In the forthcoming evolution of cellular standards such as GSM and WCDMA, the use of additional modulation techniques such as Orthogonal Frequency Division Multiplexing (OFDM) can be expected. One proposal, referred to as Super 3G or S3G, can be seen as an evolution of the 3G WCDMA standard. Super 3G will likely use OFDM as a modulation technique, and will operate on a bandwidth that spans from 1.25 MHz to 20 MHz, with data rates up to 100 megabits per second (Mb/s) possible. Therefore, for smooth migration, within the existing radio spectrum, of the “old” cellular systems to the new high-capacity, high-data rate cellular systems, user equipment will need to be capable of operation on a flexible bandwidth.
The evolution of cellular standards will thus introduce new challenges in the design of terminals. Besides the need to support higher data rates, the variable bandwidth also puts new requirements on the receiver and transmitter. Furthermore, the mix of different access technologies within the same radio spectrum places new requirements on inter RAT operations, i.e., how the terminal handles handover (HO) and cell search among cells, including those that use different access technologies.
Conventionally, inter RAT operations involve adapting the receiver to the bandwidth and HO/cell search parameters of a certain cellular system, performing the cell search, measurements, etc., using the access technology of that cellular system, then adapting the receiver to a different access technology of another cellular system, and performing the cell search, measurements, etc., using that different access technology. During an ongoing communication session, such as a speech or data call, supported by a first access technology, if a handover to another cell that uses a second access technology becomes necessary, for reasons of capacity or coverage, the ongoing communication session is interrupted in order to permit cell search/measurements on the second access technology. In WCDMA systems, this is referred to as compressed mode, where an ongoing WCDMA communication session is interrupted so the terminal can measure on GSM. Interruptions in ongoing communication sessions reduces throughput and, with solutions such as WCDMA compressed mode, also reduce capacity. These reductions in throughput and capacity are disadvantageous from a system perspective.
Interruptions in ongoing communication sessions can be avoided if the terminal is provided with multiple receiver branches, including multiple antennas, multiple front end receivers, etc. This permits the ongoing communication session to continue on one receiver, adapted to a first access technology, while cell search/measurements are simultaneously performed using the other receiver, adapted to a second access technology. This two-receiver approach is expensive from the terminal perspective.
With Super 3G, the complexity of inter RAT operations is expected to be even greater as compared to inter RAT operations in WCDMA/GSM, because of the possibility of handovers to WCDMA/GSM on the same or adjacent carrier frequencies with different bandwidths.
It is therefore desirable to provide for an inter RAT operation that is more efficient than the conventional solutions.