The present application claims priority from Japanese applications JP-2007-100024 filed on Apr. 6, 2007 and JP-2006-243522 filed on Sep. 8, 2006 the contents of which are hereby incorporated by reference into this application.
The present invention relates to a wireless communications system that employs OFDM (Orthogonal Frequency Division Multiplex) for wireless communication and to a system for implementing cellular communication. This technology can prevent a rapid increase in the interference power to a base station that is a problem when data transmission is started.
Research and development of a wireless communications system that employs OFDM is under way. In OFDM, transmission data is created in the frequency domain, is converted to signals in the time domain via IFFT (Inverse Fast Fourier Transform), and is transmitted as wireless signals. The receiving side converts the signals in the time domain to the signals in the frequency domain through FFT (Fast Fourier Transform) to retrieve the original information. When communication is performed, the transmission power of a terminal on the reverse link must be controlled to control the interference power to a base station.
IEEE802.20, which is a standardization organization, proposes an OFDM-based wireless system, and IEEE C802.20-06/04 defines the reverse link power control method described above.
3GPP, which is a standardization organization, proposes an OFDM-based wireless system as LTE (Long Term Evolution), and 3GPP TR25.814 V7.0.0 (2006-06) defines the reverse link power control method described above.
3GPP2, which is a standardization organization, proposes an OFDM-based wireless system as LBC (Loosely Backwards Compatible), and 3GPP2 C30-20060731-040R4 defines the reverse link power control method described above.
The transmission power control of a terminal in LBC is that a terminal increases or decreases the T2P (Traffic-to-Pilot) gain ΔP according to the index OSI (Other Sector Interference), which indicates the interference to each sector, to adjust the transmission power of the OFDM signal. Here, a sector refers to a beam-based logical division unit of a base station, and a terminal directly communicates with a sector. A T2P gain, which indicates the magnitude of the CDMA pilot transmission power versus the magnitude of the OFDM data channel transmission power, is defined by the transmission power per OFDM sub-carrier, that is, by the power spectrum density.
The following describes the transmission power control based on OSI with reference to
A terminal detects OSI transmitted from sectors defined as OSIMonitorSet and performs operation according to a policy that the T2P gain ΔP is increased when the OSI is 0 and the T2P gain ΔP is decreased when the OSI is 1 or 2. The OSIMonitorSet refers to a pre-defined set of neighboring sectors except the RLSS. More specifically, for each of the sectors included in the OSIMonitorSet, the terminal decides whether to increase or decrease the T2P gain ΔP based on the detected OSI, assigns weight to this value using the propagation attenuation from each sector to the terminal so that the contribution of a nearer sector becomes larger, and adds up the weighted values. Let the calculated value be Dw. If Dw is equal to or smaller than a threshold, the terminal decreases the T2P gain ΔP by a predetermined value.
If Dw is equal to or larger than another threshold, the terminal increases the T2P gain ΔP by a predetermined value. If Dw does not satisfy either condition, the terminal does not change the T2P gain ΔP. The operation described above controls the transmission power of a terminal as shown in
On the other hand, when a terminal has data to transmit, the terminal first requests the RLSS to assign the communication resources of the reverse link via an R-REQCH(Reverse Request Channel) 1003. The RLSS that receives the R-REQCH assigns the sub-carrier information and the packet format information, which will be used on the reverse link, to the terminal via an RLAM (Reverse Link Assignment Message) on an F-SCCH (Forward Shared Control Channel) 1004. The terminal transmits data via an R-DCH (Reverse Data Channel) 1005 using the resources specified by the RLAM.
In the transmission power control such as the one described in the BACKGROUND OF THE INVENTION, there is a possibility that a terminal suddenly starts communication at a high rate and with a large power when data transmission is started if the interference received by the neighboring sectors is small. Therefore, the interference given to non-RLSS sectors from the terminal increases more rapidly when data transmission is started than when data is being transmitted and, so, the communication quality such as PER (Packet Error Rate) is sometimes degraded. This is because each sector decides the OSI based on the interference condition at a particular time without considering interference status variations after deciding the OSI and, so, there is sometimes a difference between the interference status of each sector at the time each sector sent an OSI notification and the interference status of each sector at the time each terminal performs communication using the transmission power decided based on the OSI detected by the terminal.
