Due to recent high interest of the 4th generation mobile communication system at home and abroad, developments of systems that satisfy the requirements thereof are being actively progressed. In particular, Orthogonal Frequency Division Multiplexing (OFDM) has been attracted by one of systems that are adapted to use in the 4th mobile communication system because of high transmission efficiency and simple channel equalization.
As typical multiple access schemes based on OFDM, there are OFDM-Frequency Division Multiple Access (OFDM-FDMA), OFDM-Code Division Multiple Access (OFDM-CDMA) and OFDM-Time Division Multiple Access (OFDM-TDMA). Among them, the OFDM-FDMA scheme allocates a different subchannel every user; and therefore, no Multiple Access Interference (MAI) is issued in intra cell and the number of subchannels and modulation level can be effectively changed depending on a transmission rate required by each user and channel circumstance. Thus; this scheme is advantageous to adaptive loading. In addition, since the OFDM-FDMA scheme is suitable for use of a great number of subcarriers, it can be applied to wireless communication systems with cells of wide area having relatively large time delay spread. And also, the OFDM-FDMA has drawn attention as a multiple access scheme for the 4th mobile communication system because it is suited to systems that support high vehicular speed.
Meanwhile, duplex schemes are largely classified into Frequency Division Duplex (FDD) and Time Division Duplex (TDD).
The FDD scheme allows an uplink and a downlink to use different frequency bands wherein an interval between the frequency bands is enough as large as interference therebetween is negligible. This FDD scheme is mainly employed in macro cells that must provide relatively high time delay spread and high vehicular speed.
The TDD scheme shares a same frequency band for an uplink and a downlink, which is distinguished by setting a different time. This scheme can support services with asymmetric data transmission rate by modifying the number of slots of uplink and downlink and doesn't need to do a separate channel estimation of uplink and downlink by reciprocity characteristic of channel. However, such a scheme needs a guard time owing to round trip time and is mainly utilized in cells such as micro cells or hot-spot due to power problem.
The 4th mobile communication system is required to support high transmission rate and secure a variety of service qualities by using limited wireless resources. For this, there has been a need for developments of cell planning and resource allocation algorithms to increase cell capacity and efficiently allocate wireless resources to users.
On the other hand, new services are needed for mobility guarantee and high speed transmission under wireless environments. According to this need, technical development and standardization works for portable internet service of 2.3 GHz band and Mobile Broadband Wireless Access (MBWA) service are being progressed in the country.
As mentioned above, the OFDM scheme is one of the most spotlighted techniques due to the high transmission efficiency and simple channel equalization.
Meanwhile, the most important property, which decides the performance of an OFDM multiple access-based system in cellular environments where mobility is considered, is Frequency Reuse Factor (FRF). This FRF implies the number of frequency sets used per cluster of mobile phone cells. In mobile phone business, it is important to accommodate the maximum subscribers with limited allocation frequencies, which is indirectly meant by FRF. As property of wireless scheme itself, an increase of FRF is possible by dividing service area into cells, micro-cells, pico-cells, etc. and again subdividing antenna sectors within a same cell, and with technique such as smart antenna, except for its own improvement.
If FRF is set as “1,” it may be most ideal in view of throughput of base station because it uses all wireless resources; but there May be serious performance degradation due to inter-cell interference caused by FRF “1.”
To implement FRF “1” while solving the performance degradation problem by the inter-cell interference, there is developed a flash-OFDM system by the Flarion company. This flash-OFDM system employs a scheme that maximally prevents the performance degradation by the inter-cell interference with Low Density Parity Check (LDPC) channel code by using a frequency hopping scheme that changes subcarriers of OFDM with a constant pattern. In addition, there is a scheme that randomly punches subcarriers to decrease collision of themselves and their adjacent cells and so as to implement FRF “1.”
In the systems that maintain FRF as “1”, however, the performance degradation still exists at the boundary of cells where channel condition is poor due to the inter-cell interference by increase of traffic load. Accordingly, as method for securing the performance of user terminals in an area where channel condition is poor such as the boundary of cells, together with improvement of frequency efficiency, there is a frequency reuse partitioning scheme.
The frequency reuse partitioning scheme is one of effective methods capable of elevating the frequency efficiency.
FIG. 1 is a view of describing a general frequency reuse partitioning scheme.
Basic idea of the frequency reuse partitioning scheme is that cells 111 to 117 are divided into an inner cell 101 and an outer cell 102 based on the length between a base station and a user terminal or an intensity of pilot signal sent from the base station to the user terminal, and then different FRFs are applied to the inner cell 101 and the outer cell 102.
As may be seen from FIG. 1, if the user terminal exists in the outer cell 102 area, subchannels with FRF “7” are allocated to cells 111, 112, 113, 116; and if the user terminal is within the inner cell 101 area, subchannels with FRF “1” are assigned to cells 114, 115, 117.
As mentioned above, the reason the channels with different FRFs are allocated to the inner cell 101 and the outer cell 102 is because it may be an importance factor that restricts cell radius of cellular system for the following: in case of terminals near to a base station, channel state is good since power loss by path loss is less than that of terminals far from the base station, while, in case of terminals around the boundary of cells, performance degradation is occurred and data transmission rate is constrained due to serious affection by power loss by path loss and inter-cell interference.
Therefore, in use of the frequency reuse partitioning scheme, channels with low FRF are assigned to the user terminals 114, 115, 117 in the inner cell 101 area to secure a proper level of service quality since their channel state is generally good, thereby enlarging the capacity of cell. Meanwhile, channels with high FRF are allocated to the user terminals 111, 112, 113, 116 in the outer cell 102 area because their channel state is relatively poor so that a radius of cell can be expanded and thus the same level of service quality and data transfer rate as that of the terminals in the inner cell can be secured, in case of the user terminals located at the boundary of cells.
In addition, wireless resource allocation methods have been recently studied to effectively employ limited frequency resources while alleviating inter-cell interference.
If it is assumed that channel is stationary and channel response of user is accurately perceived by transmission end, it is known that a combined scheme of water filtering and adaptive modulation technique is optimal one. This water filtering technique has been mainly developed only in single-user systems or multi-user systems that support constant resource allocation: for example, a system using TDMA or FDMA allocates slots or frequency channels for a given time for each user and then applies an adaptive modulation technique for channels of each user.
In the multi-user OFDM scheme that adopts the adaptive modulation technique based on the constant resource allocation as described above, however, there is a drawback in that it cannot know optimal resource allocation that can be provided by actual system. The reason is because there are many unused channels when water filling algorithm is applied due to presence of subchannels that suffers from deep fading or subchannels to which high power cannot be given in view of frequency selective channel characteristic.
However, there may be an instance where channel appeared as deep fading to one user may not be appeared as deep fading to another user. Generally, if the number of users is increased, a probability that each of subchannels constituting OFDM is appeared as deep fading channel to all users is gradually decreased. In other words, a multi-user diversity gain can be acquired by bearing independent channel as the number of users is increased.
Consequently, there has been required a study of an improved scheme capable of enabling an optimal resource allocation by dynamically allocating relatively good channel to each user based on channel information of all users and then applying an adaptive modulation scheme using those channels.