ZTE´s WCDMA HSDPA System

ZTE´s WCDMA baseband chip with proprietary intellectual property rights uses the most advanced Application-Specific Integrated Circuit (ASIC) manufacturing technique. It, together with the fastest speed Digital Signal Processing (DSP) chip, enables the same baseband processing module to support the baseband processing functions of both the R99/R4 system and HSDPA system, and perfectly satisfies the requirements of the HSDPA system for the hardware processing capability of the base station. Adopting ZTE´s V3 series base stations, an operator can update its R99/R4 network to HSDPA without adding any new hardware to the network.

More information at:
http://wwwen.zte.com.cn:8080/English/03product/list1.jsp?CateID=941

The High-Speed Downlink Packet Access (HSDPA) technology is introduced to the WCDMA system to improve its low access speed and wireless capacity. Such technology provides WCDMA operators with the ability to develop data services with low cost and high speed. ZTE´s V3 series base stations (including B09, B09A, B06C, B03C, B03R, BBUA and B01C) are designed based on the integration of HSDPA and R99/R4, fully supporting properties of HSDPA.

1 Necessity to Introduce HSDPA System
The data services based on 2G systems such as GSM, CDMA and PHS are developing slowly because such services are restricted by the technical limitations of the 2G systems. Such limitations include low-speed wireless packet access, high cost and not being able to meet users´ requirements fully for the rate of wireless access. In the coming 3G era, the nature of 3G technology is to provide broader bandwidth for data services than 2G. Therefore, such technology can fully satisfy the users´ requirement for the rate of wireless packet access and further increase the revenue of operators´ data services. Taking Japan´s 2G PDC network and 3G FOMA network as examples, we can clearly see the growing trend towards data services in Figure 1.

    In Japan, the data traffic of the subscribers of the 3G FOMA network is 6-7 times bigger than that of the subscribers of the 2G PDC network. Due to the fast development of the data services based on the 3G network, the ARPU of individual FOMA subscribers has increased from 9,610 Yen in the second quarter to 10 270 Yen in the forth quarter in 2004. In contrast, data services based on the 2G network developed slowly because the low bandwidth couldn´t meet the requirements of the subscribers, and the ARPU of individual PDC subscribers decreased from 8 040 Yen in the second quarter to 7 730 Yen in the forth quarter in 2004.

    However, compared with the current popular Wireless Local Area Network (WLAN) and future World Interoperability Microwave Access (WiMAX) system, the WCDMA R99/R4 system provides high-speed data access services with much slower access speed, and the cost of transmitting megabits in WCDMA system is much higher than that in WLAN and WiMAX systems. In order to offset the defect, the HSDPA technology has been added in the WCDMA R5 system to enable the 3G network to provide similar access speed and cost as that of WLAN system. This therefore encourages more subscribers to utilize the data services and boost the revenue on data services for operators.

2 HSDPA TechnologyCompared with the original WCDMA
R99/R4 system, the HSDPA technology introduces new technology such as short transmission time interval (TTI=2 ms), Adaptive Modulation Coding (AMC) and Hybrid Automatic Repeat Request (HARQ), and adds three physical channels HS-PDSCH, HS-SCCH and HS-DPCCH for the transmission of data and signaling of HSDPA system. In addition, an MAC-hs entity is added to Node B of the HSDPA system for quick packet scheduling.

    The HSDPA system has the following working process:

• When User Equipment (UE) accesses an HSDPA network to carry out high-speed data download, the UE periodically reports the Channel Quality Indicator (CQI) to Node B to indicate the data rate the UE can support under the current wireless environment. (The CQI includes adopted coding and modulation strategy and the number of multicodes).

• Based on the CQI reported by the UE, Node B, in order to transmit data with the highest possible speed to the UE, determines the coding and modulation model to transmit to the UE in the next time interval.

• Having received the data from Node B, the UE reports the ACK/NACK message of each data packet to Node B to make it know when and by which way to retransmit the data.
  

3 Evolution from WCDMA R99/R4 to HSDPA System
The HSDPA system is not a replacement of the original WCDMA R99/R4 system, but an add-on HSDPA function to it. The two are converged, so operators can smoothly update their original WCDMA R99/R4 networks to the HSDPA system. In doing so, the following problems need to be solved:

• Deployment strategy of the WCDMA R99/R4 and HSDPA networks

• Handover strategy between the WCDMA R99/R4 and HSDPA networks

• Code resource sharing strategy of the WCDMA R99/R4 and HSDPA networks

3.1 Deployment Strategy of WCDMA R99/R4 and HSDPA Networks
The co-existence strategies of the HSDPA system and the original WCDMA R99/R4 system are limited to the following two:

• HSDPA and WCDMA R99/R4 share the same carrier frequency.

• HSDPA and WCDMA R99/R4 use different carrier frequencies to set up separate networks.
  

    Under the condition that HSDPA and WCDMA R99/R4 systems share the same carrier frequency, the two systems work at the same frequency point, share the resources of base stations like transmit power and channelized codes, exert advantages of both systems under unified scheduling, and enable the subscribers to enjoy seamless overall voice and data services. This co-existence strategy maximizes the utility of base station hardware resources, avoids the difficulty of UE selection and stays in different carrier frequencies, therefore ensuring minimal investment by WCDMA network operators and quick network construction.

    Under the condition that HSDPA and WCDMA R99/R4 systems utilize different carrier frequencies, the HSDPA system provides high-speed data services only at its frequency band, and the voice services are still provided by the original WCDMA R99/R4 system. The advantage of adopting this strategy is that HSDPA and WCDMA R99/R4 systems can set up separate networks without interrupting with each other. However, since HSDPA needs a separate carrier, the cost of network construction is high and the sharing level of the resources of the base stations is low.

