HSDPA & LTE Escort Sustained Development of TD-SCDMA

Zhang Jianhui, Datang Mobile

1.                                    ...
WCDMA HSDPA. Supposing it is a multi-carrier TD-SCDMA HSDPA system, if TS0 on the
complementary carrier is assumed to tran...
3.                                        TD-SCDMA                                              LTE

As recent years has w...
improving spectrum efficiency or power efficiency, such as link self-adaptation, multi input and
multi output (MIMO) and p...
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  1. 1. HSDPA & LTE Escort Sustained Development of TD-SCDMA Zhang Jianhui, Datang Mobile 1. Foreword It is widely known that the specialized trial for TD-SCDMA industrialization wound up in the middle of this year has collected the evidence that TD-SCDMA is certainly capable of independent networking in large scale and provided a good opportunity for some cutting-edge key technologies based on TDD to be firmly validated. As all-sided breakthroughs have been made in TD-SCDMA industry and its large-scale commercial use has been ensured for the near future, if an operator, especially a Greenfield operator, decides upon a technology most appropriate for its 3G network, he has to consider a very important factor whether this 3G technology proposed by China is capable of sustainable development. This article provides a brief description of the features and strengths of TD-SCDMA HSDPA/HSUPA Enhanced and TD-SCDMA LTE, and explains that TD-SCDMA system will take a technical evolution path as planned, aiming at a smooth migration to a new generation of mobile communication network over time. 2.TD-SCDMA HSDPA/HSUPA Enhanced In view of the ever-mounting mobile data services nowadays, it is predicted that the asymmetry of uplink and downlink traffic, which has not been seen in the past dominated by the traditional wireless voice services, finally become highlighted. And therefore a wireless system is also required to have some features suitable for data service transmission, such as high throughput, high outburst and high reliability. Compared with 2G, 3G technologies have been enhanced substantially in terms of packet data transmission, but the fast-paced growth of market demands will render the 2Mbps peak rate defined for 3G far from perfection. To cope with that along with the development of 3G, 3GPP has included important enhanced technologies in R5 and R6 respectively, namely, High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA). HSDPA integrates some wireless enhanced technologies and could improve the peak downlink data rate based well above the current level. HSDPA is also compatible with FDD and TDD, with much similarity in the way of deployment. The basic bottom-layer key technologies used include Adaptive Modulation and Encoding (AMC), Hybrid Automatic Repeat Request (HARQ), fast scheduling algorithm and etc. Although HSDPA is deployed the same way either for FDD WCDMA or TDD TD-SCDMA and both of the systems could realize network migration only through upgrading software, rending changes to the legacy network architecture unnecessary, some advantages can be found in TD- SCDMA HSDPA compared with its rival upon an evaluation in an overall manner. The advantage to be pointed out first is the high spectrum efficiency that the theoretical peak rate can reach 2.8Mbps when the ratio of upstream and downstream time slots of TD-SCDMA single-carrier (1.6MHz) HSDPA is 1:5. The peak rate of 16.8 Mbps that could be reached within a 10MHz bandwidth (bandwidth for a pair of carriers in WCDMA system) is higher than 14.4 Mbps for
  2. 2. WCDMA HSDPA. Supposing it is a multi-carrier TD-SCDMA HSDPA system, if TS0 on the complementary carrier is assumed to transmit data, the absolute data rate is up to ((N-1)X 3.3 + 2.8) Mbps (N is the number of carriers), where this system is superior to WCDMA HSDPA due to its higher spectrum efficiency; meanwhile a multi-carrier TD-SCDMA HSDPA system is more compatible with different service packages and the improvement of the integral scheduling efficiency to bring about more convenience and benefits to an operator. In addition, WCDMA requires the 5MHz bandwidth for upstream and downstream traffic to be available continuously if HSDPA is to be provided within 10MHz bandwidth. On the contrary TD-SCDMA allows 6 separate 1.6MHz carriers, which is particularly suitable for an environment with limited carrier resources. Furthermore, a TD-SCDMA HSDPA network based on narrow bandwidth carrier gains much superiority over WCDMA HSDPA for a HSDPA and R4 co-existence solution in terms of resource utilization, networking cost, complexity in managing mobility and flexibility. Before the industrialization of TD-SCDMA Enhanced is in full swing, the TD-SCDMA Alliance has drawn a rough roadmap with milestones for the technology evolution and has offered a relatively uniformed planning for the schedule of it commercial use, based on which the specific technology evolution roadmap is at the sole discretion of each vendor. Some domestic entities are now on a fruit-yielding stage as to TD-SCDMA R & D work, among which Datang Mobile, the leader in both TD-SCDMA standard and the industry at all times has retained its leadership in the enhanced evolution technologies as always. On the occasion of the Beijing International Communication Expo in Oct. 2005, Datang Mobile demonstrated its successful case that a single carrier data rate in air interface reached 2.8 Mbps based on self-developed TD-SCDMA HSDPA equipment and some typical 3G services like VoD and high-rate FTP downloading were on the show, which acquired the wide recognition and high opinion from the insiders. In view of the R&D status quo and the landmarks as planned, Datang Mobile is expected to be the first to make TD-SCDMA HSDPA system available for commercial use in mid 2006, so as to meet the requirements of operators in their endeavor to deploy HSDPA in hot spots of data services at the initial stage of 3G deployment. As the mass use potential of HSDPA is getting clearer with each passing day, other TD-SCDMA Alliance members have stepped up the R&D efforts in HSDPA system and terminals. Based on the current performance of HSDPA, it could satisfy the users’ appetite for data services within a certain period after the kickoff of 3G deployment. As to HSUPA, the standard formulation work of 3GPP R6/R7 is still under way. Although there is difference between the uplink and downlink power resources and other characteristics, which is the reason why HSDPA is not fully applicable for uplink, the key HSUPA technologies as we refer to still include HARQ and fast scheduling algorithm. It is predicted that HSUPA will be phased in sometime in the future if appropriate as 3G network operation gradually matures to generate more service demands. The enhanced technology of TD-SCDMA not only improves the data-rate, but lowers the operation costs for each users due to better spectrum efficiency and expands network coverage and capacity. Meanwhile it guarantees the backward compatibility of the system. Undoubtedly, all these features are extremely essential that operators have to consider when looking at the performance-price ratio for 3G network deployment.
