In today's 3G networks, voice calls are often in the so called “soft handover state”, which means that the radio network controller sends and receives the data for a voice call to and from several base stations simultaneously. The mobile then receives the voice call data stream from all three cells simultaneously and combines the received signals. While this wastes some bandwidth on the backhaul, these soft handovers often help to reach mobiles at the cell edge, which receive one or more cells with a similar strength.
For HSPA data transmissions, however, the soft handover is not used as the base stations autonomously decide when to schedule data on the air interface and thus, it is pretty difficult to synchronize the transmission over the air interface of several cells. In LTE, such a direct cooperation between cells is also not foreseen for the moment, even though there might be some benefits for mobiles at the cell edge. For LTE Advanced, however, people seem to have started thinking about it again. Instead of sending the same data stream over two or more cells, however, they are thinking about a cooperative MIMO scheme, i.e. each base station sends a different data stream and the mobile then analyses each data stream separately. The result would be a higher throughput for that mobile.
As Moray Rumney points out in his recent book, though, such a cooperative MIMO scheme would be quite challenging to implement in the network. First, it would put quite a demand on backhaul capacity and second, data would have to be exchanged between the base station with a delay of a millisecond or less. O.k., it is still some years away and technology advances, but I tend to agree with him, that's quite challenging to do. What do you think?
UL collaborative MIMO: great! the network is simply capturing more of the UE’s tranmitted energy. If we ignore the backhaul for minute, this should result in higher UL spectral efficiency.
On the DL, however, Collaborative MIMO (read Soft handoff)requires additional resources on both the radio and the backhaul. Radio-wise it seems a bit like N>1 frequency reuse. A UE in 2 way handoff needs 2x the spectral resource from the network. Calling it a ‘MIMO stream’ doesn’t stop it being a reusable resource.
So collaborative MIMO may improve cell edge performance…..but does it (like many techniques before it) come at the cost of average cell Tput?