k. Rather, from a user perspective, the data rate 4.5.2.1 Downlink scheduling provided over the radio interface should simply be as 'high as possible.' Actually, even in case of typical 'con- In the downlink, transmissions to different mobile stant-rate' services such as voice and video, (short-term) terminals within a cell are typically mutually orthogo- variations in the data rate are often not an issue, as long as nal, implying that, at least in theory, there is no in- the average data rate remains constant, assuming aver- terference between the transmissions (no intra-cell aging over some relatively short time interval. In such interference). Downlink intra-cell orthogonality can be cases, that is when a constant data rate is not required, an achieved in time domain, Time Division Multiplexing alternative to transmit-power control is link adaptation (TDM); in the frequency domain, Frequency-Domain by means of dynamic rate control. Rate control does not Multiplexing (FDM); or in the code domain, Code aim at keeping the instantaneous radio-link data rate Domain Multiplexing (CDM). In addition, the spatial constant, regardless of the instantaneous channel condi- domain can also be used to separate users, at least in tions. Instead, with rate control, the data rate is a quasi-orthogonal way, through different antenna ar- dynamically adjusted to compensate for the varying rangements. This is sometimes referred to as Spatial channel conditions. In situations with advantageous Division Multiplexing (SDM), although it, in most channel conditions, the data rate is increased and vice cases, is used in combination with one or several of the versa. Thus, rate control maintains the Eb/No ~ P/R at above multiplexing strategies and not discussed further the desired level, not by adjusting the transmission power in this chapter. P, but rather by adjusting the data rate R. This is illus- For packet data, where the traffic often is very bursty, trated in Figure 4.5-lb. it can be shown that TDM is preferable from a theoreti- It