Discussion on HSDPA Technical Principles

HSDPA refers to high-speed downlink grouping access. It is a modulation and demodulation algorithm proposed by 3GPP In R5 Protocol to meet the asymmetry of upstream/downstream data services, it can increase the downstream data service rate to 10 Mbps without changing the structure of the constructed WCDMA network. This technology is an important technology for improving downstream capacity and data service rate in the later stage of WCDMA network construction. The Application of HSDPA technology can fully meet the needs of operators for capacity expansion during the 3G network maturity period.

I. HSDPA technology and performance

HSDPA uses adaptive encoding and modulation (AMC adaptive modulation and coding) and fast hybrid automatic retransmission (HARQ hybrid ARQ) to increase the rate of downlink grouping data and reduce the latency) and quick scheduling technology. In fact, the above three technologies belong to the Link adaptive technology, and can also be seen as a further improvement of the variable Spread Spectrum Technology and Power Control Technology in WCDMA technology.

(1) adaptive coding and modulation (AMC)

AMC selects the appropriate modulation and encoding methods based on the changes in the wireless channel. The network side selects the most suitable downlink modulation and encoding methods based on the user's instantaneous channel quality and current resources, to achieve the highest possible data throughput. When a user is in a favorable communication location (such as close to NodeB or has a line of sight), user data transmission can adopt high-order modulation and high-rate channel encoding methods, such as 16QAM and 3/4 encoding rate, in this way, a high peak rate is obtained. When a user is in an unfavorable communication location (for example, the user is located at the edge of the residential area or the channel is in deep decline ), on the network side, low-order modulation and low-rate channel coding schemes are selected, for example, the following code rates are used to ensure communication quality.

(2) HARQ Technology

HARQ technology can improve system performance, flexibly adjust the Effective encoding rate, and compensate for the error code caused by link adaptation. HSDPA combines AMC and HARQ technologies to achieve better link adaptive performance. HSDPA first provides a rough data rate selection scheme through AMC, and then uses HARQ technology to provide precise rate mediation, so as to improve the accuracy of adaptive adjustment and resource utilization. The definition of HARQ mechanism itself is an error control solution that combines FEC and ARQ. There are many forms of HARQ mechanism, and three HARQ mechanisms are mainly used in HSDPA technology: TYPE-I HARQ, TYPE-II HARQ, TYPE-III HARQ. You can select the corresponding HARQ mechanism based on system performance and device complexity.

(3) fast scheduling

The scheduling algorithm controls the allocation of shared resources and determines the behavior of the entire system to a large extent. Scheduling should be based on the channel conditions, while considering the amount of data to be sent and the priority level of the business, and give full play to the capabilities of AMC and HARQ. The scheduling algorithm should transmit data to users with the best channel conditions in an instant, so that the highest user data rate and maximum data throughput can be achieved at each instant, but at the same time, we need to take into account the level and fairness of each user. To better adapt to rapid channel changes, HSDPA puts the scheduling function unit in NodeB instead of RNC, and shortens TTI to 2 ms.

II. Introduction of HSDPA

HSDPA technology is introduced in R99 system, and MAC-hs entities are added to the MAC layer. MAC-hs is in NodeB instead of RNC, it is mainly responsible for handling HARQ operations and fast scheduling algorithms. HSDPA adds three new physical channels to UTRAN of R99:

(1) HS-DSCH channel: downlink, responsible for transmitting user data, channel sharing mode is mainly time division multiplexing and code division multiplexing

(2) HS-SCCH channel: downlink, responsible for transmitting the control information necessary for HS-DSCH Channel Decoding

(3) HS-DPCCH channel: uplink, responsible for transmitting necessary control information, mainly for ARQ response and downlink quality feedback information

The HSDPA function greatly modifies NODEB and RNC algorithm and protocol software, with little impact on hardware. If the HSDPA function upgrade requirements (such as 16QAM, buffer and processor performance) are taken into account in the original device, hardware upgrade is not required to implement the HSDPA function. You only need to upgrade the software, therefore, many manufacturers claim to support the HSDPA feature through software upgrades. The implementation of this function is not too difficult. The key is the implementation performance. Therefore, the real performance after implementation of HSDPA needs to be verified.

Iii. Performance Testing considerations

The introduction of HSDPA technology requires functional verification and performance testing in stages. There are two points to note: first, the processing performance achieved by the current hardware will determine the future HSDPA performance, the performance of the upgraded HSDPA needs to be clarified. In addition, the impact of the HSDPA technology on the network performance when implementing the HSDPA function. In view of the current development status of HSDPA technology, it is recommended to adopt a phased Performance Test Scheme to verify the network performance of HSDPA technology, and study how to introduce HSDPA through performance tests, provide technical preparations for the future development and evolution of WCDMA network capacity and performance.

However, no manufacturer in the industry can provide the commercial HSDPA function. Some Existing products of some manufacturers already have the ability to process HSDPA in hardware and are ready to upgrade the software to HSDPA. However, the introduction of HSDPA also needs to consider the progress and capabilities of terminal R & D. major manufacturers are also negotiating the R & D schedule with terminal equipment developers and chip design companies for interoperability testing.

Iv. Conclusion

HSDPA technology was officially introduced in the 3GPP WCDMA standard Release '5 version, thus completing the HSDPA technology standardization work. However, the freeze of Release '5 also marks the maturity of HSDPA technical standards. According to the initial planning of the 3GPP Organization, each Release version should be officially released on an annual basis.

Through the introduction and analysis of HSDPA, we can see that the HSDPA technology, as a WCDMA enhanced wireless technology, will improve the system's spectrum efficiency and code resource efficiency, is an effective way to improve network performance and capacity. HSDPA not only effectively supports non-real-time services, but also supports some real-time services, such as streaming media services.

1. Impact of HSDPA technology on WCDMA networks and Improvement Measures
2. HSDPA Wireless Network Planning