a transceiver transmitting packets, at least one packet requested as a response to the request. The method further comprises estimating the quality of at least one packet having failure in reception; and controlling the transmission power of a retransmission according to the estimated quality of the at least one packet having failure in reception.
According to another aspect of the invention, there is provided a method of controlling transmission power in a radio system using a transmission power control, the method comprising: communicating between a network infrastructure and at least one user terminal of the radio system using a connection over a radio interface; transmitting, from the at least one user terminal receiving packets during communication, a request to retransmit at least one packet having failure in reception; retransmitting, from the network infrastructure transmitting packets, at least one packet requested as a response to the request. The method further comprises estimating the quality of at least one packet having failure in reception; and controlling the transmission power of a retransmission according to the estimated quality of the at least one packet having failure in reception.
According to another aspect of the invention, there is provided a network infrastructure in a radio system using a transmission power control, wherein at least one user terminal and the network infrastructure are configured to communicate using a connection over a radio interface, the at least one user terminal receiving packets during communication is configured to transmit a request of retransmission of at least one packet having failure in reception, and the network infrastructure transmitting packets is configured to retransmit at least one packet requested as a response to the request. The network infrastructure further comprises an estimator configured to estimate the quality of at least one packet having failure in reception; and a controller configured to control the transmission power of a retransmission according to the estimated quality of the at least one packet having failure in reception.
According to another aspect of the invention, there is provided a network infrastructure in a radio system using a transmission power control, wherein at least one user terminal and the network infrastructure are configured to communicate using a CDMA connection over a radio interface, the at least one user terminal receiving packets during communication is configured to transmit a request of retransmission of at least one packet having failure in reception, and the network infrastructure transmitting packets is configured to retransmit at least one packet requested as a response to the request. The network infrastructure further comprises means for estimating the quality of at least one packet having failure in reception; and means for controlling the transmission power of a retransmission according to the estimated quality of the at least one packet having failure in reception.
According to another aspect of the invention, there is provided a radio system using a transmission power control, wherein at least two transceivers are configured to communicate using a connection over a radio interface, the at least one transceiver receiving packets during communication is configured to transmit a request of retransmission of at least one packet having failure in reception, and the at least one transceiver transmitting packets is configured to retransmit at least one packet requested as a response to the request. The radio system further comprises an estimator configured to estimate the quality of at least one packet having failure in reception; and a controller configured to control the transmission power of a retransmission according to the estimated quality of the at least one packet having failure in reception.
According to another aspect of the invention, there is provided a radio system using a transmission power control, wherein at least one user terminal and the network infrastructure are configured to communicate using a connection over a radio interface, the at least one user terminal receiving packets during communication is configured to transmit a request of retransmission of at least one packet having failure in reception, and the network infrastructure transmitting packets is configured to retransmit at least one packet requested as a response to the request. The radio system further comprises an estimator configured to estimate the quality of at least one packet having failure in reception; and a controller configured to control the transmission power of a retransmission according to the estimated quality of the at least one packet having failure in reception.
According to another aspect of the invention, there is provided a base station in a radio system using a transmission power control, wherein at least one user terminal and the network infrastructure are configured to communicate using a connection over a radio interface, the at least one user terminal receiving packets during communication is configured to transmit a request of retransmission of at least one packet having failure in reception, and the network infrastructure transmitting packets is configured to retransmit at least one packet requested as a response to the request. The network infrastructure further comprises means for estimating the quality of at least one packet having failure in reception; and means for controlling the transmission power of a retransmission according to the estimated quality of the at least one packet having failure in reception.
According to another aspect of the invention, there is provided a radio network controller in a radio system using a transmission power control, wherein at least one user terminal and the network infrastructure are configured to communicate using a connection over a radio interface, the at least one user terminal receiving packets during communication is configured to transmit a request of retransmission of at least one packet having failure in reception, and the network infrastructure transmitting packets is configured to retransmit at least one packet requested as a response to the request. The network infrastructure further comprises means for estimating the quality of at least one packet having failure in reception; and means for controlling the transmission power of a retransmission according to the estimated quality of the at least one packet having failure in reception.
According to another aspect of the invention, there is provided a computer program product encoding a computer process for controlling transmission power in a radio, wherein at least one user terminal and the network infrastructure are configured to communicate using a connection over a radio interface, the at least one user terminal receiving packets during communication is configured to transmit a request of retransmission of at least one packet having failure in reception, and the infrastructure transmitting packets is configured to retransmit at least one packet requested as a response to the request. The process further comprises: estimating the quality of at least one packet having failure in reception; and controlling the transmission power of a retransmission according to the estimated quality of the at least one packet having failure in reception.
