king turns indoor system with low mobility, where the quality only in transmitting, thus creating a TDMA-like operation varies slowly with time. with inter-user orthogonality in the time domain. Although the round-robin scheduler is simple, it is far from the optimal scheduling strategy. 4.5.2.4 Acquiring on channel-state However, the transmission power in a terminal is information limited and therefore additional sharing of the uplink resources in frequency and/or code domain is required. To select a suitable data rate, in practice a suitable This also impacts the scheduling decisions. For example, modulation scheme and channel-coding rate, the trans- terminals far from the base station typically operate in the mitter needs information about the radio-link channel power-limited region, in contrast to terminals close to the conditions. Such information is also required for the base stations which often are in the bandwidth-limited purpose of channel-dependent scheduling. In case of region (for a discussion on power-limited VS. bandwidth- a system based on frequency-division duplex (FDD), limited operation, see Chapter 4.3). Thus, for a terminal only the receiver can accurately estimate the radio-link far from the base station, increasing the bandwidth will channel conditions. not result in an increased data rate and it is better to only For the downlink, most systems provide a downlink assign a small amount of the bandwidth to this terminal signal of a predetermined structure, known as the and assign the remaining bandwidth to other terminals. downlink pilot or the downlink reference signal. This On the other hand, for terminals close to the base station, reference signal is transmitted from the base station an increase in the assigned bandwidth will provide with a constant power and can be used by the mobile a higher data rate. terminal to estimate the instantaneous downlink chan- nel conditions. Information about the instantaneous downlink conditions can then be reported to the base 4.5.2.3 Link adaptation and 