One radio access technology in present day wireless communication systems is the hierarchical so-called Wideband Code Division Multiple Access (WCDMA), see FIG. 1. Within WCDMA, a terminal or user equipment (UE) communicates with one or several Node Bs. The Node B refers to a logical node that is responsible for physical-layer processing such as error correction coding, modulation and spreading, as well as conversion from base band to the radio-frequency signal transmitted from the antenna(s). A Node B is handling transmission and reception in one or several cells. Further, a radio network controller (RNC) controls multiple Node Bs and is responsible from call setup, quality of service handling and management of radio resources in the cells for which it is responsible. In addition, the so-called Automatic Repeat Request (ARQ) protocol handling retransmissions of erroneous or missing data is located in the RNC.
The processing in WCDMA is structured into different layers, with the radio link control (RLC) at the top of the protocol stack, followed by the media access layer (MAC) and the physical layer. The MAC layer offers services to the RLC in the form of so-called logical channels. The MAC layer can multiplex data from multiple logical channels. It is also responsible for determining the transport format of the data sent to the next layer, the physical layer. The interface between the MAC and the physical layer is specified through so-called transport channels over which data in the form of transport blocks (TB) are transferred. In each transmission time interval (TTI), one or several transport blocks are fed from the MAC layer to the physical layer, which performs coding, interleaving, multiplexing, spreading etc. prior to data transmission. The different protocol layers are configured by the radio resource control (RRC), which performs admission control, handover decisions, and active set management for soft handover.
The introduction of high-speed downlink packet access (HSDPA) enhances the WCDMA downlink packet-data performance and capabilities in terms of higher peak data rate, reduced latency, and increased capacity by including higher-order modulation, rate control, channel-dependent scheduling, and so-called hybrid ARQ (HARQ) with soft combining. The HARQ enables a terminal or user equipment to request retransmission of erroneously received transport blocks, effectively fine-tuning the effective code rate and compensating for errors made by the link-adaptation mechanism. A corresponding development for the uplink has been implemented through the so called enhanced uplink or high-speed uplink packet access (HSUPA), which improves the WCDMA uplink capabilities and performance in terms of higher data rates, reduced latency and improved system capacity. The combination of HSDPA and HSUPA is commonly referred to as high-speed packet access (HSPA).
In HARQ for HSUPA for each transport block received in the uplink, a single bit is transmitted on the so-called E-DCH Hybrid ARQ Indicator Channel (E-HICH) from the Node B to the UE to indicate successful decoding (ACK) or to request a retransmission of the erroneously received transport block (NACK). The E-HICH is a downlink dedicated physical channel, carrying the binary HARQ acknowledgements to inform the UE about the outcome of the E-DCH detection at the Node B. The Node B transmits either ACK or NACK, depending on whether the decoding of the corresponding E-DCH transport block was successful or if a retransmission is requested. To not unnecessarily waste downlink transmission power, nothing is transmitted on the E-HICH if the Node B did not detect a transmission attempt; that is, no energy was detected on the E-DCH Dedicated Physical Control Channel, E-E-DPCCH, or E-DCH Dedicated Physical Data Control Channel, E-DPDCH. E-DPDCH is used to carry the E-DCH Transport Channel and E-DPCCH is used to carry the control information related with the E-DCH.
Recently, the HSUPA has been further extended with multiple input multiple output (MIMO) in order to increase peak data rates through multi-stream transmission. The term MIMO is commonly used to denote the transmission of multiple layers or multiple streams as a mean to increase the data rate possible in a given channel. In case of HSUPA in frequency division duplex (FDD), the additional use of MIMO introduces the possibility for a UE to simultaneously transmit up to two Media Access Control Protocol Data Units, MAC PDUs, (e.g. MAC i/is) (transport blocks) in a same TTI on orthogonal beams e.g. virtual antennas.
This introduces new problems when HARQ schemes are used. Consequently, there is a need for methods and arrangements to enable improved retransmissions for MIMO HSUPA in FDD.