12 Flexible layer one

3GPP45.001General descriptionGSM/EDGE Physical layer on the radio pathRelease 17TS

With the Flexible Layer One (FLO), the physical layer offers transport channels to the MAC sublayer of Layer 2 (see 3GPP TR 45.902). Figure 4 shows the radio interface protocol architecture around FLO. On transport channels, transport blocks (TB) are exchanged between the MAC sublayer and the physical layer on a Transmission Time Interval basis (TTI). A transport channel is characterized by how the information is transferred over the radio interface. FLO is configured by Layer 3.

Figure 4: Radio interface protocol architecture around the physical layer for FLO

In the following subclauses, the new concepts and definitions introduced by FLO are explained. The multiple access and timeslot structure of section 5, the frequency hopping capability of section 6, the modulations of section 8, the transmission and reception of section 9, and the other layer 1 functions of section 10 remain unchanged and can be used as such by FLO.

12.1 Set of transport channels

The offered transport channels are Dedicated CHannels (DCH). A DCH can be either full rate (DCH/F) or half rate (DCH/H) depending on the rate of the dedicated basic physical subchannel on which they are used.

12.2 Transport block structure

A summarised description of the transport block structure for FLO appears in table 2, in terms of net bit rate, length and recurrence of blocks.

Table 2: Transport block structures

Type of transport channel	net bit rate (kbit/s)	block length (bits)	block recurrence (ms)1
DCH/F	0,05 – 68,5	1 – 1370	20
DCH/H	0,05 – 34,1	1 – 682	20
NOTE 1: or transmission time interval (TTI).

12.3 Channel organisation

The channel organization for FLO uses the 26‑frame multiframe structure, as described in figure 2 of section 5.3, where T depicts a TDMA frame that can be used to transmit transport block(s).

12.4 Transport channel coding/multiplexing for FLO

The coding/multiplexing unit of FLO is a combination of error detection, forward error correction, rate matching, multiplexing and interleaving.

The transport channels offered by FLO (DCHs) are used to transmit data flows with a negotiated QoS over the radio interface. A number of transport channels can be active at the same time and multiplexed at the physical layer. The configuration of a transport channel is denoted the Transport Format (TF). A number of different transport formats can be associated to one transport channel. Layer 3 controls the configuration of the transport formats. Only a limited number of combinations of the transport formats of the different TrCHs are allowed. A valid combination is called a Transport Format Combination (TFC). The set of valid TFCs is called the Transport Format Combination Set (TFCS). In every radio packet, the Transport Format Combination Indicator (TFCI) tells which TFC is used.

The following coding/multiplexing steps can be identified:

– CRC attachment: error detection is provided on each transport block through a cyclic redundancy check (CRC). Layer 3 configures the size of the CRC to be used. Code blocks are output from the CRC attachment.

– Channel coding: after CRC attachment, the code blocks are processed through channel coding (1/3 rate convolutional code), producing encoded blocks.

– Rate matching: in rate matching, bits of an encoded block on a transport channel are repeated or punctured to ensure that the total bit rate after TrCH multiplexing is identical to the total channel bit rate of the assigned basic physical channel. Outputs from the rate matching are called radio frames. The rate matching produces one radio frame per encoded block, i.e. per TrCH.

– Multiplexing of transport channels: for every radio packet to be transmitted, 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).

– TFCI mapping: the coded TFCI is appended at the beginning of the CCTrCH to form a radio packet.

– Interleaving: the radio packet is interleaved and then mapped on bursts. The interleaving can be either block diagonal or block rectangular and is configured by Layer 3.