부산대학교 도서관

We consider a Gaussian multiple-access channel with random user activity where the total number of users ℓ n and the average number of active users k n may be unbounded. For this channel, we characterize the maximum number of bits that can be transmitted reliably per unit-energy in terms of ℓ n and k n . We show that if k n log ℓ n is sublinear in n, then each user can achieve the single-user capacity per unit-energy. Conversely, if k n log ℓ n is superlinear in n, then the capacity per unit-energy is zero. We further demonstrate that orthogonal-access schemes, which are optimal when all users are active with probability one, can be strictly suboptimal.