For example, when a terminal starts data transmission immediately after each sector sent an OSI to the terminal, the interference power received by each sector becomes larger than when the OSI notification was sent to the terminal. In such a case, the S/I (Signal-to-Interference) ratio assumed when the OSI was decided cannot be achieved with the result that the reception PER in the sector is degraded. The S/I ratio is a ratio between the signal power and the interference power. The following describes an example of communication quality degradation with reference to
The problem described above is not generated in a system that employs CDMA. For example, in cdma2000 1x EV-DO (Evolution Data Optimized) introduced in 3GPP2 C.S0024-B, it is specified that low-rate, low transmission power communication be performed when a terminal starts data transmission. This feature prevents the reception power of a sector from increasing rapidly, and the communication quality from being degraded, when a terminal starts data transmission.
To solve the problem described above, the transmission power is controlled to be smaller when a terminal does not transmit data than when the terminal is transmitting data.
To solve the problem described above, the transmission power control is performed to make it easier to decrease the T2P gain ΔP when a terminal does not transmit data even if the interference power received by a sector is small. That is, because the T2P gain ΔP is suppressed while the terminal does not transmit data, this control prevents the terminal from suddenly transmitting data with a large power. Therefore, when the terminal starts transmitting data, this control can prevent a rapid increase in the power received by a sector and avoids degradation in communication quality. Thus, the problem is solved.
To solve the problem described above, when a terminal starts transmitting data, the data transmission destination sector instructs the terminal to decrease the T2P gain ΔP. In response to the instruction, the terminal starts transmitting data with a suppressed transmission power based on the T2P gain ΔP decreased by a predetermined amount. This control can prevent a rapid increase in the power received by a sector, when the terminal starts transmitting data, and avoids degradation in communication quality. Thus, the problem is solved.
The present invention, which suppresses the transmission power when a terminal starts transmitting data, prevents a rapid increase in the interference power received by a sector and keeps communication quality while utilizing the framework of the conventional OSI-based power control mechanism.
The present invention optimizes the transmission power control of a reverse link especially in OFDMA-based cellular communication, thus preventing degradation in communication quality.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with accompanying drawings.
In general, an OFDM cellular wireless communication system comprises multiple base station devices and multiple terminals as shown in
Each sector in an OFDM cellular base station receives signals from terminals that are communicating with this sector, interference signals from terminals that are communicating with other sectors, and noises. Each sector measures the interference power and the noise power and calculates the ratio between them to obtain the IoT for the sector. Based on the calculated IoT, each sector decides the intensity of interference it receives as OSI at one of three levels, 0, 1, and 2. OSI=0, which indicates that the interference power is small, is a numeric value notifying a terminal, which is the interference source of each sector, that the transmission T2P gain ΔP may be increased. OSI=1 and OSI=2, which indicate that the interference power is large, are numeric values requesting a terminal, which is the interference source of each sector, to decrease the transmission T2P gain ΔP. In particular, OSI=2, which indicates that the interference is very high, is provided to force a terminal, which is the interference source of each sector, to decrease the transmission T2P gain ΔP. Each sector notifies OSI to a terminal via F-OSICH and F-FOSICH.
When a terminal transmits data to a sector, the terminal first uses an R-REQCH(Reverse Request Channel) to request the sector to assign the frequency/time resources for transmitting data. The R-REQCH includes information such as the buffer size of transmission data transmitted by the terminal. In response to the R-REQCH from the terminal, the sector decides the frequency/time resources to be assigned to the terminal and, based on them, creates a resource assignment information message RLAM (Reverse Link Assignment Message). The RLAM, a message used by the sector to notify the terminal about sub-carrier information and packet format information to be used by the terminal on the reverse link, is transmitted from the sector to the terminal using the F-SCCH (Forward Shared Control Channel). The terminal uses the resources, notified via the RLAM, to transmit data to the sector.
