GoTa - A Future-Oriented Digital Trunking Technology

GoTa technology offers several advantages over conventional trunking communications:

• Advanced technology: employing 3G wireless technology and advanced system structure to bring new space for service development of trunking technology
• Feature-rich services: capitalizing on the combination of the trunking service and value-added services to attract large amounts of users to access the network
• Less investment and more cost-effectiveness

1 Overview
Trunking communication is an important branch in the mobile communications area. By adopting the PTT (Push-to-Talk) mode, half-duplex single calls and group calls can be directly set up. The trunking technology is mainly employed in the field of dedicated mobile communications. Like other mobile technologies, the development of trunking technology undergoes a course from analog technology to digital technology and then to more advanced technologies, and its applied fields also have been changed from dedicated networks to common networks.

  In 2002, ZTE organized its R&D institutes at home and abroad to develop new trunking technology, and put forward the GoTa digital trunking technology system.

  GoTa, the abbreviation of "Global open Trunking architecture", adopts and optimizes the latest radio technology in mobile communications to meet the technical requirements of a trunking system. In addition, GoTa has powerful common network operation and service development capabilities to satisfy the requirements of trunking technology in the future.

2 Development History of  Trunking Communications
PTT is one important feature of trunking communication. PTT is the abbreviation of "Push To Talk" in the radio communication area. The user can just press the PTT button on the mobile terminal to call or receive calls. PTT features quick call setup and point-to-point or point-to-multipoint mode communications; the user can receive calls at any time without taking the receiver off the hook. Such a communication mode has high channel utilization rate, and is mainly applied in dedicated communication areas.

  Trunking communication or the PTT call has been developed from the wireless interphone system to analog and to today´s digital trunking system.

  In 1990s, the TDMA-based digital trunking communication system was developed. Currently, there are several system standards for digital trunking mobile communication systems in the world, some of which have been put into application; despite of the improvement, their market is quite limited. The communication modes adopted by those trunking communication systems are either TDMA or FDMA.

  In China, the national digital trunking mobile communication system
SJ/T11228-2000 was officially launched in 2000. According to the technical specifications and telecom industry development in China, the system adopts TDMA-based TETRA and iDEN systems as two recommended standards for China´s trunking communication.

  However, the rollout of digital trunking communication systems fails to address such problems as technical advancement and network operation management. The trunking market and industry development remains in the stagnant state.

  As the digital trunking system has been incorporated into the basic telecommunication services by the Chinese government, and domestic enterprises of radio communication field actively participate in the R&D of digital trunking products, it is foreseeable that China´s digital trunking will step into an orderly and healthy development phase.

3 GoTa

3.1 Technical Features
GoTa was developed to meet the demands for common networks and dedicated digital trunking communication networks.

  GoTa´s air interface has been optimized and reformed on the basis of 3G mobile communication technologies, and hence can fully satisfy the technical demands of modern trunking communication. Firstly, the call mode adopted by GoTa is the PTT voice call in trunking communication. Secondly,
GoTa defines a set of corresponding system structure and protocol stack so as to fulfill the quick connection of trunking communication system. Thirdly, to support group call, GoTa must optimize its air interfaces so that the subscribers of one group in the same community can share the same air channel when they have calls.

  GoTa features quick access, high channel efficiency, higher subscriber privacy, ease of expandability and support to multiple services.

  Based on CDMA2000 technology,GoTa also integrates abundant services like ordinary subscriber´s calls, SMS, positioning as well as wireless data services. These services combine together to provide integrated solutions for professional corporate subscribers.

3.2 Network Architecture
As shown in Figure 1, the basic network architecture of GoTa is composed of terminals, radio BSS (Base Station Subsystem) and DSS (Dispatching Subsystem).

  In order to support phone interconnection services, data services and SMS, the switching subsystem, data service subsystem and SMS center can also be connected to the basic architecture of GoTa.
Radio subsystem BSS mainly realizes the wireless access of various trunking services, data services and ordinary telephone services. Generally, BSS is composed of BTS (Base Transceiver Station) and PDC (PTT Dispatching Controller). Functionally, BTS is responsible for the modulation and demodulation of GoTa´s baseband signals and transceiving of RF signals. In addition, PDC is responsible for the allocation of radio resources, dispatching call control and power control, and supporting GoTa terminals to fulfill various handovers, convergence and distributions of trunking voice and data streams in areas with different coverage.

  While connecting with DSS, PDC also connects with multiple function interfaces, switching subsystems and data service subsystems, fully satisfying the demands of GoTa terminals for various service expansions.

