High speed engine analysis of the hottest WCDMA

2022-08-20
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WCDMA's high-speed engine analysis

the ability to support more than 2000 mobile packet data services in high-speed mold manufacturing enterprises is one of the most important features of 3G system. WCDMA R99 can provide a data rate of 384kbps, which is basically sufficient for most existing packet services. However, for many data services with high requirements for traffic and delay, such as video, streaming media and download, the system needs to provide higher transmission rate and shorter delay

in order to better develop data services, 3GPP improves the air interface from these two aspects and introduces HSDPA technology. HSDPA not only supports high-speed asymmetric data services, but also greatly increases network capacity and minimizes operator investment costs. It provides a smooth evolution path for UMTS with higher data transmission rate and higher capacity, just like the introduction of edge in GSM network. The development of HSDPA is divided into three stages, namely, the basic HSDPA stage, the enhanced HSDPA stage and the further evolution stage of HSDPA. The further evolution stage of HSDPA has not been finalized yet and is still being studied in 3GPP

basic principle WCDMA R5 version high-speed data service enhancement scheme fully refers to the design idea and experience of CDMA2000 1x EV-DO, adds a new high-speed shared channel (HS-DSCH), and adopts some more efficient adaptive link layer technologies. The shared channel makes the transmission power, PN code and other resources can be used uniformly, which can be dynamically allocated according to the actual situation of users, so as to improve the utilization of resources. 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 merge, centralized scheduling technology, etc., so as to improve the throughput of the system as much as possible

based on evolution considerations, one of the criteria followed by HSDPA design is to be as compatible as possible with the functional division between functional entities and logical layers defined in the R99 version. While maintaining the R99 version structure, a new media access control (MAC) entity MAC HS is added to NodeB (base station), which is responsible for scheduling, link adjustment, hybrid ARQ control and other functions. In this way, the system can uniformly manage users' switching between HS-DSCH channel and dedicated data channel DCH in RNC. The channel introduced by HSDPA uses the same frequency points as other channels, so that operators can flexibly configure channel resources according to the actual service conditions. HSDPA channel includes high-speed shared data channel (HS-DSCH), corresponding downlink shared control channel (HS-SCCH) and uplink dedicated physical control channel to drive the movable chuck to rotate (hs-dpcch). The downlink shared control channel (HS-SCCH) carries the control information from Mac HS to the terminal, including the identity tag of the mobile station, H-ARQ related parameters, and the transmission format used by HS-DSCH. These messages are sent from the base station to the mobile station every 2ms. The uplink dedicated physical control channel (hs-dpcch) is used by the mobile station to report the downlink channel quality status to the base station and request the base station to retransmit the wrong data block

the channel code resource mapped by the shared high-speed data channel (HS-DSCH) is composed of 15 SF codes with a fixed spread spectrum factor of 16. In addition to sharing channel resources in different periods of time, different mobile stations also share channel code resources. Channel code resource sharing enables the system to use only a subset of the channel code set when transmitting small packets, so as to use channel resources more effectively. In addition, channel code sharing also enables the terminal to start with a lower data rate capability and gradually expand, which is conducive to the development of the terminal. The channel codes allocated from the shared channel pool are allocated by RBS every 2ms according to the channel traffic of HS-DSCH. Unlike the dedicated data channels, which use soft handoff, high-speed shared data channels (HS-DSCH) use hard handoff

key technology

data service and voice service have different business characteristics. Voice services are usually sensitive to delay, and have high requirements for rate constancy, while relatively weak requirements for bit error rate; Data services, on the contrary, usually tolerate short-time delays, but have high requirements for bit error rate. HSDPA refers to CDMA2000 1x EV-DO system and takes full account of the characteristics of data services. It adopts link layer adjustment technologies such as fast link adjustment technology, fast hybrid retransmission technology combined with soft consolidation, and centralized scheduling technology

