Capacity of broadcast fading channels with variable rate and power

We obtain the broadcast (downlink) channel capacity region in fading and AWGN for time-division, frequency-division, and code-division. For all of these techniques, the maximum capacity region is achieved when the transmitter varies the data rate sent to each user as their channels vary. This optimal scheme requires channel estimates at the transmitter, dynamic allocation of timeslots, bandwidth, or codes, and variable-rate and power transmission. For both AWGN and fading channels, nonorthogonal code-division with successive decoding has the largest capacity region, while time-division, frequency-division, and orthogonal code-division have the same smaller region. However, when all users have the same average SNR, the capacity regions for all these techniques are the same. We also examine the capacity region of nonorthogonal code-division without successive decoding, and of orthogonal code-division when multipath corrupts the code orthogonality.