As a practical matter, the UPSD scheme will have about the same 75% power-save efficiency for a 20-ms CBR scenario as the HCCA scheme. With a 30-ms CBR interval, the efficiency should improve to 83%. The principal difference is that the N-body synchronization problem is traded for a one-body exercise.
Complexity arguments favor the EDCA scheme, especially because the power-save efficiency of the two approaches are similar. Two other points of comparison between HCCA and EDCA should be mentioned although they're only indirectly related to power-save procedures. These are the hidden node problem and AP interference.
Hidden nodes are stations that can receive frames from an AP, but may not be able to receive frames from all other stations associated with the AP. Without any mitigating procedures, hidden nodes violate the basic premise of CSMA. That is, the stations should sense the medium before transmitting. If sensing is imperfect because they're hidden, they'll create interference when transmitting.
Fortunately, EDCA is robust in the presence of interference and collisions from all sources including hidden nodes. Also, hidden nodes are rarely a factor in small or enterprise environments where there's good AP coverage. In outdoor environments, directional antennas can eliminate most of the hidden nodes.
It's often claimed that HCCA is immune to hidden node effects because stations transmit only when polled, thus avoiding collisions. This is true. But the polled stations must also be EDCA stations to communicate with the AP for non-CBR traffic. Thus, a hidden node remains hidden to some extent with HCCA. It's also true that HCCA APs and a CBR polling discipline are particularly vulnerable to interference from nearby APs and other stations. Any appreciable interference rate will right-shift the schedule whereas EDCA will adapt without effort. The bottom line is that the one-body solution with EDCA/UPSD is preferred to an N-Body problem with HCCA.
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