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In this paper, we study the code assignment problem in high speed downlink packet access (HSDPA) networks. In HSDPA, the spreading factor of codes is fixed at 16, and the maximum number of bits reserved for code assignment transmitted on the control channel is limited to 64 bits. This high control overhead may lead to inefficiencies in HSDPA systems when the number of mobile nodes in a cell becomes large. Existing code assignment mechanisms allow codes with different spreading factors to be allocated, rendering them inadequate for HSDPA systems. In this paper, a new segment-based code assignment mechanism is proposed. In our mechanism, 16 codes are grouped into several segments. Since the number of codes in each segment is reduced, the corresponding signaling overhead is also reduced. The load of each channel is then balanced in each segment so as to maximize the system performance.