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Radio Link Error Model for Third Generation
60%, 2000-2001

A proposal for third generation systems consist in the realization of a network completely based on IP protocol, considering a possible integration whether with version 4 and the rising version 6. The base idea is to implement an integrated network able to transmit data, voice and more generally all the multimedia traffic, with a full exploitation of the Internet technology produced in the last years. The system is based on two different parts, both working in IP: radio access and core network. This research is focused on the impact of a completely IP based network structure applied on radio access. The main problems are the following: assure high spectral efficiency, allow the control of the quality of service, allow integration between mixed applications, assure the possibility of cooperative sessions. Utilize a datagram package technique instead of classical circuit commutation used in 2G networks, needs the introduction of an header for each transmitted package whose functions are routing and communication management. If we consider the stack structure of an IP network (TCP/IP or UDP/IP) the header has a dimension about 40 byte and 60 byte respectively for IPv4 and IPv6. If, therefore, the payload of the package is little, in around of few bytes, it happens that the introduced header turns out occupy most of the band at disposal for the communication. In particular, for voice application, if the header is transmitted without a preventive treatment, the efficiency considerably diminishes as regards the current circuit commutation technique. Therefore putting opportune techniques of reduction of the header in account is necessary. Those techniques are essentially based on two methodologies: reduction of redundant (header stripping) explicit (elimination) and implicit (coding) information and compression of the current information (header compression). TCP/IP stack permit to recover transmission errors using mobile window retransmission mechanisms of the packages. This solution cannot be used in case of voice packages because of pressing temporal ties; and, therefore, is used the simplest UDP/IP stack with the possible introduction of a code for the recovery of the errors. If UDP/IP stack is considered, the header needs greater protection compared to payload, because a loss of header information cause the discard of whole package, while a loss of payload information could permit the recovery of a part of its information content. The possible introduction of a code for errors recovery increases the transmitted information which we will have to keep into account in the header compression study phase.

The project was developed in three phases:
  1.  study and proposed of an high spectral efficiency system for radio access on ideal channel for data and voice sources. It has been studied and compared different techniques of header stripping and header compression, considering the possibility to implement mechanisms of streaming and virtual circuits. We have evaluated the performances in terms of compression efficiency.
  2. study through simulations of the statistical features of the propagation channel; derivation of the errors model placing attention to the package structure (header and payload).
  3. integration of the statistical model of the errors developed in the previous phase and evaluation of methods for information protection (in particular for the header), that keep in account compression and channel features both in the same time. Evaluation of performances through simulations trying to give a description using an analytical model.

The obtained results are the following: identification of a stripping and compression header policy in UDP case without consider retransmission at RLC level; identification of a stripping and compression header policy in TCP case considering the possibility of retransmission at RLC level; set-up mechanisms of virtual circuits in the radio path and of identification of the base station functions for the interface with the core network that has TCP/IP and UDP/IP standard structure; implementation of a channel and CDMA system simulator, in multi-user regime, with power control, with slow and fast fading effects, which allow to obtain the statistic of the errors with different granularity (bit, part of package, whole package); evaluation of errors influence on classical IP package compared with the case of package exposed to stripping and compression header; identification and evaluation of opportune merit figures for the study of the final performance (misrouting probability, overhead on the core network given by misrouting, communication delays, probability of lose information etc..); study of possible introduction of protection technique of the information, with particular attention to the header; study of the modalities of use of the channel expected by the standard to implement communication totally IP.

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