«ETSI TR 101 987 V1.1.1 (2001-08) Technical Report Terrestial Trunked Radio (TETRA); Proposed Air Interface Enhancements for TETRA Release 2; Analysis and ...»
The SN-ACTIVATE PDP CONTEXT ACCEPT PDU can vary between 43 to 76 octets in length depending on the configuration requested by the MS for the PDP Context as shown in the table 14.
Two courses of action are available:
1) If the MS or SwMI detects that a layer 3 PDU is likely to suffer from excessive fragmentation then the recommendation in the Air Interface Standard clause 188.8.131.52  should be followed. The PDU should be transmitted on an existing Advanced Link or an Advanced Link should be created. This requires no change to the standard  but is an implementation issue.
2) Specifically in the case of the SN-ACTIVATE PDP CONTEXT DEMAND and SN-ACTIVATE PDP CONTEXT ACCEPT PDUs the ‘protocol configuration’ element could be sent in a separate SN-CONTEXT CONFIGURE PDU. This PDU can be used to transfer configuration data on the uplink and downlink. This SN-ACTIVATE PDP CONTEXT DEMAND and SN-ACTIVATE PDP CONTEXT ACCEPT PDUs are used to create and authenticate the context whilst the SN-CONTEXT CONFIGURE PDUs are used to negotiate the compression, protocol and other configurable elements.
Reduction in On-Air signalling
Whilst the Air Interface Standard  defines the protocols to be used, further work is required to use the protocol efficiently. For example, a PDCH (Packet Data CHannel) can be used to both exchange Packet Data and to perform CC.CMCE and MM signalling. This is using the PDCH in its capacity as an ASCCH (Assigned Secondary Control CHannel). However these issues are currently addressed at the TETRA IOP MoU. Changes are often identified within this forum and passed to the relevant TETRA Working Group for further discussion and possible inclusion into the appropriate TETRA standard.
5.4.2 Reduced Speech Delay In order to reduce the speech delay on the Air Interface a TCH could use all 18 frames in a multi-frame to transmit speech. However frame 18 is currently used to broadcast network information on the BNCH, for synchronization on the BSCH - downlink - and linearization on the CLCH - uplink, In addition, Frame 18 is also for call maintenance on both the uplink and downlink. Although the speech delay can be reduced by this method consideration must be given to the broadcast of the BNCH and BSCH on the downlink and the CLCH on the uplink, call maintenance and maintaining compatibility with TETRA Release 1 mobiles and SwMIs.
5.4.3 Recommendations The Physical Layer In order to maintain compatibility between TETRA Release 1 and TETRA Release 2 no changes are proposed to the physical layer of the Air Interface.
The MAC Layer No enhancement changes to the MAC layer are proposed.
The LLC Layer Basic Link No enhancement to the Basic Link protocol is proposed.
Advanced Link No enhancement to the Advanced Link protocol is proposed.
Layer 3 protocols Transmission of Large PDUs Due to the potentially large size of some of the SNDCP protocol PDUs it is proposed that an input paper be submitted to WG3 for discussion on how the LLC may implement an advanced link for the purpose of transmitting large PDUs.
It is also proposed an input paper specifying how protocol configuration may be implemented in smaller, unfragmented PDUs for TETRA Release 2 systems be submitted to WG3.
Reduction in On-Air signalling It is recommended that the current feedback from the TETRA IOP MoU continues to be used as the vehicle for ensuring optimization of the Air Interface protocols.
Reduced Speech Delay No feasible enhancements have been identified for speech delay reduction.
5.5 User requirements implementation issues 5.5.1 General This clause describes results of analysis and feasibility assessment of the enhancements of extended range and provision of location information.
5.5.2 TETRA LCS (Location Service) 184.108.40.206 Background Location services are starting to emerge in cellular networks and cellular positioning is gaining increasing publicity both in media and telecommunications forums. The drivers of the positioning functionality have been both regulative and commercial.
On the regulatory side the process was started in the USA, where the FCC required that 911 calls from cellular phones
shall be located after 1st of October 2001. The accuracy and reliability requirements are:
• for network based solutions, 100 m for 67 % of calls, 300 m for 95 % of calls;
• for handset based solutions, 50 m for 67 % of calls, 150 m for 95 % of calls.
