Patent Publication Number: US-2006018276-A1

Title: Resource allocation method for downlink transmission in a multicarrier-based CDMA communication system

Description:
PRIORITY  
      This application claims priority to an application entitled “RESOURCE ALLOCATION METHOD FOR DOWNLINK TRANSMISSION IN MULTICARRIER-BASED CDMA COMMUNICATION SYSTEM”, filed in the Korean Intellectual Property Office on Jul. 10, 2004 and assigned Serial No. 2004-53813, the contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a wireless communication system, and more particularly to a resource allocation method for downlink transmission in a multicarrier-based Code Division Multiple Access (CDMA) communication system.  
      2. Description of the Related Art  
      Demand for high-speed data transmission and multiple access techniques is on the rise along with the increased use of personal mobile communications, creating a surge in the development of industries based on mobile communications. To meet this demand, a new hybrid technology for combining wireless digital modulation with multiple access has been recently proposed.  
      A Direct Sequence-Spread Spectrum (DS-SS) CDMA system, which is a user multiplexing technique for efficiently sharing wireless communication channels, is known to have resistance to frequency selectivity of channels. The capacity of the DS-SS CDMA system is limited by Multiple Access Interference (MAI) resulting from incomplete auto-correlation and cross-correlation characteristics of spreading codes.  
      In flat fading channels, MAI can be removed using orthogonal codes having no cross-correlation. However, in frequency-selective fading channels, the orthogonality cannot be guaranteed due to inter-chip interference, which causes MAI and degrades system performance.  
      Orthogonal-Frequency-Division-Multiplexing-Code-Division-Multiple-Access (OFDM-CDMA) has been proposed to suppress the inter-chip interference in frequency-selective fading channels, which combines CDMA with OFDM creating a multicarrier modulation scheme.  
      In the OFDM system, multiple carriers are allocated to transmit data, so that the overall transmission rate is high and each carrier transmission rate is low. Even though the transmission rate is low in each carrier, the OFDM system can perform smooth transmission, enabling high-speed communications in multi-path channel environments.  
      The OFDM system generally transmits information of one user through one subchannel at a specific time. Technologies such as MC DS-CDMA (Multicarrier Direct-Sequence CDMA) and VSF-OFCDM (Variable Spreading Factor-Orthogonal Frequency Code Division Multiplexing) have been proposed to increase the efficiency of each subchannel in the OFDM system. Although these technologies have advantages of the physical layer by applying CDMA in channels, they provide no detailed and effective implementation method for management of wireless resources.  
      Thus, there is a need to provide a detailed and effective resource allocation method for achieving efficient resource management in the OFDM-CDMA system.  
     SUMMARY OF THE INVENTION  
      The present invention has been made in view of the above problem, and it is therefore an object of the invention to provide a resource allocation method for downlink transmission in a multicarrier-based wireless communication system, which increases system capacity by allocating the same subchannel to one or more terminals according to channel environments in downlink transmission.  
      It is another object of the present invention to provide a resource allocation method for downlink transmission in a multicarrier-based wireless communication system, which efficiently manages wireless resources by dividing the service area of a base station into two virtual cells having different radii as a function of base station signal strength, and allocating the same subchannel to one or more terminals according to the locations of the terminals.  
      In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a resource allocation method in a communication system supporting multiple access of a plurality of terminals identified using spreading codes, the method including collecting, from terminals located in a service area of a base station, information of respective received signal strengths of the terminals for channels; determining whether a terminal having a received signal strength higher than a predetermined threshold for a specific subchannel is present; and allocating the specific subchannel to at least one terminal or at least two terminals according to the determination.  
      Preferably, if a terminal having a received signal strength higher than the predetermined threshold for the specific subchannel is not present, the specific subchannel is allocated to a terminal having a highest received signal strength, among the terminals located in the service area of the base station.  
      Preferably, if a terminal having a received signal strength higher than the predetermined threshold for the specific subchannel is present, it is determined whether or not the number of terminals having a received signal strength higher than the predetermined threshold for the specific subchannel is greater than 1. If the number of terminals having a received signal strength higher than the predetermined threshold for the specific subchannel is 1, the specific subchannel is allocated to the terminal having the received signal strength higher than the predetermined threshold for the specific subchannel. If the number of terminals having a received signal strength higher than the predetermined threshold for the specific subchannel is greater than 1, the specific subchannel is allocated to a terminal having a highest received signal strength and a terminal having a second highest received signal strength, among the terminals having a received signal strength higher than the predetermined threshold.  