A first embodiment of the present invention will be described with reference to
In this embodiment, the value of Dw is smaller when data is not transmitted than when data is transmitted. This makes it more difficult for the transmission T2P gain ΔP to be increased, and makes it easier to be decreased, when data is not transmitted than when data is transmitted. As a result, because the OFDM sub-carrier transmission power of the terminal becomes smaller when data is not transmitted than when data is transmitted, the terminal starts data transmission with a smaller transmission power. Therefore, the fluctuation range in the interference power, received by the sectors other than the RLSS of the terminal that started the data transmission, is smaller than that in the conventional method and, so, the communication quality such as PER can be kept constant.
A second embodiment of the present invention will be described with reference to
In this embodiment, when data is not transmitted, the T2P gain ΔP is not increased. As a result, because the OFDM sub-carrier transmission power of the terminal becomes smaller when data is not transmitted than when data is transmitted, the terminal selects a smaller transmission power when it starts data transmission. Therefore, the fluctuation range in the interference power, received by the sectors other than the RLSS of the terminal that started the data transmission, is smaller than that in the conventional method and, so, the communication quality such as PER can be kept constant.
A third embodiment of the present invention will be described with reference to
In this embodiment, when data is not transmitted, a decrement offset is applied to the change in the transmission T2P gain ΔP. As a result, because the OFDM sub-carrier transmission power of the terminal becomes smaller when data is not transmitted than when data is transmitted, the terminal selects a smaller transmission power when it starts data transmission. Therefore, the fluctuation range in the interference power, received by the sectors other than the RLSS of the terminal that started the data transmission, is smaller than that in the conventional method and, so, the communication quality such as PER can be kept constant.
A fourth embodiment of the present invention will be described with reference to
On the other hand, before the terminal transmits data over the reverse link, the terminal uses R-REQCH to request a sector, with which reverse link communication is to be performed, to assign resources for the data transmission. In response to the resource assignment request from the terminal, the sector checks whether or not the terminal transmitted data in a predetermined period of time. The sector decides the resources, which will be used by the terminal, and transmits an RLAM, which is a resource assignment message, to the terminal via the F-SCCH. The configuration of the RLAM message in this embodiment is shown in
The PF (Packet Format) field indicates the packet format used by the terminal to transmit data. The Ext Tx (Extended Transmission) field indicates whether or not the extended transmission mode is used in which data is transmitted in multiple continuous time frames. The Suppl (Supplemental) field indicates whether or not the supplemental mode is used in which resources are additionally assigned. The PSD (Power Spectral Density) Adjust field indicates whether or not the terminal is to decrease the T2P gain by a fixed amount. When the terminal has not transmitted data for a predetermined period of time, the sector uses the PSD Adjust field of the RLAM to notify the terminal to decrease the T2P gain by a predetermined amount. Otherwise, the sector uses the PSD Adjust field of the RLAM to notify the terminal not to decrease the T2P gain by a fixed amount. The terminal receives the RLAM from the sector and uses the resources, specified in the RLAM message, to transmit data via the R-DCH(Reverse Data Channel). At this time, the transmission T2P gain ΔP is a value calculated by subtracting the fixed amount, predetermined according to the PSD Adjust field, from the ΔP value updated based on the OSI.
In this embodiment, when a terminal starts data transmission, a base station notifies the terminal to transmit data using the value, generated by decreasing a predetermined amount from the transmission T2P gain decided based on the OSI received by the terminal and, according to this notification, the terminal decreases the transmission T2P gain by the predetermined amount. As a result, the terminal uses a small transmission power when it starts data transmission. Therefore, the fluctuation range in the interference power, received by the sectors other than the RLSS of the terminal that started the data transmission, is smaller than that in the conventional method and, so, the communication quality such as PER can be kept constant.
It should be further understood by those skilled in the art that although the foregoing description has been on embodiments of the invention, the invention is not limited thereto and various change and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.
Number | Date | Country | Kind |
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2007-100024 | Apr 2007 | JP | national |
2006-243522 | Sep 2006 | JP | national |