The most
straightforward way of increasing the data rates is to use more bandwidth. Dual cell (dual carrier)
HSDPA (DC-HSDPA) in Release 8 uses two adjacent carriers in downlink to push the data rates
up twofold. DC-HSDPA networks were widely launched during 2011. The bandwidth can be
further increased to 20 MHz in Release 10 by using four carriers and even to 40MHz in
Release 11 with eight carriers. The uplink dual cell was included in Release 9 specifications. The multicarrier evolution is illustrated in
Figure below;
Operators have deployed
multiple carriers in their networks almost from the very beginning of WCDMA for
capacity reasons. Harnessing the already-deployed multicarrier infrastructure
to delivering data to one user is a cost-efficient way for increasing the
system peak data rates. The spectrum utilization is also significantly improved
because it is possible to efficiently balance the loading between carriers and
reduce the probability that some carriers don’t see any loading while some
other carriers are fully utilized due to a momentary high-data rate download.
Multicarrier links also enable frequency domain scheduling, providing system gains even if all carriers in
the system are fully utilized. Notably the system benefits of improved spectrum
utilization, more dynamic load balancing and frequency domain scheduling gains
are available even if only a fraction of the devices in the system are
multicarrier capable.
Release 9 also brought
the possibility to allocate the two carriers from two different frequency bands
to a DC-HSDPA device, and this was further extended in Release 10 so that the four
carriers can be split between two frequency bands. The figure below illustrates
the standard supported band combinations that can be aggregated to a
multicarrier capable device. As a multiband capable device is anyway built with
the RF hardware capable of receiving HSDPA on different frequency bands, it is
attractive to aim at using that same RF hardware also simultaneously for
improved data rates and better spectrum utilization.
Release 11 work is
ongoing for aggregation of a block of non-adjacent carriers to a fourcarrier HSDPA
device. The bands for which the support is worked on are Band I (2100 MHz) and
Band IV (2100/1700MHz). The practical side of this is that the supporting
device is expected to require two independently tuneable receiver chains
capable of receiving on the same frequency band. A possible implementation
option could be to use the two receivers as receiver diversity in case it is
configured to receive a set of carriers adjacent to each other and it houses
two antennas, and in the case it is configured to receive a set of carriers not
adjacent to each other it could allocate one receiver chain to receive one
frequency block and the other receiver chain to receive the other frequency
block.