fast link adjustment technology

as mentioned above, data services and voice services have different service characteristics. Voice communication systems usually use power control technology to offset the impact of channel fading on the system to obtain a relatively stable rate, while data services can tolerate delay and short-term changes in rate. Therefore, HSDPA does not try to improve the channel condition, but adopts the corresponding rate according to the channel condition. Since the HS-DSCH updates the channel status information every 2ms, the link layer adjustment unit can quickly track the channel changes and adjust the rate by adopting different coding and modulation schemes

when the channel conditions are good, H should record data during the test. S-dsch adopts a more efficient modulation method --16qam to obtain higher frequency band utilization. Theoretically, although xqam modulation method can improve the channel utilization, the difference between modulated signals becomes smaller, so higher chip power is required to improve the demodulation ability. Therefore, xqam modulation method is usually used in the case of bandwidth limitation, rather than power limitation. In HSDPA, users close to the base station usually have relatively strong signal receiving function, and can get the benefits of xqam modulation method

in addition, WCDMA is a voice data integration system. In addition to ensuring the power required by public and private channels for voice services, the remaining power can be used for HS-DSCH to make full use of the power of the base station

combined with soft merge hybrid retransmission (HARQ) technology

the terminal quickly requests the base station to retransmit the wrong data block through the HARQ mechanism, so as to reduce the impact of data errors caused by rapid adjustment of the link layer. The terminal reports the correct decoding or error of the data to the base station within 5ms after receiving the data block. After receiving the retransmission data from the base station, the terminal will make full use of the relevant information carried by the previously transmitted data block and the retransmitted data block to improve the decoding probability when decoding. When receiving the retransmission request from the terminal, the base station controls the retransmission of the same encoded data or different encoded data (further increase information redundancy) according to the error condition and the storage space of the terminal, so as to help improve the error correction ability of the terminal

centralized scheduling technology

centralized scheduling technology is the key factor that determines the performance of HSDPA. CDMA2000 1x EV-DO and HSDPA pursue system level optimization, such as maximum fan 11, safety device: passing rate of electronic limit protection area. The centralized scheduling mechanism enables the system to decide which user can use the channel and at what rate according to the situation of all users. Centralized scheduling technology makes the channel always used by users who match the channel conditions, so as to maximize the channel utilization

the changes of channel conditions can be divided into slow fading and fast fading. Slow fading is mainly affected by the distance between the terminal and the base station, while fast fading is mainly affected by the multipath effect. The data rate also has short-term jitter and long-term variation corresponding to these two changes of the channel. Data services are relatively tolerant of short-term jitter, but have strict requirements for long-term jitter. A good scheduling algorithm should not only make full use of the characteristics of short-term jitter, but also ensure the long-term fairness of different users. That is, users who can make the best use of the channel should use the channel to improve the system throughput, and users with relatively poor channel conditions should be able to use the channel within a certain period of time to ensure business continuity

common scheduling algorithms include proportional fairness algorithm, ping-pong algorithm and maximum CIR algorithm. Ping Pong algorithm does not consider the channel changes; Proportional fairness algorithm not only utilizes short-term jitter characteristics, but also ensures a certain degree of long-term fairness; The maximum CIR algorithm enables a few users with better channel conditions to get higher throughput, while most users may not get system services

impact on system performance the impact of HSDPA on system performance includes two levels: business and system throughput. Fast link layer adjustment technology maximizes the use of channel conditions and enables the base station to transmit signals at near maximum power; Centralized scheduling technology enables the system to obtain the benefits of system level multi-user diversity; High order modulation technology improves the spectrum efficiency and data rate. The comprehensive use of these technologies has significantly improved the throughput of the system. At the same time, the improvement of user rate and the use of HARQ technology improve the tcp/udp performance, thereby improving the service performance. However, the improvement of business performance is related to the business model

as a WCDMA R5 version high-speed data service enhancement technology, HSDPA improves the data throughput and service performance of the system by using time-division shared channel, fast link adjustment, centralized scheduling, HARQ and other technologies, while ensuring the forward compatibility of the system. In addition to adding the corresponding MAC module in RBS, it will not have any other impact on the system structure, which is conducive to the flexible deployment of the system. (end)

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