Mobile broadband services over cellular systems are gaining momentum with the introduction of HSPA (High Speed Packet Access) radio bearers in networks. There is also an increased interest in IMS (IP Multimedia Subsystem) based services, such as VoIP (Voice over IP), PoC (Push-to-talk over Cellular) and presence. HSPA introduces the possibility of downloading and uploading data with speeds of several Mbits/s.
As traffic demands increase, the networks are planned denser. A general trend is to move antennas down from radio towers to building rooftops, and even down to the street level, to support higher end-user bit rate and system capacity demands. For street-level users, the propagation environment has changed to often comprise LOS (line of sight) situations. Moving antennas downs can result in less coverage per cell, but a significant increase in cell capacity can also occur due to lower interference. In general, the orthogonality is better when the channel consists of fewer taps.
In a scenario where the interference from other cells is minor, given a fixed SIR (Signal to Interference Ratio) target, changed channel orthogonality forces the DPCH (Dedicated Physical Channel) power to change accordingly. When a UE increases its power, the overall interference is increased, thus the cell can serve fewer UEs. In another scenario, if the interference from other cells is significant, a change in orthogonality will not be as significant for the cell capacity.
In any wireless network including an HSPA network, the following goals are desired among others—high throughput, high quality (less errors) and large number of users. In many ways, these are competing goals necessitating tradeoffs. For example, to achieve high throughput, the speed of data transmission can be increased. But this generally corresponds to higher rate of errors. To reduce errors (i.e., achieve high quality), a robust modulation scheme can be utilized, but this generally reduces throughput. One can increase transmission power of a UE to increase throughput and/or reduce errors, but this increases interferences for other UEs which reduces throughput and/or quality of their transmissions.