HSDPA通信技术

HSDPA Technology and Its Potential Applications
Zhang Junfeng
(Mobile Division of ZTE Corporation, Shenzhen 518004, China)
Since WCDMA, a third generation mobile communication system, is based on R99 version, it only can provide a maximum transmission rate of 2 Mb/s, which is far from sufficient for current demand from users for bandwidth-intensive mobile multimedia services. Due to the competition with WLAN and the following WiMAX technology, R99 is facing a dilemma that it might get outdated immediately after it is put into commercial use. As a result, 3GPP Organization has diverted the technology development path towards HSDPA (High Speed Downlink Packet Access), which is able to meet user's requirements for transmission bandwidth in stages. From a technical angle, it is seen that a High-Speed Downlink Share Channel (HS-DSCH) has been employed in HSDPA. By installing the functional entity into Node B and applying the shortened sub-frame, 16QAM, Adaptive Modulation Coding (AMC) and the crucial technology combined with HARQ (Hybrid Automatic Repeat Request), this revolutionary new technology can improve the downlink capability and reduce delay in the 3G network anaphase construction by improving the downlink data traffic rate to 13.9 Mb/s. Combined with the new technologies, such as Orthogonal Frequency-Division Multiplex (OFDM) or Multi-Input Multi-Output (MIMO), HSDPA technology is able to evolve to a post-3G system as well as enlarge the data flow.
Introduction of HSDPA Technology
1Structure of HSDPA Channel
As compared to R99 version, more physical channels and transmission channels are added in HSDPA.
(1)C hange of Physical Channel.
The High-speed downlink physical share channel (HS-PDSCH) is used to load
HS-DSCH, which is mainly used to load the dedicated packet data and some of the third layer signaling. HS-PDSCH is employed with the fixed SF=16 physical channel. HS-SCCH (Shared Control Channel for HS-DSCH) is applied with the fixed code rate (60 kb/s, SF=128), which is used to load the downlink signaling related to HS-DSCH. The spread frequency (SF= 256) of uplink HS-DPCCH carries the feedback signaling, including Hybrid ARQ Acknowledgement (HARQ-ACK) and Channel Quality Indicate (CQI) feedback, related to downlink HS-DSCH transmission.
(2) Change of Transmission Channel
HS-DSCH is a data loading transmission channel, on which the relative downlink signaling information is transmitted to UE (User Equipment), such as the modulation information, coding information and retransmission information, etc. After detecting and decoding HS-SCCH channel, UE is able to receive HS-DSCH data. HS-DPCCH (Dedicated Physical Control Channel for HS-DSCH) transmits the feedback signaling that includes HARQ-ACK and CQI. ACK/NACK (Acknowledgement/Negative Acknowledgement) field is encoded to 10 bits repeatedly and is transmitted in one time-slot. The CQI indicate field occupies 20 bits and supports up to 32-level CQI.
(3) Characteristics of Transmission Channel
• One HS-DSCH channel processes one CCTrCH (Coded Composite Transport Channel) and decodes from one CCTrCH.
2 Key Technologies of HSDPA
(1)Adaptive Modulation and Coding Technology
The AMC technology is used to change the modulation mode and code rate according to the channel situations. In systems where AMC technology is employed, the subscribers at favorable locations such as those near BTSs (Base Transceiver Station) can use high-order modulation and high code rate (such as 16QAM, a fairly big size of data block). In this way, the deficiency of base station downlink transmission dynamic range can be overcome and used completely. In contrast, the modulation order and code rates of the users at unfavorable locations, such as those far from the BTS, will be lower (such as QPSK or a fairly small size data
block). AMC technology can be applied to improve the rates for the subscribers at favorable locations so as to increase the average throughput of cells. In addition, it helps reduce interference by changing the modulation mode rather than altering the transmission power, which means that the repaid power control of HS-PDSCH channels related to HSDPA is cancelled.
Shortening the length of the sub-frames (2 ms), AMC modulation rate can be increased efficiently so as to support the quick change of wireless channels.
