The Universal Mobile Telecommunication System (UMTS) is a third generation mobile communication technology, which is designed to succeed GSM. The 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard, and provide a higher data rate and an improved efficiency. A UMTS Terrestrial Radio Access Network (UTRAN) is a radio access network of a UMTS, and an evolved UTRAN (E-UTRAN) is a radio access network of an LTE system. As illustrated in FIG. 1, a UTRAN typically comprises a number of user equipments (UE) 13a-c, each UE wirelessly connected to a radio base station (RBS) 14a, 14b, denoted a NodeB, which is wirelessly connected to a Radio Network Controller (RNC) 15, one RNC typically controlling several NodeBs.
Modern wireless communication systems for packet based communication may normally include a hybrid automatic repeat request (HARQ) functionality on the physical layer to achieve robustness against the impairments of the radio channel, and HARQ is available both in the LTE and in the UMTS. The HARQ combines forward error correction coding (FEC) with automatic repeat request (ARQ). In FEC coding, forward error correcting parity bits are computed from the information bits and added prior to transmission, to enable a correction of erroneously received information bits on the receiving side. In an ARQ scheme, the receiver detects whether a received block of information bits is correct or not using an error detection (ED) code, e.g. by a Cyclic Redundancy Check (CRC), and a positive acknowledgement (ACK) is sent to the transmitted if no error is detected. However, if an error is detected, the information is reordered by the receiver sending a negative acknowledgement (NACK) to the transmitter. Upon reception of the negative acknowledgment, the transmitter will retransmit the information.
As mentioned above, the hybrid automatic repeat request (HARQ) is a combination of FEC and ARQ, and it can be used in a selective repeat mode or in a stop-and wait mode. A block of information bits, denoted a transport block (TPB), is encoded, adding both an error-detection code, e.g. the CRC, and FEC parity bits. In a stop-and-wait HARQ, a received encoded block is decoded, and the ED-code and the FEC parity bits are used to check whether the decoding was successful or not, a successful decoding indicating that the information had been transmitted and received correctly. If the block of information is received without errors, an ACK is sent to the transmitter indicating that the transmission was successful and that the receiver is ready for a new block. On the other hand, if the block of information bits was not decoded correctly, the receiver reorders the information by sending a NACK to the transmitter, indicating that the receiver is expecting a retransmission of the same block.
A further improvement is HARQ with soft-combining, which combines the retransmissions with soft-combining functionality, according to which the receiver does not discard erroneously received blocks of information bits. Instead, it buffers their soft-bit values and combines these values with the soft-bit values of the received retransmitted blocks.
Multi-antenna techniques can increase the data rates and reliability of a wireless communication system significantly, and the performance is improved if both the transmitter and the receiver are equipped with multiple antennas. This will result in a multiple-input multiple-output (MIMO) radio communication channel, and such systems and related techniques are commonly referred to as MIMO. In a MIMO-operation or in a multi-cell operation, multiple simultaneous and independent stop-and-wait HARQ processes may take place, which leads to a better resource utilization.
The current MAC-hs, MAC-ehs, MAC-es and MAC-is protocols are all based on multiple independent stop-and-wait HARQ processes, and in the High Speed Packet Access (HSPA), the MAC-hs/ehs protocols handle the HS-DSCH (High Speed Downlink Shared Channel) and the MAC-es/is protocols handle the uplink E-DCH (Enhanced Dedicated Transport Channel). A successful reception of reordered and retransmitted transport blocks from different HARQ processes may occur at a different order than the initial, original transmission of the transport block, since a different number of retransmission may have been required before the successful reception, which could result in that data is delivered out-of-order to higher layers.
For this reason, the above-mentioned protocols all include a transmission sequence number (TSN) to enable a correct delivery of reordered data units to higher layers, by indicating the original and initial transmission order of the received reordered data units. The reordering procedure is standardized according to the current downlink MAC-hs and MAC-ehs protocols, but implemented by the network according to the uplink MAC-es and MAC-is protocols.
The size of the TSN-field for indicating the TSN is 6 bits in the conventional MAC-hs, MAC-ehs, MAC-es and MAC-is protocols. A field comprising 6 bits, each bit indicating 0 or 1, enables the field to indicate a digital number ranging between 0 and 31, thereby allowing a TSN reordering window of 32 transmission sequence numbers. However, for 1.28 Mcps TDD multi-frequency HS-DSCH operation mode, it is possible to include a field accommodating three additional bits, which is inserted in the payload at the end of a MAC-hs PDU Protocol Data Unit (PDU), possibly instead of the padding.
FIG. 2 illustrates such a conventional MAC-hs PDU Protocol Data Unit (PDU), for 1.28 Mcps TDD multi-frequency HS-DSCH operation mode, indicating the MAC-hs header 21, the payload 22, the 6 bit TSN field 23 located in the header 21, as well as the optional added extended three bit TSN field 24 located in the payload, before the padding.
As explained above, a TSN-field size of 6 bits gives a TSN reordering window that accommodates 32 transmission sequence numbers, i.e. a so-called reordering depth of 32. This may be sufficient to support up to three retransmissions before the TSN reordering window stalls, if two MAC PDUs with different TSN are transmitted per TTI (Time Transmission Interval), and this could occur either at MIMO or at dual cell operation with MAC-ehs. However, if more than two MAC PDUs are transmitted per TTI with different TSNs, the number of possible retransmissions before TSN reordering window stalling is reduced, and with Dual Cell operation with MIMO, TSN reordering window stalling will take place at the first retransmission.
This situation is illustrated in FIG. 3, indicating four MAC PDUs 31 transmitted per TTI for a dual-operation of Cell 1 and Cell 2, i.e. four active HARQ processes per TTI. At unmodified UE processing requirements, up to 24 HARQ processes could be used during one RTT (Round-Trip Time), which will consume up to 24 TSNs per one HARQ RTT, of the available 32 transmission sequence numbers. Consequently, already the first retransmission will result in the occurrence of TSN reordering window stalling.
Thereby, a continuous operation could be maintained with the 6 bit TSN-field, allowing a TSN reordering window 32, but a single retransmission may lead to TSN reordering window stalling.
If more than two carriers are used, the TSN shortage will be even more severe, and could lead to a situation where even continuous operation is not possible.
Further, it is not possible to use the MAC-hs PDU format defined for 1.28 Mcps TDD multi-frequency HS-DSCH operation mode, as illustrated in FIG. 2, since the dual cell operation and MIMO are only supported by MAC-ehs, which is octet aligned. Furthermore, the 1.28 Mcps TDD multi-frequency HS-DSCH operation mode MAC-hs PDU format is complicated to process, since the three additional and least significant bits of the extended TSN field are located at the end of the payload, before the padding, and are only available after the processing of the MAC-hs header.