TD-SCDMA, as one of the third-generation mobile communication standard systems proposed by China, has attracted close attention from all parties, especially from the domestic industry. The researchers in optical transmission networks are more concerned about the demand and impact of TD network construction on transmission bearer network, They consider the bearer technology scheme, support for transmission network planning and construction methods, as well as the integration of TD technology development with the development of optical network technologies to adapt to TD’s current and medium to long-term development.
I. TD transmission bearer network technology scheme selection
TD network development in the near and medium to long term can be divided into three stages: R4, R5, and R6 . Each stage has different service bearer protocol, interfaces and service capacities. The Iub network interface evolves from E1 to GE/FE, the Iu-CS interface evolves from STM-N/GE to GE, Iu-PS/Nb/Gn/Gi interface evolves from GE to GE/10GE. Therefore, the construction of the TD transmission network should also be based on the different technical application stages of 3G and choose the appropriate technology for implementation.
TD-SCDMA network structure is divided into two main parts: UTRAN and CN. RNC(Radio Network Controller) generally adopts high-capacity and less bureau construction, so at the transmission network level, RNC is grouped with nodes like MGW, MSC Server, GGSN, SGSN into the core layer of metro transmission network. On the other hand, NodeB, which is more numerous and scattered, along with the 3G services from NodeB to RNC can be grouped into the access layer and convergence layer of metro transmission network. The construction of UTRAN is one aspect that affects the metro transport network.
1. Transmission bearer network technology scheme discussion
(1) R4 UTRAN bearer technology scheme
After analysis and research, the basic requirements of RAN in the current TD-SCDMAR4 version are: Iub interfaces of base station equipment are mainly IMAE1 and STM-1, which are mainly used to support voice service applications and data multimedia services during the initial 1 to 2 years of network construction. Typically, they need to provide 3 to 8 E1 links. A few large-capacity base stations, which are connected to other sub-base stations or RF units using baseband pulling technology, require connection through the STM-1 interface (its capacity is related to the actual network configuration).
At the current stage, using mature technology for service transmission is the preferred solution for transmission network construction. This involves utilizing SDH(Synchronous Digital hierarchy) for service transmission to achieve high-quality service delivery. This approach offers benefits such as cost reduction, fast network construction, clear network hierarchy, and the service layer is separated from the transmission layer, making it easier to manage.
(2) IP-based UTRAN bearing technology solutions
The initial version of UTRAN used ATM transmission technology, and with the development of IP technology, IP transmission was introduced as a second optional transmission mechanism in the R5 specification. This allows for the transmission of user plane frames using UDP/IP at the Iur/Iub interface, and RTP/UDP/IP at the IuCS interface, in addition to AAL2/ATM.
To ensure flexibility in the implementation of physical layer interfaces in the operator’s network, the specification does not specify the physical layer interfaces in detail. This means there are no restrictions on the underlying physical mediums (such as E1/T1/STM-1/Ethernet, etc.), and the specific use depends on the operator itself. For the data link layer, the specification requires IP transport options to support PPP/HDLC frames, but does not exclude the use of other L2/L1 protocols (such as PPPMux/AAL***TM, PPP/AAL2/ATM, Ethernet, MPLS/ATM, etc.).
At this stage, in order to improve bandwidth utilization and ensure high quality of service for voice services, a voice and data separation transmission approach is used for transparent transmission of voice services. By making making appropriate use of technologies such as MSTP, embedded MPLS, RPR, and others, bandwidth statistics multiplexing and security isolation for data services can be achieved.
(3) CN transmission bearer network technology scheme
The core network of R4TD system has been IP-based, with interfaces mainly using high-speed POS ports and GE ports, which can be upgraded to 10GE in the later stages. Traditional SDH equipment has low capacity efficiency, and it is recommended to introduce dynamic WDM (ROADM+GSS) on top of SDH layer to efficiently handle large grain services, as shown in Figure 1.
2. base station fiber optic long-distance transmission scheme to explore
ZTE is at the forefront of the industry in TD-SCDMA base station technology, adopting the second generation distributed TD base station (BBU+RRU) technology, which has been first implemented in Qingdao’s existing network. The communication between BBU and RRU is done through optical signals, offering the following two advantages over the traditional method of extensive cable feeders to the top of the tower.
