 The AMPS system is an example of a pure FDD full-duplex FDMA system. GSM uses TDMA and FDD in combination. A given user is allocated both an uplink and down- link channel of 200 kHz (and so, FDD) separated by a wide guard band (in many countries, this gap is/was 45 MHz). The single channel in each direction is further divided into eight time-slots to support eight users using TDMA. In comparison, the UMTS, 4G, and 5G architectures allow both FDD and TDD duplexing tech- niques in their radios. 10.2.1 Impact of Synchronization in FDD and TDD Systems Irrespective of the technology generation or mobile protocols utilized, all base stations must support a frequency accuracy of 50 parts per billion (ppb) at the radio air interface. To meet this requirement at the radio, the standards specify that a frequency with an accuracy of at least 16 ppb is available at the base station. This is implemented by providing a frequency signal of that accuracy, traceable to a primary reference clock (PRC) or primary reference source (PRS). Table 10-1 Comparison of FDD-Based Versus TDD-Based Communications Parameter Due to separate frequency bands for Spectral TDD requires only one frequency channel for UL and DL, spectrum efficiency is Efficiency both UL and DL and hence is more efficient. Because transmission and reception Because transmission and reception cannot hap- happens simultaneously, FDD is pen at the same time, there is time delay in TDD, Latency more suitable for low latency and latency is higher for TDD based communication. communication. UL and DL transmission is separated by guard Guard time, which allows for base station switching A guard band is used to separate the Band/Guard time between transmitter and receiver. Because UL and DL bands. no data can be transmitted during this time, this part of the spectrum is sacrificed. Because UL and DL band size is pre- Distribution of UL or DL bandwidth can be op- Traffic determined, it is not possible to make timized by adjusting the time-slots to match Asymmetry dynamic chan