THE CAPACITY OF DOWNLINK FADING CHANNELS WITH VARIABLE-RATE AND POWER

We obtain the Shannon capacity region of the down-link (broadcast) cha nnel in fading and additive white Gaussian noise (AWGN) for time-divis ion, frequency-division, and code-division. For all of these technique s, the maximum capacity is achieved when the transmitter varies the da ta rate sent to each user as their channels vary. This optimal scheme requires channel estimates at the transmitter; dynamic allocation of t imes-lots, bandwidth, or codes; and variable-rate and power transmissi on. For both AWGN and fading channels, nonorthogonal code-division wit h successive decoding has the largest capacity region, while time-divi sion, frequency-division, and orthogonal code-division have the same s maller region. However, when all users have the same average received power, the capacity region for all these techniques is the same. In ad dition, the optimal nonorthogonal code is a multiresolution code which does not increase the signal bandwidth. Spread-spectrum code-division with successive interference cancellation has a similar rate region a s this optimal technique, however, the region is reduced due to bandwi dth expansion. We also examine the capacity region of nonorthogonal co de-division without interference cancellation and of orthogonal code-d ivision when multipath corrupts the code orthogonality. Our results ca n be used to bound the spectral efficiency of the downlink channel usi ng time-division, frequency-division, and code-division, both with and without multiuser detection.