      Preferably, if two or more terminals having a received signal strength higher than the predetermined threshold for the specific subchannel are present, the specific subchannel with maximum power is allocated to the terminal having the highest received signal strength, and the specific subchannel is allocated to the terminal having the second highest received signal strength, while gradually decreasing the maximum power allocated to the terminal having the highest received signal strength.  
      In accordance with another aspect of the present invention, there is provided a resource allocation method in a multicarrier-based mobile communication system including base stations and supporting multiple access of a plurality of terminals identified using codes, the method including dividing a service area of each base station into at least two virtual cells having different radii about a base station; and actively allocating resources to terminals located in the service area of the base station according to distribution of the terminals in the virtual cells.  
      Preferably, the radii of the virtual cells are determined based on received signal strength of the terminals for a specific subchannel.  
      Preferably, the radius of a first one of the virtual cells is determined based on maximum transmission power of a specific subchannel, and the radius of a second one of the virtual cells is previously determined based on a threshold transmission power lower than the maximum transmission power.  
      Preferably, the distribution of the terminals is determined based on received signal strength of the terminals for the specific subchannel, the received signal strength being fed back from the terminals.  
      Preferably, if the terminals are all distributed in the first virtual cell, the specific subchannel is allocated to a terminal having a highest received signal strength among the terminals in the first virtual cell.  
      Preferably, if the terminals are distributed in both the first and second virtual cells, the specific subchannel is allocated to a terminal present in the second virtual cell.  
      Preferably, if the terminals are distributed in both the first and second virtual cells and one terminal is present in the second virtual cell, the specific subchannel is allocated to the terminal present in the second virtual cell.  
      Preferably, if the terminals are distributed in both the first and second virtual cells and two or more terminals are present in the second virtual cell, the specific subchannel is allocated to a terminal having a highest received signal strength and a terminal having a second highest received signal strength for the specific subchannel, among the two or more terminals present in the second virtual cell.  
      Preferably, if two or more terminals are present in the second virtual cell, the specific subchannel with maximum power is allocated to the terminal having the highest received signal strength, and the specific subchannel is allocated to the terminal having the second highest received signal strength, while gradually decreasing the maximum power allocated to the terminal having the highest received signal strength and gradually increasing power for the terminal having the second highest received signal strength. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  is a block diagram illustrating an Orthogonal Frequency Division Multiplexing-Code Division Multiple Access (OFDM-CDMA) system, to which a resource allocation method according to a preferred embodiment of the present invention is applied;  
       FIG. 2  is a conceptual diagram illustrating a method for dividing the service area of a base station into virtual cells in a resource allocation method according to a preferred embodiment of the present invention;  
       FIG. 3  is a flow chart illustrating a resource allocation method according to a preferred embodiment of the present invention; and  
       FIG. 4  is a graph illustrating performance simulation results of the resource allocation method according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Now, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.  
       FIG. 1  is a block diagram showing an Orthogonal Frequency Division Multiplexing-Code Division Multiple Access (OFDM-CDMA) system, to which a resource allocation method according to a preferred embodiment of the present invention is applied.  
      As shown in  FIG. 1 , the OFDM-CDMA system includes a transmitter including Serial/Parallel (S/P) converters  11 , spreaders  12 , adders  13 , an OFDM modulation unit  15 , and a receiver including an OFDM demodulation unit  17  and a parallel/serial converter  19 . Each of the S/P converters  11  serial/parallel converts input information of each user. Each of the spreaders  12  multiplies each of the S/P converted signals output from the S/P converters  11  by a spreading code of a corresponding user. Each of the adders  13  adds spread signals of different users output from the spreaders  12 . The OFDM modulation unit  15  performs an Inverse Fast Fourier Transform (IFFT) on signals output from the adders  13 , Parallel/Serial (P/S) converts the signals, and inserts guard intervals into the parallel/serial converted signals to transmit the signals through channels. In the receiver, the OFDM demodulation unit  17  receives the signals transmitted from the transmitter, removes guard intervals from the received signals, and S/P converts the resulting signals, and then performs a Fast Fourier Transform (FFT) on the converted signals. The P/S converter  19  converts parallel signals output from the OFDM demodulation unit  17  on a user-by-user basis, and outputs the converted signals.  
       FIG. 2  is a conceptual diagram illustrating a method for dividing the service area of a base station into virtual cells having different radii about the base station in a resource allocation method according to a preferred embodiment of the present invention.  
      As shown in  FIG. 2 , for each of the base stations  23  and  25 , two virtual cells are defined based on a Signal to Interference Ratio (SIR), taking account into the signal strength of a subchannel which a terminal receives from the corresponding base station and interference signal strength in the same subchannel of a neighboring cell. Specifically, the service area of the first base station  23  is divided into a first cell  23   a,  which is located near the base station  23  and has a high signal strength, and a second cell  23   b  which is located distant from the base station  23  and has a relatively low signal strength. The service area of the second base station  25  is divided into first and second cells  25   a  and  25   b  in the same manner.  
