1,Foreword
In order to meet users to the transmissive performance requirements of up run, 3GPP is in HSDPA (High Speed Downlink Packet Access) The norm has started HSUPA (High Speed Uplink Packet Access) after releasing Research project. The goal of HSUPA is to improve utilization factor of the capacity of air interface of up-chain and the terminal user's experience through improving the throughput of the Residential Area and cover of high data rate. In order to support such high-speed data transmission rate, HSUPA introduces into a kind of new transmission channel E-DCH (the specialized transmission channel of enhancement type) . E-DCH can used for, transmit business datum of dividing into groups, it support rate of change transmit, fast retransmission and divide into groups deployment,etc. fast. This text combines HSUPA agreement, has analyzed the characteristic of fast retransmission, has studied several kinds of schemes of carrying on fast retransmission in TD-SCDMA HSUPA system.
2,TD-SCDMA HSUPA critical technology and workflow
2.1 Critical technology of TD-SCDMA HSUPA
HSUPA is that some is wireless to strengthen the technical set, utilizes HSUPA technology to make the speed of upstream crest value improve a lot on the basis of prior art, and received greater throughput on the up-chain. TD-SCDMA HSUPA has mainly adopted the following two kinds of technology: The physical layers mix retransmission fast, because of the managing fast of Node B.
2.1.1 The physical layers mix retransmission fast
Adopt fast HARQ in HSUPA ' Mix automatic retransmission to ask) ,Technology this allow Node B ask for retransmission fast to Improper data that receive, its function is in media's access control high speed (MAC-hs) Storey of realization, this floor is stopped in Node B place. In R99 in the past, the retransmission of data packet was to control RLC retransmission under to finish by RNC. In transparent mode (AM) Leave, the retransmission of RLC involves the interface of RLC signalling and Iub to transmit, retransmission delays time to exceed 100 ms [1 ]. In this way, the retransmission time delay of fast HARQ is well below the retransmission time delay of the RLC layer, has reduced the time delay of TCP/IP and time delay sensitive business and shaken greatly.
2.1.2 Because of the managing fast of Node B
In the up run strengthens technology, in order to realize low time delay and fast allocation of resources, deployment and mix automatic retransmission, must ask close to air interface, propose, manage because of Node B fast [2 ], pass and introduce the new MAC entity in HSUPA, join MAC-e on Node B end, is responsible for deployment and HARQ by MAC-e. The literature [3 ] points out, under 1.28M TDD mode, adopts it because of this technology, can increase by 8% of system capacity in 5s game business model in duration, and to probably can increase 50% 500 ms, this because service time long, for shorten to the more non-obvious advantage have time delay. So, if the packet data traffic is under the situation of short block of the burst, the throughput of the system will adopt and get prominent improvement on the basis of the managing fast of Node B.
2.2 Workflow of TD-SCDMA HSUPA
Fig. 1 HSUPA workflow
Fig. 1 provides the workflow of HSUPA, can find out HSUPA moves the function that dispatcher and retransmission control from RNC to Node B end from the picture. The concrete step of HSUPA workflow is described as follows:
( 1) The intersection of UE and quality according to signal channel, can wait, transmit data bulk, determine speed applied to take in the buffer, with transmitted power, and send out the application to Node B.
( 2) Node B deployment device calculates each user's priority according to the dispatch algorithm that adopts in the system, line up all user in view of the above.
( 3) According to priority, Node B, according to the increment (RoT, noise over thermal) of the thermal noise of the system Or terminal throughput, and combine the speed that users apply and decide the assigned speed about each user in the queue, and send the deployment order through E-AGCH/E-RGCH and notify UE.
( 4) UE sends the data packet through E-DPDCH, and convey the corresponding control information through E-DPDCH.
( 5) Node B carries on demodulation to the data packet on E-DPDCH, and send ACK/NACK to respond to on E-HICH according to the error detection result of crc.
( 6) UE determines whether it is the retransmission or not according to ACK/NACK received, if receive NACK news, needs retransmission, until receive ACK news or reach the number of times of maximum retransmission of system set-up.
3,Comparative research of several kinds of HARQ schemes
In the intersection of HSUPA and system, throughput rate and transmit, it delays time to be the intersection of attribute and important parameter of system performance of system. After Turbo yard appear, people begin to carry on research [3 ] to 3 kinds of traditional typological ARQ retransmission mechanisms of turbo yard. This text passes the comparative research of different HARQ schemes, its characteristic of emulation, has offered feasible reference for realization of TD-SCDMA HSUPA.