  DSS mainly accomplishes trunking-dispatching services, comprising PDS (Push-to-Talk Dispatching Server) and PHR (Push-to-talk Home Register) .

  As the general control point of trunking calls, PDS mainly processes the trunking dispatching calls, including authenticating trunking subscribers, establishing various trunking calls such as private calls and group calls and judging trunking PTT requests.

  As a dispatch server, PDS also receives trunking voice data from the reverse links, and then distributes the data to corresponding forward links according to the call attributes.

  PHR manages and configures the database. It provides trunking subscribers with group registration, group member registration, local information and service authority record of the trunking members. Moreover, PHR fulfills call statistics and billing functions.

3.3 Key Technologies
As a trunking system oriented to future advanced technologies, GoTa changes the usage of air channels according to the requests of trunking group calls, and realizes multi-subscriber conversations over the same channel. In addition, GoTa forms new procedures and protocol systems according to the features of PTT calls, hence quickening call connection.

  The sharing of downlink service channels is one of the important features of a trunking system. GoTa optimizes the usage of wireless channels to realize the sharing of downlink service channels,
making the voice´s fast access.

  Another feature is that GoTa can realize fast connection of group calls and private calls. For the trunking group call services, calls can be quickly set up by adopting data channel instead of the traditional circuit domain. When setting up calls, there is no need to set up dedicated PPP links, thereby saving much connection time.

  In the processing course of the base station, a concurrent processing mode is recommended to be adopted, so as to shorten the subscribers´ connection time. For the called in-group members,
PDS finds their local information from PHR, and transfers it to PDC so that PDC can quickly page other in-group subscribers in the trunking system.

 There is no point-to-point PPP connection in GoTa trunking system;
however, it can still provide the "always-on" mode for group subscriber´s calls thanks to the public trunking message channel CTML, sharing trunking data channel STDL and F-SCH, which effectively quickens the access of trunking group calls.

3.4 Technical Advantages
CDMA is a more advanced wireless multi-address technology. In recent years, the limited spectrum resources have been intensely conflicting with the increase of mobile subscribers. Such problem cannot be effectively solved only by narrowing channel bandwidths or dividing the service cells into smaller ones. It is a good method to solve the deficiency of spectrum resources by adopting the Spread Spectrum Technology (SST) for code division multiple addressing and through the multi-system sharing of spectrum resources by utilizing low power spectrum density of spreading signals. That is why 3G mobile communications usually adopt CDMA technology.

  Currently, GoTa is the only trunking mobile communication system based on CDMA technology in a real sense.

  However, many renowned international companies are aware of the importance to develop trunking functions based on the 3G mobile communication technology, and some standardization organizations such as 3GPP2 even have incorporated the trunking application of radio resources into the standards. Therefore, it is possible for new trunking technologies to come into being.

  Compared with the traditional digital trunking, GoTa has the following salient technical advantages:

• Leading radio technology and high spectrum utilization
• Simultaneously satisfying the applications of trunking dedicated and common networks
• More powerful service development capability
• Low cost for system and terminals, fully satisfying large-scale network construction

3.5 Service Provision
GoTa mainly provides trunking services based on PTT calls. As a half-duplex communication mode, PTT calling comprises point-to-point and point-to-multipoint services, including group calls, private calls, dynamic regrouping, forced insertion/release, delayed access, talk prompt and called display.
In addition, a GoTa system also provides phone interconnection services, multiple supplementary services and wireless broadband data services.

3.6 Terminals
GoTa terminals have been rolled out by ZTE, together with the launch of GoTa trunking communication system. GoTa terminals support generic trunking services, abundant supplementary services and data services. In order to meet the demands of special trunking subscribers, ZTE will launch series of terminal products in succession, including vehicle mounted terminals and dedicated terminals.

  The G800 digital trunking handset, ZTE´s proprietary new type digital trunking terminal, is applicable to GoTa digital trunking network. Leveraging ZTE´s years of experience in handset design and state-of-the-art technologies, the G800 handset features salient performance, powerful functionalities, strong durability, great flexibility and human design.

  G800 digital trunking handsets adopt ZTE´s proprietary digital trunking communication module MG800A as the core communication module. The appearance design, structure design, hardware design and software development of G800 handsets were all independently developed by ZTE.

   The basic buttons on GoTa terminals are the same as those of ordinary handsets. The PTT button, on the side of a handset body, is mainly used for trunking call services. Usually, the PTT button is at the notable position with large volume so that subscribers can press on it directly and conveniently.

Manuscript received: 2004-02-26