Enabling & Support

Packet data transmission over a shared transmission channel

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ESA / Enabling & Support / Space Engineering & Technology

509 - Abstract of the offer:

The method proposed by the European Space Agency is related to an enhanced Random Access (RA) technique dubbed by the inventors Contention Resolution Diversity Slotted Aloha (CRDSA). It involves generating two or more replicas for each data packet to be transmitted. The replicas are emitted through a transmission channel, where they are separated by lags and/or random frequency. Looking for a company interested in a license agreement to industrialize the invention.

Description of the offer:

The Contention Resolution Diversity Slotted Aloha (CRDSA) key idea is to transmit two replicas of the same packet at random locations within the same frame as in DSA but with a little extra signaling to point to the "twin" packet location. In case of successful packet reception at the gateway this extra signaling information allows to locate the twin packet within the frame and to accurately cancel it in addition to the one successfully decoded. To be remarked that using a powerful Forward Error Correcting (FEC) code and an adequate signal to noise ratio, there is non zero probability of correctly detecting the packet even in presence of a collision. By iterating the process a number of times some of the initially lost packets can be recovered and the DSA performance enhanced. It has been found that by repeating 3-4 times the same packet in the frame and exploiting the received packet power unbalance (present in the system or artificially introduced), CRDSA allow to get a throughput 500 times higher than SA with a probability of lost packet around 1E-5 which is 100 times lower than SA.

Such a replica location signaling method allows the implementation, at the level of receiver, of a method of recovery of the transmitted packets. Such a method of recovery of packets is also an innovation of the present offer. In fact, it allows the simultaneous implementation of such a transmission method and this method of recovery of packets at the level of receiver constitutes the new CRDSA random access communication protocol.

Having devised a RA techniques capable to achieve RA throughput in excess of 1 b/s/Hz with packet loss ration lower than 1E-4 one can consider to extend the applicability of RA in satellite networks for:

SCADA applications;
Mobile messaging applications;
Transmission of sparse traffic/signaling packets of limited size in broadband access networks; d) Support of unpredictable real-time variable bit rate applications (e.g. VoIP, video conferencing).

SCADA and mobile messaging applications multiple access can be purely based on a RA scheme. The application of the advanced RA schemes described in this paper will allow a substantial increase in the utilization of the resources with a reduced collision probability. RA can also boost the performance of Demand Assignment Multiple Access (DAMA) protocols in the return link of a residential scenario for Internet access. DAMA has performance limitations in this scenario for the transmission of low volumes of data, that are quite frequent in the return link. In addition, DAMA cannot cope well with the transmission of Real-Time traffic, such as VoIP or video conferencing, when variable bit rate codecs are used. These flows are characterized by very tight delay and delay jitter constraints. Thus, real-time flows with variable bit rate (VBR) codecs are normally not performing well under a CF-DAMA scheme due to the long capacity reservation delays, and constant bit rate (CBR) traffic sources are preferred for real-time services with the subsequent waste of return link resources. The integration of DAMA with efficient and highly reliable RA schemes can solve all the above limitations and improve the overall system efficiency and user experience. A reliable RA scheme can replace the Free Capacity Assignment (FCA) mechanism, that is less efficient in a consumer scenario and be seamlessly integrated with a DAMA protocol resulting in a very efficient utilization of the resources with delay and delay jitter performances comparable to those on the forward link, despite being a multiple access channel shared by a very large number of terminals. For instance, for the transmission of low volumes of data, the terminal can make use of the random access channel, thus reducing the end-to-end delay for the delivery of a small burst of data. For the transmission of larger volumes of data, where the propagation delay represents only a fraction of the transmission delay, a traditional volume based capacity request (VBDC) under DAMA scheme can be issued. For the transmission of real-time traffic flows, RA can complement the traditional rate based capacity requests (RBDC) under DAMA. While the DAMA scheme can well estimate and serve the real-time flows average aggregate data rate, the RA scheme can well serve the flow variations, thus avoiding queueing of real-time packets on the terminal side.

Innovations and advantages:

Transmission of data packets through a shared transmission channel by a plurality of users.
Generation of a different preamble for each packet and the preamble in each replica of the packet.
Transmission of data packets with a determined periodicity, specific to each user.
Power unbalance among the received random access packets.
Forward Error correcting codes while keeping packet loss probability very low.
Improved the random access channel performance.
CRDSA RA scheme can be integrated with conventional DAMA protocols.