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Short answer: One of the advantages of CDMA over TDMA. Long answer: In TDMA or AMPS, due to spectrum reuse, a given slot on a given frequency channel can't be used by neighboring cells. So when a phone which is in a call moves from one cell to another, at a certain point it has to switch between cells. In AMPS and TDMA it will be commanded by the system to change frequencies, all at once. This is called a hard handoff, so called because it's all or nothing: the transition is a hard one. In CDMA, on the other hand, all the cells operate on the same frequency. The phone still has a single RF receiver which converts radio frequency down to baseband, but behind that it has a rake receiver with multiple fingers. Since all the cells operate on the same frequency, the single RF receiver picks up all of those which are within range. The phone then assigns fingers from the rake receiver to various signals, and these are added together to create the full signal the phone utilizes. Sometimes these are multiple paths from the same cell. For instance, if there's a direct route from the cell to the phone, and in addition the signal travels to a large building and reflects off it before reaching the phone, then the CDMA phone can utilize both of these signals for additional clarity. This is called multipath. (Similar conditions degrade TDMA and AMPS performance.) But even more useful is when the phone is about halfway between two cells. While in a call, the phone is not only handling its transport of data back and forth to the cell, but it's also actively looking for other cells. When it finds one whose signal strength is good (on the same frequency, remember) it will inform the cell system of this. The cell system might decide at that point to route the call through both cells simultaneously. The specification actually permits a phone to talk to six cells at once, though no phone currently in existence has this capability. So when a CDMA phone in a call moves from one cell to another, the handoff process happens in multiple steps. First the phone notices the second cell, and the cell begins to carry the call on both cells. As the phone continues to move, eventually the signal strength from the one the phone is moving away from will drop to the point where it isn't useful any longer. Again, the phone will inform the cell system of this fact, and the system will drop the original cell. Thus it isn't an all-or-nothing transition, which is why it is called soft. |
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