Typical wireless communication systems adopt resource scheduling technology in order to effectively use limited wireless resources. The resource scheduling technology adopted in the wireless communication system includes resource allocation according to the transmission direction of data, resource allocation considering the reuse efficiency, or the like.
The resource allocation according to the data transmission direction may include allocation of resources for a downlink from a base station to a wireless terminal (hereinafter, referred to as “downlink resources”), and allocation of resources for an uplink from the wireless terminal to the base station (hereinafter, referred to as “uplink resources”). In addition, the resource allocation considering the reuse efficiency means resource allocation in its own service area by considering interference with respect to nearby service areas.
The typical wireless communication system adopts an adaptive modulation and coding (AMC) scheme and a channel sensitive scheduling scheme in order to effectively allocate the wireless resources and improve the transmission efficiency.
The AMC scheme refers to technology in which the amount of transmission data is adjusted according to the channel status in order to transmit a lot of information while maintaining the reception error probability at a desired level. According to this, scheduling for reducing or increasing the amount of transmission data in response to the channel status may be provided.
The channel sensitive scheduling scheme refers to technology in which a user in a good channel status is selectively served among a plurality of users. The channel sensitive scheduling scheme may selectively provide a service to a user in a good channel status among a plurality of users to relatively increase the system capacity. Such an increase in the system capacity is called a “multi-user diversity gain.”
In other words, according to the AMC scheme and the channel sensitive scheduling scheme, by receiving feedback about partial channel quality information (CQI) from a receiver, transmission/reception information between selected terminals and base stations is allocated to limited wireless resources at a time determined to be most effective, and the resources allocated to the terminals are applied with a proper modulation and coding scheme.
Meanwhile, vibrant studies about broadband wireless communication systems, such as LTE Release 8/9, are in progress. In the broadband wireless communication system, the modulation scheme of transmission data and the coding rate of error correcting codes may be determined to be suitable for the channel environment before the data is transmitted.
The modulation and coding scheme (MCS) level for the downlink or uplink transmission of the broadband wireless communication system may be implemented in various ways, and typically, it may be determined in the following manner.
In the case of the downlink, when channel quality information and a reply signal to downlink data, i.e., acknowledgement (ACK)/negative acknowledgement (NACK) signal, are received from the terminal, the base station may determine the channel status of the terminal on the basis of the channel quality information and the ACK/NACK signal. In addition, the base station may detect an MCS level corresponding to the determined channel status with reference to an MCS determination table including predetermined MCS level information according to the channel status. Next, the base station may transmit downlink data to the terminal using the detected MCS level.
Although the uplink may be implemented in various ways, typically, the base station may receive uplink channel quality information {sounding reference signal (SRS)} from the terminal, and may measure an SRS-based SINR from the average interference-over-thermal (IoT) of PUSCH of each cell. In addition, the base station may receive a reply signal to uplink data, i.e., an acknowledgement (ACK)/negative acknowledgement (NACK) signal, and may determine the channel status of the terminal on the basis of the channel quality information (the SRS-based SINR) and the ACK/NACK signal.
In addition, the base station may detect an MCS level corresponding to the determined channel status with reference to an MCS determination table including predetermined MCS level information according to the channel status. Next, the base station may transmit downlink data to the terminal using the detected MCS level.
Alternatively, in the case of the uplink, the channel status of the terminal may be estimated based on only the reply signal to the uplink data, i.e., the ACK/NACK signal, without considering separate uplink channel information, and the MCS level corresponding to the determined channel status of the terminal may be detected with reference to the MCS determination table including predetermined MCS level information according to the channel status.
As mentioned above, according to the prior art, the base station determines the MCS only by interference information measured in each cell, uplink pilot strength information received from the terminal, and the ACK information without considering interference of the actual terminal channel environment.
A difference of the methods for determining MCS between the downlink and the uplink stems from the fact that the interference of the downlink can be preliminarily estimated because the terminal directly measures the interference and reports the same in the channel quality information to the base station, whereas the interference of the uplink cannot be estimated because interference signal sources, which are dynamically changed, cannot be accurately recognized in the uplink.
Particularly, in the case of the uplink, the terminal allocated with the resource in the neighboring cell varies dynamically, so the interference signal source is not fixed. Therefore, it is impossible to estimate an optimal MCS on the basis of real time SINR information, so two MCS determination methods may be applied for implementation.
That is, the MCS level is determined by referring to interference strength information that is averaged in a cell unit in order to determine the SRS-based SINR information, which is referred to in determining the MCS, or by estimating the channel quality only through the ACK/NACK determination in a receiver without the help of the SRS-based SINR information.
In this case, since an adaptive modulation and coding (AMC) determining unit determines the MCS level by considering only limited channel quality information and limited (not real time/not in a UE unit) interference information, which is averaged in a cell unit, in estimating the MCS, the uplink frequency efficiency becomes quite low, compared with the downlink.
Because an actual interference signal source dynamically changes a lot according to the scheduling result of the nearby (neighboring) cells, whereas since an interference estimated value relies on a cell-specific average expected value, the channel quality information just depends on the ACK/NACK information, and the open-loop rate control (OLRC) is inefficiently operated without an accurate reference so that the uplink frequency efficiency may be lowered.
According to the conventional AMC determination method, the AMC_SINR is determined in accordance with the cell-specific average expected value.
Meanwhile, the CoMP standard, which is under discussion as a study-item or a work-item in the LTE-Advance system after the LTE system, plans to introduce the coordinated multi-point (CoMP) transmission/reception technology or the multi-cell multiple input multiple output (MIMO) technology, in which neighboring cells cooperate with each other by considering the instant channel and traffic situation of the terminal located in the cell-boundary (hereinafter, referred to as a boundary terminal).
According to the downlink method of the coordinated scheduling/coordinated beamforming (CS/CB) CoMP among the CoMP technologies, when a plurality of base stations (BS) communicate with the terminals through antenna beamforming, the terminal may select the antenna beamforming of the base station to thereby increase the capacity of the boundary terminal.
At this time, each terminal may select the antenna beamforming of each base station such that the signal of a serving base station becomes a maximum value, and the interference signal from the nearby base stations is a minimum value. Here, the CS/CB CoMP cooperative base stations transmit data to only its own terminals rather than the terminals in the neighboring cooperative cells.
The CoMP technology requires a lot of information, which is to be transmitted through the backhaul for the cooperation of the base stations, and complicated scheduling and CoMP signal processing calculation for the resource allocation. However, since the CoMP technology has an advantage of raising the cell boundary and cell average capacity compared to the ICIC technology, it is spotlighted as the cell cooperative scheduling technology with reduced implementation complexity.
The inter-cell cooperative scheduling technology, which can be implemented in the uplink, may exemplify the technology in which the terminals are separated as interfering/non-interfering UEs using long-term channel strength information in each terminal, and inter-cell cooperative scheduling of an interference-coordination function is performed. In the uplink system, if such an advanced inter-cell cooperative scheduling introduces the effective MCS estimation method based on the real time interference, in addition to the benefit of the inter-cell cooperative scheduling, the inefficient uplink MCS determination method can be improved to considerably enhance the uplink frequency efficiency.
In the uplink system, if the advanced inter-cell cooperative scheduling does not introduce the effective MCS estimation method based on the real time interference, the inefficient uplink MCS determination method may not be improved and there may be a limit in enhancing the uplink frequency efficiency.