frame a 32 kbit s channel uses 2 tdma slots per frame and a 64 kbit s channel uses 4 tmda slots per tdma frame as shown in fig1 5 fig1 5 assumes that there is no space division multiple access however it may be possible to support more than one user on a given tdma slot if the users are geographically separated and the transceiver can take advantage of that separation to form spatial beams in order to increase the overall capacity of the system and to ensure viable deployment of the system in various interference levels propagation environments and possible required transmission ranges pwan may use various coded modulation schemes rates under benign channel conditions an efficient high rate code may be used if conditions get worse a low rate coded modulation scheme is used this is especially important considering the high availability requirements for a wireless local loop system as an example we consider the use of a bandwidth efficient rate 3 4 16 qam scheme lower rate codes can also be used the base transmits information to multiple rus in its cell this section describes the transmission formats for a 16 kbit s to 64 kbit s traffic channel together with a 1 kbit s to 4 kbit s link control channel lcc from the base to a single ru the 16 kbit s link is achieved by assigning one tdma slot per tdma frame the tdma frame is 3 ms long thus the effective data.