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Tham khảo tài liệu 'quality of service and resource allocation in wimaxfig part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 116 Quality of Service and Resource Allocation in WiMAX Note that I contains aN subcarriers. The remaining 1 a N subcarriers are shared by the three sectors in an orthogonal way such that each base stations c has at its disposal a subset Pc P as in Protected of cardinality N. If user k modulates a subcarrier n e Pc then process Wk n m will contain only thermal noise with variance Ớ2. Finally I u PA u PB u Pc 0 1 . N 1 . Also assume that channel coefficients HCc n m ngNk are Rayleigh distributed and have the same variance Pk E IHk n m 2 Vn e Nk. This assumption is realistic in cases where the propagation environment is highly scattering leading to decorrelated Gaussian-distributed time-domain channel taps. Under all the aforementioned assumptions it can be shown that the ergodic capacity associated with each user k only depends on the number of subcarriers assigned to user k in subsets I and Pc respectively rather than on the specific subcarriers assigned to k. The resource allocation parameters for user k are thus i The sharing factors 7k j Yk P defined by 7k j card I n Nk N yC P card Pc n Nk N . 14 ii The powers Pkj Pkp transmitted on the subcarriers assigned to user k in I and Pc respectively. We assume from now on that Yki and Yk P can take on any value in the interval 0 1 not necessarily integer multiples of 1 N . Remark 3. Even though the sharing factors in our model are not necessarily integer multiples of 1 N it is still possible to practically achieve the exact values of Yk j Yk P by simply exploiting the time dimension. Indeed the number of subcarriers assigned to user k can be chosen to vary from one OFDM symbol to another in such a way that the average number of subcarriers in subsets I Pc is equal to YkiN Yk PN respectively. Thus the fact that Ykj Yk P are not strictly integer multiples of 1 N is not restrictive provided that the system is able to grasp the benefits of the time dimension. The particular case where the number of subcarriers is restricted
