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Soft Combining of FACCH-Blocks based on LAPDm Re-Transmissions

IP.com Disclosure Number: IPCOM000103543D
Original Publication Date: 2005-Apr-16
Included in the Prior Art Database: 2005-Apr-16
Document File: 3 page(s) / 963K

Publishing Venue

Siemens

Related People

Juergen Carstens: CONTACT

Abstract

In GSM systems, signaling information and speech or data traffic is transmitted via the air interface. All transmitted frames are protected via forward error correction against distortions on the air interface, which is caused for example by noise, inter-symbol interference, or interference from other senders. At the early beginning of GSM, the forward error correction of the signaling channels was designed to be better than for the traffic channels. However, more than 10 years after introduction of GSM, a couple of new services and coding schemes with improved channel coding performance have been introduced for speech and data channels. For example some AMR (Adaptive Multi Rate) codecs use convolutional codes with memory 6 and rate 1/4 that make them very robust against distortions. On the other hand the channel coding of the signaling channels, e.g. FACCH (Fast Associated Control CHannel) has not changed. In networks with high interference level, the error rate of the signaling channels are the capacity limiting factor. It is desirable to improve the link level performance of the signaling channels, e.g. FACCH in downlink and uplink direction. Transmission of signaling messages over the air interface (Um interface) is controlled by the LAPDm (Link Access Protocol of the Dm channel) protocol. In case of failed FACCH decoding of an I-frame (LAPDm in acknowledged mode) the transmitted I-frame is repeated. In the repeated I-frame not all information bits are identical: at least a part of the L2 header has changed. It is desirable to use the information from the previously transmitted I-frame to improve the decoding of the repeated I-frame. Currently, FACCH blocks are decoded without taking into account the history of previously sent FACCH blocks.

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Soft Combining of FACCH-Blocks based on LAPDm Re-Transmissions

Idea: Dr. Rupert Herzog, DE-Muenchen; Dr. Sergio Parolari, IT-Milan; Steffen Wagner, DE-

Muenchen; Oliver Cyranka, DE-Muenchen

In GSM systems, signaling information and speech or data traffic is transmitted via the air interface. All transmitted frames are protected via forward error correction against distortions on the air interface, which is caused for example by noise, inter-symbol interference, or interference from other senders. At the early beginning of GSM, the forward error correction of the signaling channels was designed to be better than for the traffic channels. However, more than 10 years after introduction of GSM, a couple of new services and coding schemes with improved channel coding performance have been introduced for speech and data channels. For example some AMR (Adaptive Multi Rate) codecs use convolutional codes with memory 6 and rate 1/4 that make them very robust against distortions. On the other hand the channel coding of the signaling channels, e.g. FACCH (Fast Associated Control CHannel) has not changed. In networks with high interference level, the error rate of the signaling channels are the capacity limiting factor. It is desirable to improve the link level performance of the signaling channels, e.g. FACCH in downlink and uplink direction.

Transmission of signaling messages over the air interface (Um interface) is controlled by the LAPDm (Link Access Protocol of the Dm channel) protocol. In case of failed FACCH decoding of an I-frame (LAPDm in acknowledged mode) the transmitted I-frame is repeated. In the repeated I-frame not all information bits are identical: at least a part of the L2 header has changed. It is desirable to use the information from the previously transmitted I-frame to improve the decoding of the repeated I-frame.

Currently, FACCH blocks are decoded without taking into account the history of previously sent FACCH blocks.

The following methods are proposed to address these problems. There are three new approaches in order to improve the situation:

1st approach:

The equalizer output is split (soft decision values of the code bits) into two parts: one part of the code bits depends on the L2 header and the CRC (Cyclic Redundancy Check) bits of the information bit sequence. The other part of the code bits belong to the L2 payload of the information bit sequence only. If an I-frame is repeated after unsuccessful decoding, the L2 header has changed but the L2 payload is identical to the previous transmission. The code bits that belong only to the L2 payload are identical to the transmission before. Thus, in case of failed decoding of the repeated transmission the code bits that belong to the L2 payload of the previous transmission can be soft combined with the corresponding code bits of the L2 payload of the repeated transmission. Fig. 1 shows an amount of 312 code bits out of 456 that are still identical in case of a changed...