HSDPA Technology Introduction

Objective

In the three standards of 3G competition, WCDMA business in the number of operators to achieve a leading, but in its network supported by the data rate on the long-term stay in the theoretical 384kbps level, and its network construction has been slow development of the state.

In contrast, in Korea, Japan and other countries to achieve commercial CDMA2000 1X ev-do network system, The 2.4Mbps peak rate has been achieved, and its broadband Access service provides customers with a 300kbps-500kbps average download rate, which is comparable to the rate of wired broadband.

In comparison, with the realization of commercial 3G network system, in the face of the existing 3G business, WCDMA has been slightly powerless, in the data transmission rate of the huge drop, as well as the resulting business capabilities on the weak, nature makes WCDMA camp not to be outdone, must look for a catch-up CDMA2000 1X Ev-do's powerful weapon.

HSDPA (high-speed downlink packet access, high Speed downlink Packages access) technology is the most important technology to improve the high-speed downlink data transmission rate of WCDMA network, Is 3GPP in the R5 protocol in order to meet the requirements of the data business asymmetry, it can be in the construction of the WCDMA network structure on the basis of greatly improve the user downlink data Service rate (theoretical maximum can reach 14.4Mbps), This technology is an important technique to improve the downlink capacity and data service rate in WCDMA network construction. (Computer science)

1, HSDPA technology and its new features

In order to improve the performance of WCDMA system, HSDPA has made a lot of changes in wireless interface, which mainly affects the physical layer and the Transport layer:

shortened the radio frames; the new high Speed downlink channel, in addition to QPSK modulation, also uses 16QAM modulation, the combination of code division multiplexing and Time division multiplexing, the new uplink control channel, the use of adaptive modulation and Coding (AMC) to achieve fast chain is destaged matching; using hybrid automatic repeat request Harq). The media access control (MAC) dispatch function is transferred to the node-b.

The HSDPA wireless frame (which is actually a sub frame in the WCDMA structure) is 2ms long, which is equivalent to the three WCDMA time slots currently defined. There are five HSDPA frames in a 10msWCDMA frame. User data transfers can be allocated to one or more physical channels within a shorter length of time. This allows the network to reconfigure its resource configuration in the time domain and in the code domain.

Downlink Transmission channel Coding

Hs-dsch evolved from the downlink shared channel (Dsch) introduced by WCDMA R99, allowing different user transmissions to be reused over time. In order to achieve higher data rate and higher spectral efficiency, the fast power control and variable broadening coefficients in Dsch are replaced by the Hs-dsch of short packet length, multi code operation and AMC and Harq in R5.

According to the R99 1/3 Enhancement Encoder, the channel coding has been using 1/3 rate. However, the effective code rate varies according to the parameters applied in the two-stage Harq rate matching process.

In this process, the number of digits on the channel encoder output matches the total number of Hs-pdsch on the Hs-dsch map. The Harq function is controlled by a redundant version (RV) parameter. The exact set of bits on the output depends on the input bits, the output bits, and the RV parameters. When using more than one Hs-pdsch, the physical channel segmentation function divides the bits between the different physical channels. It intersects each of the physical channels separately.

HSDPA adopts orthogonal phase shift keying modulation (the technology specified in WCDMA) and uses 16 orthogonal amplitude modulation (16QAM) when the radio condition is good.

Downlink Physical channel Structure

The first time slot of the physical channel hosts the key information received by Hs-pdsch, such as the channel Code set and modulation scheme. After receiving the first time slot, the UE has only one time slot to decode the information and prepare to receive the Hs-pdsch.

The number of Hs-pdschs (or code Channels) mapped to a Hs-dsch may vary markedly between 1-15. It uses orthogonal variable broadening coefficient (OVSF) code. The number of codes and the corresponding bias information from the Hs-pdsch mapped on the given Hs-dsch are transmitted on the HS-SCCH. The multiple code (P) at the bias (0) is assigned as follows:

cch,16,0 ... Cch,16,o+p-1. The second time slot and the third time slot carry Hs-dsch channel encoded information, such as transmission code group length, Harq information, RV and constellation version, and new data indicator. Use 16-bit UE identification to cover three time slots data.

Adaptive modulation and coding

Chain is destaged matching is an important way to improve data throughput of HSDPA. The technology used is adaptive modulation and coding (AMC). In each user transmission process, the modulation coding scheme of the system is matched with the average channel condition. The transmitted signal power remains unchanged during the frame cycle, changing the modulation and coding format to match the signal quality or signal conditions currently received. In this case, users in the vicinity of BTS typically configure higher-order modulation (for example, a o.89 16QAM), but the modulation order and the code rate will decrease as the distance from BTS increases. As mentioned above, 1/3-bit-speed enhanced coding can be used to obtain different effective code rates through various rate matching parameters.

Hybrid ARQ

Hybrid automatic repeat request (HARQ) technology combines feedforward error correction (FEC) and Arq methods to save information from previous attempt failures for future decoding. Harq is a kind of hint chain is destaged collocation technique. AMC uses explicit c/i or similar measures to set up modulation and coding formats, while HARQ uses link layer validation to make retransmission decisions. On the other hand, AMC provides coarse data rate selection, while HARQ provides data rate fine-tuning capabilities based on channel conditions.

Packet scheduling function

In addition to channel coding and changes in physical and transport layers, HSDPA also implements another change to support fast transfer groupings. It moves the group debugging function from the network controller to the MAC layer in the Node-b (BTS).

The packet scheduling algorithm takes into account the wireless channel conditions (based on the CQI of all UE involved) and the number of data transmitted to different users.

2. Business Prospect Analysis

The current 3G system in terms of capacity, speed and cost is not enough to support the monthly mobile internet business, the urgent need for high-capacity, Low-cost HSDPA. The key to HSDPA's success is to see whether it can meet the needs of a monthly mobile internet business.

HSDPA mobile internet services have gone beyond cellular mobile data services into the broadband wireless access market, with applications ranging from PDAs to laptops.

At present, most WCDMA manufacturers are actively involved in HSDPA technology and equipment research and development, many manufacturers have laboratory experiments, system integration testing, began to enter the site test. It is generally believed that HSDPA will commence commercial deployment in 2006.