Technical explanation of HSDPA in WCDMA

The support capability of high-speed mobile packet data service is one of the most important features of 3G system. The WCDMA R99 version can provide a 384kbps data rate, which is basically sufficient for most existing packet operations. However, for many data services, such as video, streaming media and downloading, which require higher traffic and delay, the system should provide higher transmission rate and shorter delay.

In order to develop the data service better, 3GPP has improved the air interface from these two aspects, and introduced the HSDPA technology. HSDPA not only supports high-speed asymmetric data service, but also minimizes the input cost of operators while greatly increasing network capacity. It provides a smooth evolutionary path for UMTS's higher data transmission rate and higher capacity, as is the introduction of edge in GSM networks. The development of HSDPA is divided into three stages, namely the basic HSDPA stage, the enhancement HSDPA stage and the HSDPA further evolution stage, in which the HSDPA further evolution stage has not been finalized and is still being studied in 3GPP.

Basic principles WCDMA R5 version High Speed Data Service enhancement scheme fully references the design idea and experience of cdma2000 1X ev-do, adds a new high speed shared channel (Hs-dsch), and uses some more efficient adaptive link layer technology. The shared channel makes the resources such as transmission power and PN code can be used uniformly, according to the actual situation of the users, so as to improve the utilization of resources. The adaptive Link layer technology adjusts the transmission parameters according to the current channel conditions, such as fast link adjustment technology, fast hybrid retransmission technology combined with soft merging, and centralized scheduling technology, so as to improve the throughput of the system as much as possible.

Based on evolutionary considerations, one of the criteria for HSDPA design is to be as compatible as possible between functional entities and logical tiers defined in the R99 version. While maintaining the R99 version structure, a new media access control (MAC) entity Mac-hs is added to the NodeB (base station), which is responsible for the functions of scheduling, link adjustment and hybrid Arq control. This allows the system to be unified in the RNC for users to switch between the Hs-dsch channel and the dedicated data channel DCH management. HSDPA introduced channel uses the same frequency point as other channels, so that the operators can flexibly configure the channel resources according to the actual business situation. The HSDPA channel includes a high speed shared data channel (Hs-dsch) and a corresponding downlink shared control channel (HS-SCCH) and an upstream dedicated physical control channel (HS-DPCCH). The downlink shared control channel (HS-SCCH) hosts control information from MAC-HS to terminals, including mobile identity tags, H-ARQ related parameters, and transport formats used by Hs-dsch. This information is sent to the mobile station every 2ms from the base station. The uplink dedicated physical control channel (HS-DPCCH) is used by the mobile station to report downlink quality status to the base station and request the base station to retransmit the wrong data block. (Computer science)

The channel code resources of the shared high speed data channel (Hs-dsch) map are composed of 16 SF codes fixed by 15 spread spectrum factors. In addition to sharing channel resources at different time slots, different mobile stations share channel code resources. Channel code resource sharing enables the system to use channel resources more efficiently by using only a subset of the channel code set when the smaller packets are transmitted. In addition, the channel code sharing also makes the terminal can start from the low data rate ability, gradually expands, is advantageous to the terminal development. The channel code allocated from the shared channel pool is assigned by RBS every 2ms according to the Hs-dsch channel business situation. Unlike a dedicated data channel, a hard handoff is used between high speed shared data channels (Hs-dsch), using a soft switch.

Key Technologies

The data business and the voice business have different business characteristics. The speech service is usually sensitive to the delay, and the requirement for the rate constant is higher, while the demand for BER is relatively weaker. The data service, on the contrary, can usually tolerate short time delay, but it has high demand for BER. HSDPA reference CDMA2000 1X ev-do system, fully taking into account the characteristics of data services, the use of rapid link adjustment technology, combined with soft combination of fast hybrid retransmission technology, centralized scheduling technology, such as link layer adjustment technology.

