Processing of Transport Blocks

9.4 3G Services

9.8 Physical-Layer Procedures

9.8.1 Processing of Transport Blocks

In each Transmission Time Interval (TTI), the MAC delivers to the physical layer a given number of transport blocks for each of the transport channels multiplexed together according to a Transport Format Combination (TFC). The physical layer executes a set of processes to map the transport blocks onto the available physical resources. Figure 9.22 shows the interface between physical and MAC layer and simplified multiplexing of two transport channels. Such a minimal combination of two DCHs normally includes the logical traffic channel DTCH and logical control channel DCCH. The transport chan- nels may have a different number of TBs to transmit and are not necessarily active all the time. The time periods, TTIs, for transmitting either channel could also be different.

A process of mapping transport to physical channel is performed at the physical layer in several steps as shown in Figure 9.23 for the uplink [11]:

• Adding CRC to each transport block.

• Transport-block concatenation and code block segmentation. Concatenation is required to form a block code of a fixed size. The coded block size value depends on the channel coding type applied, 504 bit for convolutional or 5114 bits, for turbo coding. The segmentation of the transport block is required when total number of bits exceeds the maximum size of block code. Theconcatenation is used in the opposite case.

• Channel coding.

• Radio frame equalization. The equalization consists of padding required number of bits after the channel coding procedure in order to ensure that the resulting number of bits fits into multiple of the number of 10 ms frames in a TTI. This procedure ensures that equal-sized blocks are transmitted per frame.

• First interleaving. The first interleaving is executed as inter-frame interleaving with an interleaver length 20, 40 or 80 ms that is directly related to the Transmission Time Interval (TTI). First interleaving is applied to each transport channel separately but start positions of the TTIs for different transport channels multiplexed together for a single connection have to be time aligned.

• Radio frame segmentation. The frame segmentation will distribute the data coming from the first interleaving over two, four or eight consecutive frames in line with the interleaving length.

TB

TFI TFI TB

TFCI Coding and Multiplexing

Physical layer Transport layer

TrCh1 TrCh2

TB

TFI TFI TB

TFCI Decoding and demultiplexing

TrCh1 TrCh2

Transmitter Receiver

Physical Control Channel

Physical Traffic Channel

Physical Traffic Channel Physical

Control Channel

Figure 9.22 Multiplexing the transport channels.

Rate matching

Physical channel segmentation

PhCH#1 PhCH#2 Radio frame segmentation

2^nd interleaving

Physical channel mapping Channel coding

a_im1, a_im2, a_im3,∙∙∙∙∙∙, a_imA

i

b_im1, b_im2, b_im3,∙∙∙∙∙∙,b_imBi

o_ir1, o_ir2, o_ir3,∙∙∙∙∙∙,o_irK

i

c_i1, c_i2, c_i3,∙∙∙∙∙∙,c_iE

i

t_i1, t_i2, t_i3,∙∙∙∙∙∙,t_iTi

d_i1, d_i2, d_i3,∙∙∙∙∙∙,d_iT

i

e_i1, e_i2, e_i3,∙∙∙∙∙∙,e_iN

i

Rate matching TrBk concatenation / Code block segmentation

CRC attachment

Radio frame equalization

1^st interleaving

TrCH Multiplexing CCTrCH b_im1, b_im2, b_im3,∙∙∙,b_imBi

o_ir1, o_ir2, o_ir3,∙∙∙,o_irK

i

c_i1, c_i2, c_i3,∙∙∙,c_iE

i

t_i1, t_i2, t_i3,∙∙∙,t_iTi

d_i1, d_i2, d_i3,∙∙∙,d_iT

i

e_i1, e_i2, e_i3,∙∙∙,e_iN

i

a_im1, a_im2, a_im3,∙∙∙, a_imA

i

Figure 9.23 Transport channel multiplexing structure for the uplink [11].

