transmission requirements and that are experiencing different channel gains in a multicarrier system. The transmission requirements include differentcoding schemes, target P, and noise margins, which are collectively characterized by the SNR gap rl . Moreover, each user may desire to transmit at a Constant Bit Rate (CBR) or at a Variable Bit Rate (VBR). Multiuser transmit optimization requires knowledge of the channel transfer functions of all users. Hence, a feedback channel is necessary for communicating back to the transmitter the channel information estimated at the receiver of each user. Multiuser loading is practical for use in fixed wireless systems or in environments where the channels vary slowly. If transmit re-optimization is done at a slower rate than the OFDM symbol rate, the overhead in the feedback channel can be low. Two typical L multiuser OFDM scenarios are considered. For the uplink scenario, users, with individual power constraints, wish to communicate with a common receiver at the base station. This is also known as the Multiple Access Channel (MAC). For the downlink scenario, the reverse is true whereby a base L station, with a total power constraint, wishes to communicate with users. This is also known as the In both scenarios, the users are either fixed or mobile, and are assigned priorities. broadcast channel. The problem of interest is then to find the capacity region, which is characterized by the set of . , . . simultaneously achievable rates (R1, R2, RL), for communication in a multiple access or broadcast channel. In the next two paragraphs, an attempt, though not a comprehensive one, is made to give a brief overview of some of the work done in these two areas. FDMA Formulation The optimization formulation for multiuser loading with the FDMA restriction is: . . , , . . . N . N) (1, In the above formulation, L (1, L) is the user index set, is the subchannel = = c N Sl index set and is the subchannel assignment for user 1. Moreover, is a measure of priority a1 of user 1, El,, is the energy allocated to user 1 for the nth subchannel, and g l , is the ratio of the 1 magnitude squared of the channel response and the noise power spectral density for user in the nth SNR subchannel. Associated with each user is the gap. Table. Optimal FDMA solution given fixed subchannel assignments
transmission requirements and that are experiencing different channel gains in a multicarrier system. The transmission requirements include differentcoding schemes, target P, and noise margins, which are collectively characterized by the SNR gap rl . Moreover, each user may desire to transmit at a Constant Bit Rate (CBR) or at a Variable Bit Rate (VBR). Multiuser transmit optimization requires knowledge of the channel transfer functions of all users. Hence, a feedback channel is necessary for communicating back to the transmitter the channel information estimated at the receiver of each user. Multiuser loading is practical for use in fixed wireless systems or in environments where the channels vary slowly. If transmit re-optimization is done at a slower rate than the OFDM symbol rate, the overhead in the feedback channel can be low. Two typical multiuser OFDM scenarios are considered. For the uplink scenario, L users, with individual power constraints, wish to communicate with a common receiver at the base station. This is also known as the Multiple Access Channel (MAC). For the downlink scenario, the reverse is true whereby a base station, with a total power constraint, wishes to communicate with L users. This is also known as the broadcast channel. In both scenarios, the users are either fixed or mobile, and are assigned priorities. The problem of interest is then to find the capacity region, which is characterized by the set of simultaneously achievable rates (R1, R2, . . . , RL), for communication in a multiple access or broadcast channel. In the next two paragraphs, an attempt, though not a comprehensive one, is made to give a brief overview of some of the work done in these two areas. FDMA Formulation The optimization formulation for multiuser loading with the FDMA restriction is: In the above formulation, L = (1, . . . , L) is the user index set, N = (1, . . . , N) is the subchannel index set and Sl c N is the subchannel assignment for user 1. Moreover, a1 is a measure of priority of user 1, El,, is the energy allocated to user 1 for the nth subchannel, and g l , is the ratio of the magnitude squared of the channel response and the noise power spectral density for user 1 in the nth subchannel. Associated with each user is the SNR gap. Table. Optimal FDMA solution given fixed subchannel assignments
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