Fast Link Adjustment Technology

As mentioned earlier, data services and voice services have different business characteristics. The Voice communication system usually uses the power control technology to counteract the channel fading to the system influence, obtains the relatively stable rate, but the data service can tolerate the delay, can tolerate the speed the short time change. Therefore, HSDPA is not trying to improve the channel condition, but to adopt the corresponding rate according to the channel condition. Because the Hs-dsch updates the channel status information every 2ms, the link Layer adjustment unit can quickly track the channel change and adjust the rate by adopting different coding modulation schemes.

When the channel condition is better, the Hs-dsch adopts a more efficient modulation method--16QAM to obtain higher frequency band utilization. Theoretically, although the XQAM modulation method can improve the channel utilization, but because the difference between modulation signal is smaller, it needs higher chip power to improve the demodulation ability. Therefore, the Xqam modulation method is usually used in the case of limited bandwidth, but not in the case of power constraints. In HSDPA, the user's reception signal function is relatively strong, and the benefits of Xqam modulation method can be obtained.

In addition, WCDMA is the voice data integration system, in order to ensure the voice business needs of the public and dedicated channel required power, the remaining power can be used in all Hs-dsch to fully utilize the base station power.

Hybrid retransmission (HARQ) technology combined with soft merging

The terminal can quickly request the base station retransmission error data block through the HARQ mechanism, in order to mitigate the effect of the data error caused by the link layer rapid adjustment. When the terminal receives the data block, the 5MS reports the data to the base station and correctly decodes or appears the error. After receiving the retransmission data of the base station, the terminal will make full use of the relevant information which is carried by the data block and the retransmission data block, in order to improve the decoding probability. When the base station receives the retransmission request of the terminal, according to the error condition and the storage space of the terminal, it controls the retransmission of the same coded data or different coded data (further increasing the information redundancy) to help improve the terminal error-correcting ability.

Centralized scheduling technology

Centralized scheduling technology is the key factor to determine the performance of HSDPA. CDMA2000 1X Ev-do and HSDPA pursuit of the system-level optimization, such as the maximum sector pass rate, centralized scheduling mechanism allows the system to determine which users can use the channel according to the situation of all users, at what rate use the channel. The centralized scheduling technology makes the channel always be used by the users who match the channel state, so as to maximize the channel utilization.

The change of channel State has two kinds of slow fading and fast fading. Slow fading is mainly affected by the distance between terminals and base stations, while fast fading is mainly affected by multipath effect. The two variations of the data rate corresponding to the channel also exist short jitter and long time change. Data service is relatively tolerable for short time jitter, but it is stricter for long time jitter. A good scheduling algorithm should not only make full use of the short time jitter, but also ensure the long time fairness of different users. That is, both the channel can be used to maximize the use of channels to improve the throughput of the system, but also to make the channel conditions relatively poor users can use the channel for a certain period of time, but also to ensure business continuity.

The common scheduling algorithms include proportional fairness algorithm, ping-pong algorithm and Maximum CIR algorithm. The ping-pong algorithm does not consider the channel change, and the proportional fairness algorithm not only uses the Short-time jitter but also guarantees a certain degree of long time fairness; The maximum CIR algorithm makes the few users with better channel conditions get higher throughput rate, and most users may not get the system service.

The effect of HSDPA on system performance includes two business and system throughput two levels. The Fast link layer adjustment technology makes maximum use of the channel conditions, and enables the base station to be near the maximum power transmitting signal, and the centralized scheduling technology makes the system gain the benefit of the system-level multiuser diversity, and the higher-order modulation technology improves the spectrum utilization and the data rate. The combined use of these technologies greatly improves the throughput rate of the system. At the same time, the increase of user rate and the use of HARQ technology make the performance of TCP/UDP improved, which improves the business performance. However, the degree of improvement in business performance is related to the business model.

As the WCDMA R5 version High speed Data Service enhancement technology, HSDPA improves the system's data throughput and business performance by adopting the technology of time sharing channel, fast link adjustment, centralized dispatch, Harq and so on, and ensures the forward compatibility of the system, in addition to adding the corresponding Mac module in RBS, No other impact on the system structure, which facilitates flexible deployment of the system.