Third Generation Network (3G), UMTS 153

• Rate matching. The rate matching is used to ensure that resulting number of bits per radio frame fits into one of the available bit rates of the physical channel. That depends on the spreading factor and on the number of parallel code sequences being used. Rate matching is achieved by either bit puncturing or bit repetition depend- ing on whether the bit rate should be decreased or increased. Higher layers assign a rate-matching attribute for each transport channel. The rate-matching attribute com- mands the number of bits to be repeated or punctured.

• Multiplexing of transport channels. Every 10 ms, one radio frame from each TrCH is delivered to the TrCH multiplexing. These radio frames are serially multiplexed into a coded composite transport channel (CCTrCH).

• Physical channel segmentation. This procedure is executed whenever the resulting number of bits in one radio frame exceeds a maximum number per single physical channel (i.e. a spreading code sequence). In this case, the bit flow is segmented into blocks of equal size and each of them will be mapped into a different physical channel.

• The second interleaving performs intra-frame interleaving over a 10 ms radio frame over each physical channel segment separately, if applicable.

Downlink mapping is similar in procedures to uplink with a major difference in terms of insertion of DTX (Discontinuous Transmission) indication bits. The objective of DTX is to indicate the frame when there is no transmission for given transport channel. The DTX indication bits are not transmitted over the air interface; they indicate to the trans- mitter at which bit positions the transmission should be turned off. The use of DTX in the downlink direction allows for two possibilities when multiplexing several transport channels: ‘fixed’ and ‘flexible’ positions inside the radio frame at which channels are mapped. Detailed discussion of both options can be found in [12]; the major conclusion is that implementation of DTX bits in the ‘flexible’ position is more effective in terms of minimal puncturing bits required. The result of transport channel mapping can be illustrated in Table 9.11 using an example from [12].

Here, transport channel TrCH1 carries the logical traffic channel DTCH. It is multi- plexed together with the dedicated control channel DCCH=TrCH2. It is assumed that TFC=(TF2, TF1) is selected in a given TTI (2 transport blocks: TTBs) are sent for the interactive service and 1 TB for the signalling).

Table 9.11 shows results of applied procedures at each step of mapping for both trans- port channels. After CRC attachment and TB concatenation, the code block sizes 688 bits and 164 bits for TrCH1 and TrCH2, respectively. In both cases, no segmentation is required since coded blocks sizes are below the maximum values for the correspond- ing channel coding. After equalization and segmentation into the radio frame, TrCH1 is mapped onto two frames of 1038 bits/frame and TrCH2 onto four frames of 129 bits/frame.

The rate-matching procedure should adjust the total number of bits per frame to one of the possible values of a DPDCH physical channel in the uplink direction. The clos- est value is 1200 bits, corresponding to a single physical channel with spreading factor 32. As a result, 1200 – 1038−129=33 bits should be repeated. When the rate-matching attribute is equal for both transport channels, this leads to repeating 29 bits from TrCH1 and 4 bits from TrCH2. So, finally, a segment of 1067 bits from TrCH1 will be transmit- ted in each frame together with a segment of 133 bits from TrCH2.

Table 9.11 Multiplexing the DTCH and DCCH onto a single DPDCH [12].

TrCh1 TrCh2

TTI 20 ms 40 ms

TFC TF2=2 TBs TF1=1 TB

TB size 336 bits 148 bits

CRC attachment 336+16=352 bits 148+16=164 bits TB concatenation 2× (336+16) =688 bits 1× (148+16) =164 bits Channel coding Turbo r=1∕3−>

3× (688+4) =2076 bits

Conv r=1∕3−>3(164+8) = 516 bits

Radio frame size equalization

2076×10∕20=1038 bit∕frame 526×10∕40=129 bit∕frame Radio frame segmentation 1038 bit/frame 129 bit/frame

Rate matching 1039+29=1067 bit∕frame 129+4=133 bit∕frame

CCTrCH 1067+133=1200 bit∕frame=120kbps