A.1 Blind transport format detection using fixed positions

25.2123GPPMultiplexing and channel coding (FDD)Release 17TS

A.1.1 Blind transport format detection using received power ratio

For the dual transport format case (the possible data rates are 0 and full rate, and CRC is only transmitted for full rate), blind transport format detection using received power ratio can be used.

The transport format detection is then done using average received power ratio of DPDCH to DPCCH. Define the following:

– Pc: Received power per bit of DPCCH calculated from all pilot and TPC bits per slot over a radio frame;

– Pd: Received power per bit of DPDCH calculated from X bits per slot over a radio frame;

– X: the number of DPDCH bits per slot when transport format corresponds to full rate;

– T: Threshold of average received power ratio of DPDCH to DPCCH for transport format detection.

The decision rule can then be formulated as:

If Pd/Pc >T then:

– full rate transport format detected;

else

– zero rate transport format detected.

A.1.2 Blind transport format detection using CRC

For the multiple transport format case (the possible data rates are 0, …, (full rate)/r, …, full rate, and CRC is transmitted for all transport formats), blind transport format detection using CRC can be used.

At the transmitter, the data stream with variable number of bits from higher layers is block-encoded using a cyclic redundancy check (CRC) and then convolutionally encoded. CRC parity bits are attached just after the data stream with variable number of bits as shown in figure A.1.

The receiver knows only the possible transport formats (or the possible end bit position {n_end}) by Layer-3 negotiation. The receiver performs Viterbi-decoding on the soft decision sample sequence. The correct trellis path of the Viterbi-decoder ends at the zero state at the correct end bit position.

The blind transport format detection method using CRC traces back the surviving trellis path ending at the zero state (hypothetical trellis path) at each possible end bit position to recover the data sequence. For each recovered data sequence error-detection is performed by checking the CRC, and if there is no error, the recovered sequence is declared to be correct.

The following variable is defined:

s(n_end) = – 10 log ( (a₀(n_end) – a_min(n_end) ) / (a_max(n_end)-a_min(n_end) ) ) [dB] (Eq. 1)

where a_max(n_end) and a_min(n_end) are the maximum and minimum path-metric values among all survivors at end bit position n_end, and a₀(n_end) is the path-metric value at zero state.

In order to reduce the probability of false detection (this happens if the selected path is wrong but the CRC misses the error detection), a path selection threshold D is introduced. The threshold D determines whether the hypothetical trellis path connected to the zero state should be traced back or not at each end bit position n_end. If the hypothetical trellis path connected to the zero state that satisfies:

s(n_end)  D (Eq. 2)

is found, the path is traced back to recover the frame data, where D is the path selection threshold and a design parameter.

If more than one end bit positions satisfying Eq. 2 is found, the end bit position which has minimum value of s(n_end) is declared to be correct. If no path satisfying Eq. 2 is found even after all possible end bit positions have been exhausted, the received frame data is declared to be in error.

Figure A-2 shows the procedure of blind transport format detection using CRC.

Figure A.1: An example of data with variable number of bits.
Four possible transport formats, and transmitted end bit position nend = 3

Figure A.2: Basic processing flow of blind transport format detection