184 IEEE COMMUNICATIONS LETTERS, VOL. 6, NO. 5, MAY2002 Improved Channel Assignment Scheme With Access Control for the Common Packet Channel in WCDMA Systems Jae-Woo So, Student Member, IEEE, Dong-Ho Cho, Senior Member, IEEE, and Minsoo Suk, Member, IEEE Abstract—We propose a channel assignment scheme with ac- cess control for obtaining better system performance within the common packet channel (CPCH) access procedure of wideband code-division multiple-access (WCDMA) systems. The proposed scheme is analyzed taking multiple access interference into con- sideration and is compared with other schemes in 3rd Generation Partnership Project (3GPP): the basic scheme, the channel moni- toring scheme, and the channel assignment scheme without access control. The results show that the proposed scheme provides high throughput, even under high-loading conditions, since the access control algorithm based on channel load prohibits excessive inter- ference. Index Terms—Access control, channel assignment, common packet channel, WCDMA. I. INTRODUCTION C ODE-DIVISION multiple-access (CDMA) techniques have recently become popular in third-generation mobile communication systems. In the last few years, many researches have focused on the design of access control schemes [1]–[3]. However, they have considered only CDMA systems with random access channels (RACH’s) or dedicated channels (DCH’s). The wideband CDMA (WCDMA) system proposed by 3GPP (3rd Generation Partnership Project) involves the initi- ation of a new common packet channel (CPCH) which is uplink transport channel carrying small and medium-sized packets, such as short message services (SMS) and web requests [4]. The CPCH access procedure consists of an initial access phase (AP), a collision resolution phase, and a packet transmission phase, as shown in Fig. 1. In the WCDMA system, Node B (similar to a base station) has codes mapped to each CPCH, different orthogonal complex signatures used for AP preambles, and collision detection (CD) preambles. The 3GPP adopted the basic (BA) scheme based on random access for the CPCH access procedure. In the BA scheme, a user equipment (UE) decides a CPCH code at an initial access phase by randomly selecting an AP signature from signatures which have an one-to-one mapping relationship with codes. Node B sends back an ACK or a NAK according to the status of thechannel selected by the UE. If the UE receives an ACK from Node B, it enters into Manuscript received December 26, 2001. The associate editor coordinating the review of this letter approving it for publication was Prof. S. Pierre. J.-W. So and D.-H. Cho are with the Department of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology, Daejon 305-701, Republic of Korea; E-mail: israin@comis.kaist.ac.kr. M. Suk is with the School of Electrical and Computer Engineering, Sungkyunkwan University, Suwon 440-746, Korea. Publisher Item Identifier S 1089-7798(02)05086-X. Fig. 1. Access procedure of CPCH. a collision resolution phase, in which one UE is distinguished from the many UE’s that have selected the same AP preamble. At this point, the UE receiving an ACK transmits a packet. However, the BA scheme results in an inefficient use of re- sources due to the random access. A channel monitoring (CM) scheme and a channel assignment (CA) scheme have been pro- posed to promote more efficient use of resources [4]. In the CM scheme, Node B broadcasts the CPCH status information. When a new packet arrives, the UE monitors the occupancy status of the CPCH codes and chooses one by randomly selecting an AP signature from the unoccupied CPCH codes. Uplink interfer- ence is thus reduced, since the number of unnecessary requests is prohibited. However, in practice, a CPCH status broadcast in- terval exists due to the UEs power consumption and the incre- ment of downlink interference. This causes failure of the UE’s channel selection at the initial access phase due to incorrect CPCH status information. Hence, the system throughput is dra- matically degraded as the CPCH status broadcast period increases [5]. In the CA scheme, Node B assigns a CPCH code to the UE, which requests a CPCH according to the total oc- cupancy of CPCH resources. In this way, the CA scheme maxi- mizes the efficient use of resources, but has the problem that up- link interference increases excessively under high-loading con- ditions, since the number of unnecessary requests increases. In this letter, we propose a channel assignment scheme with access control for the CPCH access procedure. The object of the proposed scheme is to guarantee high throughput even under high-loading conditions. The proposed scheme is analyzed con- sidering multiple access interference and is compared with other schemes in 3GPP: the BA scheme, the CM scheme, and the CA scheme without access control. II. SYSTEM MODEL AND ACCESS CONTROL ALGORITHM For the 3GPP WCDMA system under consideration, we con- sider the uplink performance of the CPCH. We are assuming an 1089-7798/02$17.00 © 2002 IEEE