  3. 3. 3. TD-SCDMA LTE As recent years has witnessed the high-speed advancement of traditional cellular mobile technologies, some broadband wireless access technologies (eg.WiMAX-802.16e) start to get a flavor of mobility with an attempt to grab a share in mobile communication market. In this context, the mobile sector imposes new market demands, which in turns call for the improvement of 3G technologies to provide a more robust data service capacity, in order to serve the users better and compete with other technologies. This is why 3GPP and 3GPP2 have commenced the research about 3G LTE (Long Term Evolution) so as to ensure the competitive edge of 3G technologies and the dominance in the mobile market. TD-SCDMA, as one of the three 3G standards, has already kicked off its preparation work for the evolution towards next generation standards that are globally applicable as organized by 3GPP. The general targets to be met by 3GPP LTE are the higher data rate with downstream rate up to 50-100Mbps and upstream up to 30-50Mbp and a higher spectrum efficiency which is 2-4 times of the planned level of 3GPP R6; the improved data rate at the cell edge while keeping the cell coverage as is planned unchanged in the main; the wireless access network latency (user plane UE-RNC-UE) to be less than 10ms and reduced control plane latency (the objective as planned is less than 100 ms, with downstream latency excluded); supporting variable bandwidth (1.25 / 2.5 / 5 / 10 / 15 / 20MHz) to respond to the user demands for flexible data rates; supporting the collaboration between legacy 3G system and the systems based on non-3GPP specifications; enhanced MBMS; reduced CAPEX and OPEX; reduced costs for R6 UTRA air interface and network migration; system and terminals with complexity, costs and power consumption of reasonable level; supporting enhanced IMS and core network; ensuring the backward compatibility to the largest extent and appropriate balance between better system performance and larger capacity; effectively supporting multiple services, packet services (e.g. VoIP) in particular. The system shall provide the optimum services to the terminals in pedestrian traffic and at the same time support terminals in high vehicular traffic; the system shall operate on both symmetrical and asymmetrical frequency bands and support the adjacent-frequency coexistence in the multi-vendor environment. The path of 3GPP LTE spreads into two branches: FDD and TDD. Comparatively speaking, the TDD mode allows a more flexible spectrum allocation. FDD needs not only frequency spectrum in pair but the sufficient duplex interval. LTE is hungry for bigger bandwidth, imposing more burdens on the operators if they want to acquire broadband spectrum in pair as required. So we see more possibility that the spectrum fit for broadband wireless communications is allocated not in pair, and as a matter of fact only the unpaired spectrum is available in some cases. TDD is the most possible or even the only candidate solution in the unpaired spectrum environment. In addition, it supports the flexible asymmetrical services such as modifying the upstream and downstream traffic ratio in one frame. Many new types of services are inherently asymmetrical where TDD is the most suitable solution. TDD boasts other advantages like improving significantly the system performance by leveraging symmetry of channel. The feature of channel symmetry in TDD mode can be applied to many cutting-edge technologies with a view to
  4. 4. improving spectrum efficiency or power efficiency, such as link self-adaptation, multi input and multi output (MIMO) and pre-equalization. And thanks to channel symmetry, TDD has the unique strengths in the following aspects, including lowest latency by using open-loop self-adaptation and control; largely simplified signaling and control information and easier implementation of some other leading technologies like MIMO. Among the LET solutions being evaluated by 3GPP, there are two TDD LET solutions submitted, in which Datang Mobile was the major composer. One solution based on CDMA stresses more the connection with the legacy technologies in use and is open for development and migration in UTRA. Another solution based on OFDM has included OFDM that is more suitable for high data- rate transmission in both uplink and downlink and further exploited the strengths and unique features of TDD. This solution is ready for full research and evolution in E-UTRA, of which the evolution path adopts a frame structure that largely resembles TD-SCDMA to satisfy all requirements imposed by 3GPP LTE in terms of capacity, coverage, latency, co-existence of various systems and backward compatibility. This has laid a solid foundation for TD-SCDMA system to migrate smoothly from current LCR-TDD towards LTE TDD.     4. Conclusion All in all, TD-SCDMA will acquire the capability for sustainable development and the competitive edge in the long run during its migration towards enhanced technology and LTE. Meanwhile we are convinced of the unique feature of TDD, which is more suitable for bearing mobile broadband services that TD-SCDMA and its evolution technologies are still among the mainstream international standards for mobile communications. And by now TD-SCDMA has seen the encouraging results and dynamic development momentum either in related standard and its industrialization, which will undoubtedly bolster up the confidence of those operators about TD- SCDMA system when deciding on the applicable technologies for 3G deployment.