Lately, I’ve been thinking a bit how different wireless systems deal with the fact that the power output of a mobile phone is much lower than the power output of the base station. In practice this means that uplink data rates per mobile phone can not reach the same level as in the downlink. Most systems today use a different frequency ranges for uplink and downlink (FDD, frequency division duplex) with the same bandwidths. This means that if only a single mobile can transmit in uplink direction at a time, bandwidth is wasted due to the power limitation.
UMTS / HSUPA / E-DCH
3G networks use Code Division Multiple Access (CDMA) in both uplink and downlink. This means that several mobile phones can send their data at the same time to the base station, each with a different code. The base station knows the code of each terminal and is thus able to extract the simultaneous data streams from the single incoming signal. This way, the data rates of all mobiles can be added up and the uplink is used very efficiently, despite the limitation in uplink power. A single mobile is not able to fully use the available bandwidth due to the power limit. If several terminals communicate with the base station, however, as is usually the case, the uplink frequency band can be used to its limit. This method applies to both 3G UMTS and 3.5G HSUPA (aka E-DCH) as they both use dedicated bearers.
The WiMAX air interface uses Orthogonal Frequency Division Multiplexing (OFDM) in both uplink and downlink direction. Basically, the OFDM approach splits the total available bandwidth into independent sub-channels and data is sent simultaneously over these sub-channels. As UMTS/HSPA terminals, WiMAX terminals are also power limited and therefore face the same problem. Contrary to the code division approach described above, WiMAX assigns different sub-channels in the uplink to different terminals. Thus, each terminal can focus it’s power on fewer sub-channels. In other words a terminal can put more power in a sub-channel if it doesn’t have to use all of them. Other sub-channels not used by the terminal are assigned to other terminals. This means that several terminals in effect communicate with the base station in uplink direction simultaneously.
The comparison shows that both UMTS and WiMAX have interesting ways to ensure that several mobile terminals can communicate with a base station simultaneously in the uplink direction to counter the restricted power output and to use uplink resources efficiently. The way it is done, however, is quite different.