      In this cell environment, terminals located in the first cells  23   a  and  25   a  can transmit data at a higher rate than terminals located in the second cells  23   b  and  25   b.    
      Terminals located in each cell transmit virtual cell location information to the base station, and the base station allocates resources to the terminals using the cell location information received from the terminals.  
      A detailed description will now be given of a method for allocating resources for downlink data transmission in the cell environment configured as described above.  
       FIG. 3  is a flow chart illustrating the resource allocation method according to a preferred embodiment of the present invention.  
      First, each terminal receives pilot symbols of all subcarriers transmitted from the base station, measures an SIR of each of the subcarriers, and reports the received signal strength of each subcarrier to the base station through a feedback message in the uplink.  
      In  FIG. 3 , at step S 301 , the base station receives the feedback message from each terminal, compares a received signal strength (P strength ) of a specific subchannel, the information of which is included in the feedback message, with a predetermined threshold (P threshold ) at step S 302 , and counts the number of terminals (N G ) that have a received signal strength (P strength ) higher than the predetermined threshold (P threshold ) at step S 303 .  
      At step  304 , the base station then determines whether or not the number of terminals (N G ) is zero. That is, at step  304 , the base station determines whether or not there is a terminal having a received signal strength (P strength ) in the specific subchannel higher than the predetermined threshold (P threshold ). If there is no terminal having a received signal strength (P strength ) in the specific subchannel higher than the predetermined threshold (P threshold ), i.e., if the received signal strength of all terminals for the specific subchannel is lower than or equal to the predetermined threshold, the base station determines that all the terminals are located in the second cells  23   b  or  25   b  of  FIG. 2 , and allocates the specific subchannel to the terminal having the highest received signal strength at step S 305 . Here, the base station allocates maximum available power to the specific subchannel for the terminal having the highest received signal strength.  
      If a terminal has a received signal strength (P strength ) greater than the predetermined threshold (P threshold ) for the specific subchannel (i.e., N G ≠0) at step  304 , the base station determines if there is more than one terminal (N G &gt;1) with a received signal strength (P strength ) greater than the predetermined threshold (P threshold ) at step S 306 . If the number of terminals (N G ) is 1, the base station allocates the specific subchannel to the only terminal having the sufficient received signal strength (P strength ) that is greater than the predetermined threshold (P threshold ) at step S 307 . The fact that only one terminal has a received signal strength (P strength ) great enough to exceed the threshold indicates that the terminal is located in the cell  23   a  or  25   a  and the remaining terminals are located in the cell  23   b  or  25   b.    
      If the number of terminals (N G ) with the required received signal strength is greater than 1, the base station allocates the specific subchannel to the terminals with the highest and second highest received signal strengths at step S 308 .  
      At step S 308 , the base station allocates the specific subchannel with the maximum power to the highest received signal strength terminal, and then allocates the same subchannel to the second highest received signal strength terminal. At the same time, power allocated to the terminal having the highest received signal strength is decreased while power to the terminal having the second highest received signal strength is increased. This procedure is performed until the combined output power of the two terminals is maximized.  
       FIG. 4  is a graph illustrating performance simulation results of the resource allocation method according to the present invention.  
      This simulation is performed 1000 times in an environment with an orthogonal factor θ of 0.2 by allocating 256 subchannels and 30 terminals to each cell in a multi-cell environment. In this simulation, the average throughput per subchannel of the conventional resource allocation technique where one terminal is allocated to one subchannel, and the average throughput per subchannel of the resource allocation technique according to the present invention, where two terminals are allocated to one subchannel using the proposed cell division method, are compared.  
      It can be seen from  FIG. 4  that the throughput per subchannel is increased by allocating two terminals to one subchannel if the orthogonal factor θ has a relatively small value, i.e., if θ=0.2.  
      The present invention provides a resource allocation method for downlink transmission in a CDMA system, particularly, an OFDM-CDMA system, which has the following features and advantages. Taking into consideration locations of all terminals in the service area of a base station, subchannels and power are dynamically allocated to the terminals, thereby enabling efficient resource management.  
      In addition, in the resource allocation method according to the present invention, the service area of the base station is divided into two virtual cells based on received signal strengths of the terminals for a specific subchannel, and the same subchannel (i.e., the specific subchannel) is allocated to one terminal or at most two terminals according to distribution of the terminals in the virtual cells, thereby significantly increasing system capacity.  
      Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.