The invention provides several advantages. The transmission power in retransmission can be adjusted as a function of the quality of an unsuccessful signal or a portion of signal. This decreases interference and increases throughput while guaranteeing the quality of connection.
In the following, the invention will be described in greater detail with reference to the embodiments and the accompanying drawings, in which
Let us first study
The core network may, for example, correspond to the combined structure of the GSM (Global System for Mobile Communications) and GPRS (General Packet Radio Service) systems. The GSM network elements are responsible for the implementation of circuit-switched connections, and the GPRS network elements for the implementation of packet-switched connections, some of the network elements being, however, shared by both systems.
A mobile services switching centre (MSC) 100 enables circuit-switched signalling in the radio system. A serving GPRS support node (SGSN) 101 in turn enables packet-switched signalling. All traffic in the radio system may be controlled by the MSC 100.
The core network may have a gateway unit 102, which represents a gateway mobile service switching centre (GMSC) for attending to the circuit-switched connections between the core network and external networks, such as a public land mobile network (PLMN) or a public switched telephone network (PSTN). A gateway GPRS support node (GGSN) 103 attends to the packet-switched connections between the core network and external networks, such as the Internet.
The MSC 100 and the SGSN are connected to a network infrastructure 104, such as radio access network (RAN). The network infrastructure 104 may be a unique unit or it may include at least one base station controller 106 controlling at least one base station 108. The base station controller 106 can also be called a radio network controller, and the base station can be called a node B. A user terminal 110 communicates with at least one base station 108 over a radio interface.
The user terminal 110 can communicate with the base station 108 over air interface. Data in packets contain address and control data in addition to the actual traffic data. Several connections may employ the same transmission channel simultaneously. A packet-switching method is suitable for data transmission where the data to be transmitted is generated in bursts. In such a case, it is not necessary to allocate a data link for the entire duration of transmission but only for the time it takes to transmit the packets. This reduces costs and saves capacity considerably during both the set-up and the use of the network.
A CRC (Cyclic Redundancy Check) can be performed in a base station to check if a packet (or a frame) is correctly received or has a failure in reception. In the case of a successful reception of a packet, the network infrastructure 104 (base station or radio network controller) may measure the quality of the received signal. That may take place such that the base station measures the quality of the received signal and sends the radio network controller 106 a signal 204 having information on the quality. The quality can be measured as the quality of service and it can be indicated by, for instance, frame reliability using CRC (Cyclic Redundancy Check), BER, FER, soft information from a decoder, Eb/N0, etc.
The target SIR1target of the first transmission can be adjusted by an outer-loop power control algorithm, which in prior art can be expressed as follows: SIR1target(n+1)=SIR1target(n)±ΔPC[dB], where n is an index of the iteration and ΔPC[dB] is the size of the step to increase or decrease the transmission power in decibels. The increase and the decrease of the transmission power can be performed using separate steps and the power may have an upper limit and a lower limit. Hence, the power control can be expressed more accurately, for example, as:
SIR1target(n+1)=min{SIR1target(n)+ΔPC
SIR1target(n+1)=max{SIR1target(n)−ΔPC
where min{x, y} means the minimum value among the elements x and y, n is an index of the iteration, ΔPC
The desired FER1target of the first transmission varies typically between 10% and 50%, and the step size of the ΔPC
The network infrastructure 104 in turn may change the target SIR according to formula (1) or (2). This can take place such that the radio network controller 106 sends the base station a signal 206 having an effect on the target SIR. If the value of the quality of service is below the quality target value, which may be true in the case of failure in reception of a packet, the network infrastructure may increase the target SIR in the base station. As a result of this, the average transmission power of a retransmission of a packet in prior art may be higher than during the initial transmission of the packet. If the value of the quality of service is above a target value, the network infrastructure may decrease the target SIR in the base station, which lowers the average transmission power with respect to interference. This may take place when a packet is received successfully.
The base station 108 checks whether the reception of packets is successful by decoding the packet. If the reception is successful, i.e. the decoding succeeds, the base station 108 continues receiving packets from the subscriber terminal 110 without retransmissions. If, on the other hand, the reception fails, i.e. the decoding does not succeed, the base station 108 transmits a request to retransmit at least one packet having failure in reception. Additionally, according to the present solution the network infrastructure 104 estimates the quality of each packet having failure in reception. The quality can be estimated as a frame error rate FERestfailure, for example, as follows:
where Π represents multiplication of the elements in the product, i is the index of the elements, N is the number of symbols, u represents hard decisions of symbols in a packet (or in a frame), λ represents soft decisions of the symbols output by the decoder, and exp is the natural exponential function the base of which is the Neper number e (e≈2.71828). The elements
represent probability of a received symbol and each symbol can be expressed as a bit or as a combination of bits. It should be obvious to a person skilled in the art that other approaches of estimating the frame error rate can also be utilized.