3.1 Traditional HARQ scheme
Traditional the intersection of HARQ and technology according to the intersection of retransmission and the intersection of data packet and including content difference their, can divide into 3: Type- H-ARQ, Type - Ⅱ - ARQ and Type- H-ARQ. Have done detailed description to these 3 kinds of mechanisms in the literature [4 ].
These 3 kinds of fundamental HARQ mechanisms are very abominable in the signal channel condition, in order to prevent ceaseless retransmission, need to set up the number of times of a maximum retransmission on the receiving end, if reach the number of times of retransmission, regardless of whether or not decode correctly, will decode the result to export to users. Then carry on the transmission of the next data packet. In addition, there are two kinds of the mode of combination before the receive side decipher of these 3 kinds of basic HARQ coding schemes. A kind of CC that is traditional, Chase makes up and decodes, it is the same to divide into groups each retransmission in its characteristic, and before the decipher, need passing signal to noise ratio SNR weighted a plurality of and receiving and dividing into groups to pay making up. This kind of method can bring the diversity gain, but because complexity is higher, it is less to study and applied at present. Another kind is IR, namely it is redundant to increase and make up decoding, it is not the repeating to the retransmission divide into groups in its characteristic briefly of the whole yard of piece, but need to increase some supplementary knowledge. Take Type - Ⅱ H-ARQ as examples here, recommend its yard to amalgamate the way.
Set basic data of sending end as m1, 1, m1, 2, m1, 3, m1, 4, m1, 5 ,The data are after turbo code:
It is the carriertointerference ratio that the first redundant edition reaches the receiving end that aboving to hit α 1, α 2 is the carriertointerference ratio that the second redundant edition reaches the receiving end. While amalgamating, as to bits in location of a bit, successively two redundant editions had not destroyed it (destroy bit used 0 to represented) ,Amalgamate according to the maximal ratio; A redundant edition is destroyed, the amalgamation result is worth the correspondent bit of redundancy edition that has not been destroyed; Both redundant editions are destroyed, the amalgamation result is 0.
3.2 CA-HARQ scheme
Know by the intersection of system and definition of throughput rate, can make it it corrects true check bit of decipher to be the essential useful information not to receive, and the extra check bit is the redundant garbage. The number of reducing the useless redundancy bit conveyed can improve throughput rate. It is easy to find out, have only considered the different typing the hole site of redundant edition in traditional Type - Ⅱ HARQ retransmission, the signal channel state without considering this, the speed is limited to several pieces of fixed value, have caused the waste of systematic throughput rate. Therefore, Mantha has put forward a kind of new retransmission scheme CA-HARQ (Capacity-Approaching HARQ) ,Increase progressively and transmit check digit with the minimum range, until receiving the good true decipher. The number of the so transmissive redundant check bit can minimize, thus make throughput rate reach maximum [5 ]. At the transport data, convey the information bit at first, if can't receive correctly, and then convey check digit. The system passes PSI (Parity Spreading Interleaver) Realize the extraction of check digit. PSI divide into groups the intersection of turbo and check digit of encoder output, receive behind the NACK each time, choose different check-up bit traffic of subset. Number and position of the check digit of each subset can be presumed in a flexible way, thus realize the extraction of the arbitrary speed. For example, the 1st retransmission uses 5% of the check digit, the retransmission of the 2nd time uses 10% of the check digit, analogize sequentially. The extraction matrix of PSI is shown in Table 1:
Table 1 PSI collects matrix
Collect it in matrix, " 1 " Show this is transmitted, " 0 " Show that is destroyed, will not be transmitted. Tij represent collect matrix the intersection of i and competent j, list element, times of bits number that convey.
System model of CA-HARQ. Assume there is not the feedback channel by mistake, CRC check digit is long enough, can neglect the error probability not detected. Sending end get systematic location and expand check digit is only location of transmitting system send for the first time, after turbo code, the receiving end carries on turbo decipher, if the mistake happens, send NAK signal to the sending end, require retransmission. Send to the port and expand check digit and encode and export the check digit interleaver PSI Coder at this moment, chosen the bit sent according to state and feedback signal of the signal channel. PSI demoder which corresponds to the receiving end collects the rule according to PSI, reduces the check bit in the corresponding position at first, then combine redundant information of retransmission this, amalgamate the maximal ratio previous transmissive Improper data, carry on turbo decipher. According to CRC value that turbo decipher outputs, the ones that judged and received, to the fault.
Fig. 2 CA-HARQ system model