A similar requirement to the FCC 911 is being considered by the EU called E112. To address this and other requirements for terminal location, a group of interested parties has been formed called the Location Interoperability Forum. The purpose is to define, develop and promote through the global standards bodies and specification
organizations - a common and ubiquitous location services solution. Such a solution is intended to:
• define a simple and secure access method that allows user appliances and Internet applications to access location information from the wireless network irrespective of their underlying air interface technologies and positioning methods.
• promote a family of standards-based location determination methods and their supporting architectures that are based on Cell Sector-ID, Cell-ID and Timing Advance, E-OTD (GSM), AFLT (IS-95) and MS-Based Assisted GPS.
• establish a framework for contributing to the global standards bodies and specification organizations to define common methods and procedures for the testing and verification of the LIF recommended access methods and positioning technologies.
The demands for the positioning are not only driven by regulation, but there is also increased pressure from the market.
Numerous commercial and non-commercial applications are closely following the progress on standardization and technology. To be fully competitive with the other cellular systems TETRA has to be able to provide positioning functionality. The overall system to provide the information could be called TETRA LCS, TETRA Location Service.
220.127.116.11 Location dependent services (LDS) Providing geographical co-ordinates does not have much value as such, but when related to some location dependent service (LDS) the information will have greater importance. The situation is totally different with the traditional GPS where positional co-ordinates, speed and heading are the main information that is utilized. In a telecommunication environment, tracking a real-time position for navigational purpose is not considered feasible.
There are many different categories of LDSs such as emergency call locationing, information services, network administration, fraud control and billing control. To support all of these purposes it is beneficial if the system is able to provide an open interface for application developers. In that case there will be interest in developing applications to take the full advantage of the system.
For the TETRA community LCS will provide effective means to compete with other systems. Plenty of applications can be run when location information will be available. The public safety segment, with its responsibility for emergency services, will have increased value from the use of location information. For professional TETRA operators LCS will enable great possibilities in numerous areas. GPS is already being used in fleet management systems where TETRA is currently used only as a bearer for the transmission of positional co-ordinates. LCS could provide an integrated and more flexible solution.
18.104.22.168 Location issues related to TETRA When considering implementation of location services to TETRA network there are some important aspects that must be considered. Firstly the viable location method(s) that could be implemented in the TETRA system have to be investigated. A second issue of importance is the additional signalling needed for those systems.
At the moment there are numerous location methods that have been investigated for other cellular systems. Those methods can be divided roughly into two categories: cellular network based methods and GPS-based methods. In considerations each method has to be balanced taking the received performance and implementation costs into account.
It is obvious that the location method will require additional signalling within TETRA network. New messages will include at least position request and position response.
When considering formats for the messages the following has to be considered:
• Is there a need to specify TETRA-specific formats?
• Is there a possibility to take the advantage of using the formats specified for other systems?
If TETRA specific formats are seen as important, then the need has to be well justified since the workload to standardize the formats will be significant. Additionally with TETRA-specific messages there is a great risk to lose the interoperability with other systems and networks.
ETSI 32 ETSI TR 101 987 V1.1.1 (2001-08) Using mainstream formats both within network and between separate networks will allow interoperability and will also provide an open and widely supported interface to application developers. Further it can be assumed that when comparing the required standardization efforts the difference between TETRA-specific and mainstream formats is significant.
22.214.171.124 Location Accuracy and Update Rates for Emergency Service Users A study undertaken for the UK Home Office on efficient transfer of AVL data over TETRA networks ascertained the requirements in table 15.
Many other organizations use AVL, but the required accuracy and update rates are not yet known although EPT/WG1 is expected to be able to provide such information during 2001.
Generic Location Requirement
• Accuracy better than 50 m
• Indication of fix quality
• Timely rule based updates consistent with reasonable airtime cost
• Associated status (256 statuses is adequate)
• Rapid update rate during alarm conditions
• Speed to an accuracy of 5 km/hr
• Direction to an accuracy of 10 degrees
• Time of fix to better than 1 minute
Rule Based Systems Most AVL designers are now using rules based update systems. This allows the mobile unit to decide when to transmit an AVL report, obviating the need for polling. The rules are programmed in when the unit is installed and can be modified by commands sent over the air from control.
