rence among spread-spec- trum radios within a network of cells is the key to achieving high overall capacity for that network. 3.5 THE POTENTIAL CAPACITY OF DIRECT-SEQUENCE SPREAD-SPECTRUM CDMA IN HIGH-DENSITY NETWORKS For practical commercial spread-spectrum radios, the potential capacity dis- cussed in the previous section is that achievable with direct-sequence spread- ing. In principle, frequency-hopping spreading can achieve similar results but requires fast hopping implementation, which is less practical in low-cost commercial applications. Thus, this discussion is limited to direct-sequence spread-spectrum radio systems. To understand the apparent paradox of achieving higher overall capacity with spread-spectrum radios than with conventional radios, consider a com- parison of TDMA versus CDMA for a single-star network as shown in Figure 3.4. Assuming the same fixed total bandwidth available for both TDMA and CDMA, Figure 3.4 shows the number of channels on the hori- zontal axis and the intolerable level of interference on the vertical axis. Initially, consider the performance of a single radio or multiple radios located at the cell site that receive TDMA or CDMA signals from mobile units surrounding the cell. TDMA is a time-dimension-limited system in which there can be no addi- tional users when all slots have been assigned. As time slots are filled by increasing numbers of TDMA users, there is no interference caused by one mobile radio to the reception of another mobile radio at the cell site. The number of TDMA users can increase until the number of dimensions (in this case, time slots) is exhausted. It is not possible to increase the number of TOLERABLE INTERFERENCE THRESHOLD NUMBER OF CHANNELS Figure 3.4. TDMA versus CDMA. Commercial Applications users beyond the dimension limit without causing an intolerable amount of interference to reception of a mobile unit at the cell-site receiver. Thus, as shown in Figure 3.4, the TDMA system has a "brick wall" limit on capacity. Spread-spectrum radios can tole