(1) Solving the problem of complicated cables and difficult construction.
(2) BBU and RRU are separated, providing flexibility and convenient to the network, which solves various problems related to server rooms and power supply.
Usually, direct fiber optic connection is used for transmission between BBU and RRU . However, after analysis, in the application scenario where BBU:RRU is 1:N, networking with coarse wavelength division equipment and replacing bare fiber with wavelength can save a large amount of fiber resources and realize the utilization and reuse of the fiber already laid in 2G network, resulting in improved network scalability. In addition, it avoids the need for laying new fiber optic cables in densely populated urban areas and ensures rapid network construction. Figure 2 and Figure 3 show the comparison of the application effects of fiber direct connection and coarse wavelength division schemes in macro base station and micro base station application environments, respectively.
In summary, the TD supporting transmission network mainly adopts MSTP technology to realize access, processing and scheduling of TDM and data services. Additionally, WDM is moderately introduced in the core layer and between RRU-BBU to achieve efficient transmission and scheduling of large-scale data services while saving fiber resources. This solution can meet the current construction demand of TD and also adapt to the dynamic development of TD in the medium and long term.
Ⅱ TD transmission network construction methods to explore
Does the existing transmission network already meet the demand for TD network construction? Is it necessary to re-plan and construct the transmission network? These are questions that network planners and implementers must consider. The following will compare the existing network with the required transmission network to support TD.
From the point of view of site deployment, some TD base stations are not located at the same address as 2G base stations due to the limitations of coverage capacity and planning methods.
The “BBU + RRU” distributed base station method mostly used in dense commercial areas and Olympic venues will lead to a sharp increase in bandwidth demand. However, some areas of the existing network are close to saturation, making it challenging for the remaining bandwidth to support the new service demand of the TD network. In addition, due to the dramatic increase in 2G and large customer services over the past few years and the sudden and unbalanced nature of these services, “bottlenecks” have emerged in the network’s overall scheduling. Despite certain regional networks having substantial capacity, issues such as low utilization of network resources and insufficient network services security have become increasingly prominent.
Early transmission network mainly provides 2M channel services with low interface rates and limited type. Low-end equipment lacked the ability for smooth upgrades, and had poor processing capacity for data class service, particularly for handling large-scale data services with low efficiency.
The TD network is still in the nature of trial and is yet to reach large-scale commercialization. The continuous technical evolution, base station upgrades, and planning adjustments of the TD network will bring about fluctuations in the existing network and have adverse effects on existing 2G services and large customer services.
Taking into account various aspects such as TD network site planning and TD technology development forecast, it is recommended to plan an independent TD supporting transmission network, with new networks as the main focus and the moderate adoption of WDM technology.
III. Long-term development trend of TD transmission network
In recent years, the rapid development of data services in the telecommunications industry, business IP has become a major trend. Data multimedia services, especially voice and video IP, has made significant progress, resulting in the gradual transformation of the transmission network from TDM-based signal bearer to IP-based signal bearer.
Currently, the technically mature and widely used MSTP(Multi-Service Transport Platform) technology emphasizes relying on SDH platform. MSTP utilizes the redundant circuit (time slot) resources of SDH network to realize transparent transmission of data services, especially Ethernet services. Building upon this foundation, MSTP gradually evolves the deepens its functionalities, such as adding L2 switching, embedded RPR functions and MPLS functions, etc. However, with the evolution of 3G IP and the maturation of related technologies and standards, along with the maturity of packet transport technology, standards and industry chains, the construction of metro transport networks based on packet transport technology, supplemented by high-capacity WDM (Optical Cross-Connect) transport backbone networks based on the existing fiber optic network structure, has become an important development trend for the future, see Figure 4.
As the transition of TD networks to an all-IP architecture is a long-term process, the market application of MSTP is expected to maintain a certain level of stability before 2010. Additionally, WDM equipment systems also need to comply with the needs of packet transmission and expand the service bearing capacity. In this context, IP over WDM is a direction that we need to pay attention to.
