1.  Field of the Invention
The present invention relates to a traffic measurement device, which measures traffic volume at a base station that communicates by connecting a radio link with a mobile station using code division multiple access and a traffic measurement method. 
2.  Description of the Related Art
Mobile communication systems widely used at present provide a mobile communication service with the entire service area divided into radio zones that are referred to as cells.  This type of mobile communication system includes a plurality of base stations that cover the cells, and mobile stations that communicate by connecting radio links with these base stations.  The scale of the equipment provided at each base station differs depending on the traffic volume of the cell covered by that base station.  Accordingly, system engineers designing mobile communication system design base station equipment by closely estimating traffic demand. 
Conventionally, before base stations are placed, system engineers estimated the traffic demand at each base station based on, for example, the population of the district in which the base station is to be placed or the type of district, namely whether it is a commercial zone or residential zone, or the like.  Furthermore, after the base stations are actually placed and mobile communication service has started, system engineers measured how many users are using each base station, namely, the actual traffic volume at each base station.  System engineers then decided whether to expand or scale down the equipment provided at each base station based on the measured actual traffic volume. 
For example, in a mobile communication system adopting a frequency division multiple access (FDMA) or time division multiple access (TDMA) as the multiple access where radio links connect the base stations and mobile stations, the actual traffic volume was grasped by counting the number of connected radio links (number of channels). 
On the other hand, in a called third generation mobile communication system that uses code division multiple access (hereafter referred to as CDMA) as the multiple access, each base station observes communication carried out at neighboring base stations as interference.  Thus, the traffic volume at a base station is the sum of the number of mobile stations actually connected to the base station (hereafter referred to as the actual number of users), which can be determined from the number of radio links (number of channels) connected between the base station and the mobile stations, and the number determined by converting the interference volume from the neighboring base stations into the number of radio links (number of channels) connected between the base station and the mobile stations, namely, the number of mobile stations connected to the base station (hereafter referred to as the converted number of users).  In other words, in the case of using CDMA, the traffic volume at a base station is not defined only by the actual number of users determined from the number of radio links, but is also subject to the communication conditions of the neighboring base stations. 
Consequently, at a base station using CDMA, whether a new call is received or not is determined according to the assumption that the interference volume at the base station corresponds to the traffic volume on the uplink and that the transmission power at the base station corresponds to the traffic volume on the downlink (Japanese Patent Application Laid-open No.  Hei.  8-191481, International Publication No.  WO98/30057). 
Furthermore, it has been verified through computer simulation that the corrected value of the interference volume on the uplink of a base station using CDMA corresponds to the traffic volume (Ishikawa and Iwamura, “Estimation Method of Interference Power Distribution and Call Blocking Rate in W-CDMA Reverse link”, 2000 The Institute of Electronics, Information and Communication Engineers General Conference, B-5-31, March 2000). 
In addition, to design a mobile communication system in consideration of radio wave propagation, a technique of estimating the radio wave propagation conditions of the service area by simulating the radio wave propagation based on the specifications of the base stations or mobile stations or topographical data using a computer may also be used (Fujii, Asakura, and Yamazaki, “Cell Design System for Mobile Communications”, NTT DoCoMo Technical Journal Vol.  2, No.  4, pp.  28-34, January 1995, and Ohmatsuzawa and Yamashita, “Total Support System for Base Station Design”, NTT DoCoMo Technical Journal Vol.  4, No.  1, pp.  28-31, April 1996).  With this technique, data such as altitude data, topographical data, or traffic volume is stored for each minutely divided area, and the signal-to-noise power ratio (SNR) at each reception point or the traffic volume at each base station is calculated based thereto. 
However, the method of assuming that the interference volume at a base station on the uplink and the transmission power at the base station on the downlink respectively correspond to the traffic volume had the following problems.  Though the interference volume or transmission power corresponded to the traffic volume so that the traffic volume increased if the interference volume or transmission power increased, this value was not directly proportional to the actual number of users, and thus was not the actual traffic volume itself.  Accordingly, the traffic volume could not be accurately grasped from the interference volume or transmission power, and the load tasked to the base station by the increasing traffic volume (hereafter referred to as the traffic load) could not be adequately grasped. 
Furthermore, with the method of determining the value that corresponds to the traffic volume by correcting the interference volume on the uplink of the base station using CDMA through computer simulation, the reception power per mobile station had to be measured and the entire limit capacity needed to be grasped in advance.  Essentially, there were many parameters difficult to ascertain.  Accordingly, since the traffic volume itself could not be accurately grasped and the traffic volume could not be easily determined with this method, actual utilization was difficult.  In addition, this method could not be used to determine the value that corresponds to that traffic volume on the downlink.  The technique of estimating the radio wave propagation of the service area was also not capable of grasping the accurate traffic volume itself at a base station and estimating the traffic load.  Furthermore, various data needed to be stored, and the traffic volume could not be easily determined.