    ZTE´s V3 series WCDMA base stations support both the sharing of the same carrier frequency and utilization of different carriers of the HSDPA and WCDMA R99/R4 networks. ZTE suggests that HSDPA and R99/R4 sharing the same carrier frequency is rather appropriate at the early stage of 3G network construction due to the minimal traffic of data services. When a 3G network develops to the stage of maturity with many subscribers, ZTE suggests constructing an individual network supporting only Packet Switching (PS) data cards on a separate frequency, and in the meantime using the strategy of HSDPA and R99/R4 sharing the same frequency to attract common handset subscribers on other frequencies.

3.2 Handover Strategy of WCDMA R99/R4 and HSDPA Networks
The throughput of the HSDPA system has a close relationship with wireless transmission conditions from the user´s location. With the increasing distance between a user and a base station, the HSDPA throughput for this user will decrease sharply, as shown in Figure 2.

    From Figure 2 we can see that the throughput of the HSDPA system is much better than that of the R99 system when a user and a base station are in a short distance. However, when the distance reaches 45% of the cell radius, the throughout of the HSDPA system decreases to similar levels of that of R99 system. When the user is at a distance of more than 70% of the cell radius, the throughout of HSDPA system drops dramatically compared with that of the R99 system.

    ZTE suggests a reasonable handover strategy between HSDPA and WCDMA R99/R4 systems according to the simulation to HSDPA system and the field tests. The strategy lets an HSDPA user utilize HSDPA technology to increase data traffic at a short distance from the base station, and switch his HSDPA service to a DCH PS one at around 50% radius of the cell. Besides, when the user moves to an area covered by the HSDPA system of another base station, the DCH PS service can be changed back to the HSDPA service by switching the DCH PS service to the HS-DSCH channel. By utilizing such a strategy, operators can maximize the advantages in HSDPA properties, while fully utilizing the resources of the original R99/R4 network to ensure uninterrupted high-speed wireless data access.

    ZTE´s V3 series WCDMA base stations support multiple HSDPA switching methods as indicated in
the following:

• Channel switching from DCH to HS-DSCH

• Channel switching from HS-DSCH to DCH

• Channel switching from HS-DSCH to FACH

• Channel switching from FACH to HS-DSCH
  

    Therefore, the V3 series can not only guarantee the coordination of the new established HSDPA system and the original WCDMA R99/R4 system, but also fully utilize the advantages of the two systems respectively.

3.3 Code Resource Sharing Strategy of WCDMA R99/R4 and HSDPA Networks
Since HSDPA supports high-speed downlink wireless access, it has a great demand for code resources. The code resources of the WCDMA R99/R4 system can support a maximum of 123-124 12.2 k voice subscribers, besides the overhead for public channels. When upgrading to the HSDPA system, as shown in Figure 3, the available codes for the R99/R4 network will decrease because a portion of the code resource has been reserved for the HSDPA system.

    From Figure 3 we can figure out the linear relationship between the capacity of the R99/R4 and HSDPA systems and the distribution of code resources. Therefore, a static distribution solution can be used to distribute the codes at the early stage of building a WCDMA network, when there are no intense demands for either R99 voice services or HSDPA data services. When the WCDMA network develops to a certain mature stage, dynamic distribution of the codes can be used to utilize fully the code resource in the system.

    Supporting both static and dynamic code distribution solutions, ZTE´s V3 series HSPDA systems can satisfy various requirements of the operators for code distribution at different network building stages. Under the condition of guaranteeing the quality of voice and HSDPA services, the V3 series fully utilize the system resources to meet the requirements for long-term development of the WCDMA network.

4 ZTE´s WCDMA HSDPA Products
ZTE´s WCDMA baseband chip with proprietary intellectual property rights uses the most advanced
Application-Specific Integrated Circuit (ASIC) manufacturing technique. It, together with the fastest speed Digital Signal Processing (DSP) chip, enables the same baseband processing module to support the baseband processing functions of both the R99/R4 system and HSDPA system, and perfectly satisfies the requirements of the HSDPA system for the hardware processing capability of the base station. Adopting ZTE´s V3 series base stations, an operator can update its R99/R4 network to HSDPA without adding any new hardware to the network.

    Besides, ZTE´s V3 series Radio Network Controller (RNC) and core network equipment with strong capabilities also provide concrete support for the solid development of an HSDPA network. The RNC equipment supports two-way 1.523 Gb/s data traffic, and its processing capability is three times higher than the equipment from other vendors. It fully meets the processing and exchanging capability requirements for RNC as PS services and especially HSDPA services develop further in the future. The capabilities of the core network equipment capability also fully satisfy requirements of HSDPA. For example, the Gn, Gi interfaces of the Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN) allow a maximum data traffic of 2 Gb/s, fully supporting the high-speed package access ability of the HSDPA system.

    In addition to the work of HSDPA infrastructure, ZTE plans to launch its own developed handsets and data card products that support HSDPA in the first quarter of 2006, making good preparation for operators to launch and promote HSDPA services.

5 Conclusions
In one word, the HSDPA network is a trend in the development of future WCDMA networks. Its properties of high rate and low cost will strongly push the development of 3G data services, and bring huge profits to operators. ZTE has been top-ranked worldwide in the R&D of HSDPA system equipment and hand phones, and is fully prepared for operators to deploy HSDPA networks. It will definitely play an important role in the future construction of HSDPA systems.

Manuscript received: 2005-10-10