If the estimated frame error rate FERestfailure of a packet with a decoding failure has a higher value than the target frame error rate FERtarget (FERestfailure>FERtarget), i.e. the estimated quality is worse than a target quality, the network infrastructure 104 may control the value of the target SIR according to the quality of the packet having failure by forming a new target SIR (SIRrtarget) for the retransmission of the received packet, for example, as follows:
where j stands for the retransmission index of a packet, J is the number of HARQ transmissions of a packet, SIRrtarget,j is the target SIR for the jth retransmission, the subscript target means transmission target, SIRr
If the estimated quality is better than a target quality, the transmission power in retransmission can be controlled by setting the retransmission target SIR at its minimum value (SIRrtarget=SIRmin).
If the decoding of a packet is in error after all retransmissions, the value of the parameter t can be updated in the network infrastructure 104 as follows:
t(n+1)=max(t(n)−Δslope
If the decoding of a packet is correct, the parameter t may be updated as follows:
t(n+1)=min(t(n)+Δslope
where n is a TTI (Transmission Timing Interval) index, tmin is the minimum value of the parameter, tmax is the maximum value of the parameter. The parameter Δslope
The range of the parameter Δslope
The SIR target and hence the transmission power in retransmission can be controlled by controlling the effect of the quality of the at least one packet having failure in reception on the SIR target. As shown in equations (6), (7) and (8), the effect can be adapted according to a success or a failure in reception by iterating the parameter t of equation (5). The adaptation may be used to weaken the effect of the estimated quality on the retransmission target SIR if a transmission or a retransmission succeeds as expressed in equation (7). In this way, the change in a value of a retransmission target SIR with respect to the estimated quality may decrease. The adaptation may also be used to strengthen the effect of the estimated quality on the retransmission target SIR if the last retransmission has a failure in reception as expressed in equation (6). In this way, the change in a value of a retransmission target SIR with respect to the estimated quality may increase.
The iteration of the parameter t in equations (6) and (7) enables the SIR target to be adapted with variations relating to data rates, environment and imperfections in the inner loop power control.
In
After step 406, if the transmission of the received packet is not the first transmission, it is checked whether it is a question of the last retransmission of a faulty packet in step 414.
If it is a question of a last retransmission in step 414, it is checked, whether the packet is correctly received in step 416. If the packet is properly received, the parameter t is updated according to equation (7) in step 418 and the transmission and the reception continue in step 400.
If it is a question of a last retransmission in step 414, and the packet has failure in reception when checked in step 416, the parameter t is updated according to equation (6) in step 420. The transmission and the reception continue in step 400.
If it is not a question of the last retransmission in step 414, it is checked whether the packet has failure in reception or not in step 422. If the packet is properly received, the parameter t is updated according to equation (7). The transmission and the reception continue in step 400.
If it is not a question of the last retransmission in step 414, and the packet has failure in reception when checked in step 422, a frame error rate relating to the packet is estimated in step 424 according to equation (4). In step 426, the estimated frame error rate FERestfailure is compared with the target frame error rate FERtarget, and if the estimated frame error rate FERestfailure has a higher value than the target frame error rate FERtarget, a new target SIR for retransmission is formed according to equation (5) in step 428. The transmission, the retransmission and the reception continue using a new retransmission target SIR in step 400. If the estimated frame error rate FERestfailure has a lower value than the target frame error rate FERtarget, a new target SIR for retransmission is formed by setting the minimum value for the target SIR in step 430. The transmission, the retransmission and the reception continue in step 400.
Chase combining where a retransmitted packet is similar to the originally transmitted packet can be used to implement HARQ. To further improve performance, it is also possible to use incremental redundancy (IR), where a retransmitted packet comprises new redundancy bits. To utilize HARQ, the receiving transceiver can be equipped with a buffer memory in which faulty packets can be stored (in
Method steps of the power control can be implemented as software run in a microprocessor. A partial equipment implementation alone or with the software can also be applied, especially using ASIC (Application Specific Integrated Circuit). Hence, a computer program product encoding a computer process for controlling transmission power can be provided, the process implementing the method. The computer program product may be embodied on a computer program distribution medium. The computer program distribution medium may include known ways in the art for distributing software, such as a computer readable medium, a program storage medium, a record medium, a computer readable memory, a computer readable software distribution package, a computer readable signal, a computer readable telecommunication signal, and a computer readable compressed software package.
Even though the invention is described above with reference to examples according to the accompanying drawings, it is clear that the invention is not restricted thereto but can be modified in several ways within the scope of the appended claims.
Number | Date | Country | Kind |
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20045244 | Jun 2004 | FI | national |