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Tuesday, June 26, 2012

Getting further integration between HSPA and LTE with carrier aggregation

LTE networks are expanded and upgraded with the more advanced HSPA+ features in order to cater to the ever-increasing appetite for wireless data. Due to the major investments in the HSPA+ infrastructure and the vast and rapidly increasing HSPA+ based mobile broadband device penetration the two networks can be foreseen to coexist in parallel for years to come. The evolution of both HSPA+ and LTE standards has introduced aggregation of carriers for higher data rates, better load balancing and increased spectrum utilization, and since the dawn of LTE, the standard support for radio level interworking for HSPA and LTE radios has been included. A natural continuation of such development is to tighten the interworking even further and introduce similar aggregation of carriers between the two radio access technologies.

The HSPA+LTE aggregation allows for transmitting data to one user simultaneously using both the HSPA and the LTE radios for maximal utilization of the available spectrum and the deployed equipment. This is considered beneficial especially in the environment where the spectrum that needs to be shared between the two radio access technologies is not abundant, and the deployed HSPA and LTE capacities and user data rates suffer from spectrum crunch. One example of such deployment is the 900 MHz for HSPA and the 800 MHz for LTE which are both seen attractive bands for building the coverage due to the low frequency but also suffer from very limited spectrum availability. With aggregation of the two bands it is possible to provide the high data rates expected from the LTE services while at the same time maintain coverage for the HSPA devices.

The same gain mechanisms that have been seen beneficial for Multicarrier HSDPA as well as LTE Carrier Aggregation can be benefited from by aggregating HSPA with LTE. At low or medium load,
HSPA+LTE aggregation is able to take advantage of the unused resources leading to significant data rate increases both at the cell edge and the cell center for the carrier aggregation capable devices. In addition, the carrier aggregation enables fast (millisecond level) load balancing across the carriers thus improving the data rates of all users.

A number of possible network architectures can be foreseen for HSPA+LTE aggregation, and are briefly touched upon in this white paper. Most promising architecture options are seen with co-located multiradio base stations with the base station (NodeB + eNodeB) acting as the data aggregation point, and simultaneously maintaining the existing network architecture for the devices connecting to the network with one radio system at a time only. This architecture can utilize some of the already deployed RF hardware in the base station, while new baseband functionality managing the data flow is required. On the device side, receiver radio architectures capable of multiband carrier aggregation should be suitable also for HSPA+LTE aggregation.

While Dual-Cell HSDPA is already in commercial operation, and higher levels of HSPA carrier aggregation as well as LTE carrier aggregation are part of 3GPP specifications existing today, HSPA+LTE aggregation is currently not standardized. Although conceptually straightforward and building on already standardized concepts, HSPA+LTE aggregation is a major feature, with a standardization effort comparable to that of LTE carrier aggregation.

This white paper will highlight some of the key aspects and architecture options available for aggregating HSPA and LTE carriers. Individually within both the HSPA and the LTE evolution, downlink and uplink carrier aggregation as well as co-site and inter-site aggregation have been considered. For both radio access technologies, the co-site aggregation has been standardized first and inter-site aggregation is currently being worked on.
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