e, the up- per half of a band may be used for transmission and the lower half of the band may be used for reception. In FDD, the guard band does not affect the throughput of the user because it is like a highway median strip-a separation of the "lanes" of traffic going in different directions. TDD uses a single frequency for both uplink and downlink and assigns different time-slots for transmission and reception. For example, the system may allow an individual user to transmit for 4 ms and then require them to receive for the next 4-ms time-slot. Guard times (or guard periods [GP] are used to separate the transmit and receive time-slots and al- low time for the radio to switch between transmit and receive modes (which may take several mi- croseconds). Additionally, the guard time reflects the propagation time between the two radios. For example, if the user equipment (UE) transmitter is 10 km away, the signal will take 33 us to get from handset to cell site, then the receiver at the cell site must allow 33 us for the signal to arrive before it can use the same frequency to transmit. For short range, this is a trivial problem, but for longer range, using guard time in this way con- sumes more and more of the available time for signal transmission-reducing the effective through- put of the system. On the other hand, although using smaller guard times provide higher throughput, that only works in combination with strict synchronization between the handset and the cell site. There are many advantages and disadvantages of FDD- and TDD-based implementations and, de- pending on the deployment requirements, mobile operators adopt one over the other. Table 10-1 captures a quick comparison of FDD versus TDD (UL is uplink, and DL is downlink). In summary, mobile telecommunication systems have evolved to use various combinations of mul- tiple access and duplexing techniques. Some examples: Advanced Mobile Phone Service (AMPS) was the first cellular system to assign a pair of fre- quencies to facilitate full-duplex operation.