(2) HARQ Technology
HARQ is a kind of link adaptive technology. ARQ implies automatic retransmission. HARQ is a technology combining Forward Error Correction Coding (FEC) and
Auto-Retransmission Request (ARQ). FEC can improve transmission reliability. However, the excessive Error Correction Bits might decrease the throughput while the channels are in good condition. In case of low Bit Error Ratio (BER), ARQ can get ideal throughput. In this case, combining with FEC can form a hybrid ARQ. Error Correction Bit and Error Detection Bit will be included in each of the sending data packets. The errors will be corrected automatically in case that the error bits in receiving packet can be corrected, otherwise, initializing retransmission will be done in case of severe errors, which exceed FEC's capability. Basically, HARQ can be divided into three categories, namely, HARQ Type I, HARQ Type II and HARQ Type III. Currently, Rel99 version is transmitted and controlled in RLC (Retransmission Without Combining) and it supports HARQ Type I (HARQ on the software layer). Due to the low feedback speed, the efficiency is not high and the overall flow and QoS will be affected. As a result, HARQ Type II and HARQ Type III on the physical layer need to be used in HSDPA.
The processing priorities of ARQ technology in HARQ Type II and HARQ Type III are both set in their physical layers. They are different in their incremental redundancy modes.
The simple retransmission packet is replaced by incremental redundancy. If the first decoding attempt fails, the transceiver is requested to retransmit data with redundancy information. No packets are discarded and the decoder works on a fairly
low bit ratio combining all data packets. The retransmitted packets are not the same as the original transmission packets. In order to correct the error, the retransmitted packet carry part of the attached redundancy information, which result in stronger FEC codes through combining the data packets received previously.
Generally, the IR (Incremental Redundancy) solution is classified into two catalogues, namely partial IR and Total IR. In Partial IR, also known as
H-ARQ-Type-lll, every retransmission can be self decoded and data can be retrieved regardless of applying the combination process. In Total IR, also known as H-ARQ-Type-ll, the retransmitted redundancy information only includes redundancy information instead of the system bits. As a result, every transmitted code cannot decode itself and the data can only be retrieved by combination.
There is a particular case of HARQ Type III, namely, CC (Chase Combining), which is also called H-ARQ-type-lll with one redundancy type. In this type, the transceiver is simply asked to retransmit data packets. In order to get the time diversity gain, the decoder at the receiving end combines these repeated transmission packets and decodes according to the weight of the received SNR (Signal Noise Ratio). This is a method, which can be realized simply and has a bright future. The implementation of HARQ Type II and HARQ Type III in standards is mainly defined by Xrv parameters. The "s" value can be 0 or 1, i.e., either the system bit or the non-system bit can assume priority to be retransmitted. The repeat mode can be changed for every retransmission process.
The Stop-and-Wait Protocol is normally used packet scheduling function by RNC. This has the advantages of closely accessing users and timely response to scheduling with a 2 ms TTI (Transmission Timing Interval) length.
Generally, the packet scheduling of HSDPA can be classified as follows: The time-based Round Robin Mode, by which each user is served orderly and obtains the average distributed time. However, the flow that each subscriber receives is not exactly the same because of their different locations.
The flow-based Round Robin Mode, by which each user is served according to a certain order regardless of the disparity of locations to ensure that every user can
obtain the same flow.
The maximum C/l Mode, by which the system traces the radio channel fading characteristic of every user according to the size of the radio channel C/l, ensures the priority of each user and guarantees that the served subscriber can get the maximum C/l at every time. It is the best distribution mode, which can provide the maximum throughput. However, it is possible that some subscriber cannot be satisfied because of the unfair characteristic.
The partially fair mode, which combines the merits of the above-mentioned scheduling modes, can not only satisfy the majority of subscribers but also ensure high system throughput to a certain extent. It is a practical scheduling mode. There are different algorithms that can realize the partially fair mode. Generally, parameters such as downlink channel quality, user buffer queue length and user average scheduling time, etc., are involved.
(5) Other Functions of MAC-hs
Apart from the functions of scheduling and HARQ, MAC-hs also can provide the following functions, as shown in Figure 1.
• Flow Control Function It co-exists with the flow control functions of
MAC-c/sh or MAC-d. Flow control is provided between the two entities, such as MAC-c/sh and MAC-hs or MAC-d and MAC-hs. This function restricts the second layer signaling delay and reduces data rejection and retransmission. Flow control provides an independent priority grade for each MAC-d flow.
• TFRC (Transport Format Resource Combination) Selection Function The function is to select suitable transmission format and resource combination for
HS-DSCH.。