annels and allocating a subchannel to each user with which the user's data is sent. The radio then modulates the data on a carrier wave at the subchannel frequency. Note that FDMA can also (rapidly) assign the user to a new frequency in the band, which is a way to re- duce interference and improve service-a technique known as frequency hopping. TDMA is a technique to divide a single frequency channel into multiple time-slots. These time- slots can be used by different transmitters; for example, user 1 uses time-slot 1 and user 2 uses time-slot 2. TDMA allows the entire channel bandwidth to be used by each user in turn during their given time-slot. Each radio modulates the data on a carrier wave at the frequency of the channel, but only during their assigned transmission time-slot. CDMA works on a principle where spread-spectrum techniques are used to distinguish between different users sharing a frequency-each user has access to the entire bandwidth for the entire time duration. Many different signals with unique spreading codes can be modulated onto a single carrier to support multiple users. Each radio modulates the data on a carrier wave at the frequency of the channel but only using the assigned spreading code. Now that the techniques for access to the radio resource have been covered, the next step is to see how the separation between transmission and reception is handled to enable simultaneous two-way communication, or full duplex. As illustrated in Figure 10-3, radio systems implement two key techniques to enable full duplex: frequency division duplex (FDD) or time division duplex (TDD). (((p))) Guard (((2))) FDD uses two frequency bands at the same time: TDD uses same frequency for uplink and f1 for uplink and f2 for downlink downstream but different time slots Figure 10-3 FDD Versus TDD FDD allocates a channel in two different frequency ranges-one channel is used for each direc- tion of transmission: uplink and downlink. A guard band separates the two frequency bands to avoid any interference. For exampl