A typical set of rules is as follows:
• Time since last report generated.
• Distance travelled since last report.
• Response to a poll command from the communications centre.
• Manually generated report (from the user/equipment).
• Whether the vehicle has stopped/started moving.
• Reaching a reporting point (waypoint).
• Change of status of vehicle crew (entered through the data terminal).
• Operation of panic button or switch input.
• Change of status or other input signal.
• Loss or restoration of GPS coverage.
• Restoration of coverage of radio network.
• Change of AVL operating profile.
The main disadvantage of a rule-based system is that the receiving application can never be sure that the location information is up-to-date unless a method of acknowledgement is used for all messages.
126.96.36.199 Location methods The key question of the TETRA location service is the method to be used. There are various methods that have been discussed within telecommunication arena. Since each of these methods will have both pros and cons, the methods with their key characteristics are presented here.
Cell ID The simplest method for estimating location in a network is to utilize the data used by radio resource management, such as the identification of the serving base station - Cell ID. The accuracy will be the area of a base station coverage, i.e.
the MS can be positioned to be within boundary of a certain cell which means that the accuracy is dependent on the cell size. At the moment the cell identity is not necessarily known by the SwMI, but it is estimated that with moderate additional work, the information can be obtained.
Received Signal Strength (RSS)
In the Received Signal Strength method MSs measure the received signal levels of serving and neighbouring base stations. The received signal strength is a function of BS power, distance and propagation conditions. Distance to a mobile station can be estimated if BS Power level and propagation conditions are known. The position of an MS can be estimated when the measurement information of a few base stations are combined. Since the end result is highly related to propagation conditions, the accuracy will have great variations but may be a viable enhancement to increase the accuracy of the Cell ID method. Implementation of methods based on received signal strengths requires co-ordination and signalling between base stations.
Timing Advance/Round Trip Time (TA / RTT) For GSM a Timing Advance-based method has been considered. In GSM, with the aid of timing advance information a distance with an accuracy of 550 m could be estimated. However to locate also the direction would require TA information from at least of three serving base stations which is not feasible since there is only one serving base station at a time. The idea of standardizing timing advances to several base stations was never adopted in GSM. In TETRA there is no timing advance information.
RTT, Round Trip Time, also referred as Round Trip Delay (RTD), has been suggested for use with UMTS. RTT in UMTS has been claimed to have better accuracy than the TA method of GSM. In the RTT method the propagation time forth and back from BS is measured and the estimate of distance is calculated. This method requires tight synchronization between BS and MS.
For TETRA, both TA and RTT methods are limited to a location accuracy of 1-2 km due to the TETRA symbol length and MS/BS synchronization requirements (see EN 300-392-2, clause 7.6 ).
Angle of Arrival (AoA) In the Angle of Arrival method the signal transmitted by the MS is measured by base stations. By using smart antennas the direction of an incident wave can be estimated. To estimate the position requires at least two AoA measurements from separate sites. Implementing AoA method requires co-ordination between base stations, which generates additional signalling load in the network. Both software and hardware changes are needed in the network. AoA does not need any changes to the MS.
As a method AoA is relatively accurate in good conditions, but reflections and Non Line of Sight will cause significant decrease in performance.
Time of Arrival (ToA) The Time of Arrival method is based on measuring the arrival times of MS signals at different base stations in the network. ToA requires a common and accurate timing reference and also co-ordination between measurement equipment. For an MS no changes are required. Since TETRA does not offer any common and accurate time base the implementation would require additional timing references and synchronization mechanisms. Also control logic would need to be implemented to take care of the measurement triggering and provision of results.
Since a number of base stations would be measuring the same burst from the MS and the location calculated using triangulation by computing the time differences, the location accuracy would depend on the accuracy of the timing synchronization between the measuring base stations (typically synchronized by GPS), and the base stations timing resolution.
NOTE: For GSM a typical accuracy has been estimated at between 50-150 m - see Ericsson Review No. 4, 1999.