5.8 Physical channels for the 3.84 Mcps MBSFN IMB option

25.2213GPPPhysical channels and mapping of transport channels onto physical channels (TDD)Release 17TS

Physical channels are defined by a specific carrier frequency, scrambling code, channelization code and in some cases a time start & stop (giving a duration). Scrambling and channelization codes are specified in [8]. Time durations are defined by start and stop instants, measured in integer multiples of chips. Suitable multiples of chips also used in specification are:

Radio frame: A radio frame is a processing duration which consists of 15 slots. The length of a radio frame corresponds to 38400 chips (10 ms).

Slot: A slot is a duration which consists of fields containing bits. The length of a slot corresponds to 2560 chips.

Sub-frame: A sub-frame corresponds to 3 slots (2 ms).

The default time duration for a physical channel is continuous from the instant when it is started to the instant when it is stopped. Physical channels that are not continuous will be explicitly described. In the case of 2 ms physical channel duration, the physical channel is active for only one 2 ms sub-frame (7680 chips) per radio frame. A physical channel of 2 ms duration may start at one of 5 start instances per radio frame. These correspond to 0 ms, 2 ms, 4 ms, 6 ms or 8 ms following the commencement of the radio frame and are denoted as sub-frames 0, 1, 2, 3 and 4 respectively.

Transport channels are described (in more abstract higher layer models of the physical layer) as being capable of being mapped to physical channels. Within the physical layer itself the exact mapping is from a composite coded transport channel (CCTrCH) to the data part of a physical channel. In addition to data parts there are also channel control parts and physical signals. For the IMB option, both a continuous and a discontinuous pilot physical channel shall be transmitted using specific OVSF channelisation codes.

The IMB option is only applicable for dedicated carrier MBSFN operations in which all TDD slots of the radio frame are configured in the downlink direction. All physical channels are common and downlink only.

Figure 18iA: Downlink transmissions in all TDD slots

5.8.1 Transmit diversity

Transmit diversity is not applicable to IMB physical channels for MBSFN operations.

5.8.2 Common physical channels

The common physical channels used on a dedicated carrier for the IMB option are P-CPICH, T-CPICH, P-CCPCH, S-CCPCH frame type 1, S-CCPCH frame type 2, SCH and MICH.

5.8.2.1 Primary Common Pilot Channel (P-CPICH)

The primary common pilot channel (P-CPICH) is a fixed rate (30 kbps, SF=256) downlink physical channel using QPSK modulation and carrying a pre-defined bit sequence in which all bits are set to logical "0". The P-CPICH is transmitted continuously on all slots of the radio frame. Figure 18iiA shows the frame structure of the P-CPICH.

Figure 18iiA: Frame structure for Primary Common Pilot Channel

The P-CPICH has the following characteristics:

– The same channelization code is always used for the P-CPICH, see [8];

– The P-CPICH is scrambled by the primary scrambling code, see [8];

– There is one and only one P-CPICH per MBSFN cluster;

– The P-CPICH is broadcast over the entire MBSFN cluster.

5.8.2.2 Time-multiplexed Common Pilot Channel (T-CPICH)

The time-multiplexed common pilot channel (T-CPICH) is composed of a set of 15 SF=16 physical channels using 16-QAM modulation, each carrying a pre-defined pilot bit sequence of length 64 bits. All of the channelization codes used to carry T-CPICH are OVSF codes as defined in [8] and are orthogonal to the P-CPICH. The T-CPICH chip-level sequence has a length of 256 chips and is transmitted at the end of each slot of the radio frame. The T-CPICH is not transmitted during the first 2304 chips of each slot. The structure of the T-CPICH is shown in figure 18iiiA.

Figure 18iiiA: Structure of the Time-multiplexed Common Pilot Channel (T-CPICH)

The T-CPICH has the following characteristics:

– The T-CPICH is scrambled by the same scrambling code as P-CPICH

– There is one and only one T-CPICH per MBSFN cluster;

– The T-CPICH is broadcasted over the entire MBSFN cluster

The UE may use the T-CPICH as the phase reference for all downlink physical channels.

The pilot bit sequences carried on T-CPICH are defined as a function of the scrambling code index used for the MBSFN cluster and the slot index in which the T-CPICH is transmitted. With index n of the primary scrambling code as defined in [4] and with the index i = 0 … 14, of the slot in which the T-CPICH is transmitted, the T-CPICH pilot bit sequences B⁽ⁿ⁾_T-CPICH,0 … B⁽ⁿ⁾_T-CPICH,959 are defined in table CD.1 of annex CD. For each slot index i, the bit sequences B⁽ⁿ⁾_T-CPICH,0 … B⁽ⁿ⁾_T-CPICH,959 are a concatenation of the 15 bit sequences b⁽ⁿ⁾_T-CPICH,0,m … b⁽ⁿ⁾_T-CPICH,63,m carried on each OVSF code C_ch,16,m (see [8]) with m = 1 … 15 such that:

{ B⁽ⁿ⁾_T-CPICH,0 , B⁽ⁿ⁾_T-CPICH,1 , … B⁽ⁿ⁾_T-CPICH,959 } = { {b⁽ⁿ⁾_T-CPICH,0,1, b⁽ⁿ⁾_T-CPICH,1,1 … b⁽ⁿ⁾_T-CPICH,63,1 },…

{b⁽ⁿ⁾_T-CPICH,0,2, b⁽ⁿ⁾_T-CPICH,1,2 … b⁽ⁿ⁾_T-CPICH,63,2 },…

…{b⁽ⁿ⁾_T-CPICH,0,15, b⁽ⁿ⁾_T-CPICH,1,15 … b⁽ⁿ⁾_{T-CPICH,63,15}} }

The OVSF code C_ch,16,0 is not used by T-CPICH.

5.8.2.3 Primary common control physical channel (P-CCPCH)

The Primary CCPCH is a fixed rate (30 kbps, SF=256) downlink physical channels used to carry the BCH transport channel. The BCH transport channel has a fixed transport format combination, hence the Primary CCPCH does not support TFCI. The P-CCPCH uses QPSK modulation.

Figure 18ivA shows the frame structure of the P-CCPCH. The P-CCPCH is not transmitted during the first and last 256 chips of each slot. Instead, Primary SCH and Secondary SCH are transmitted during first DTX period and T-CPICH is transmitted during the last DTX period.

Figure 18ivA: Frame structure for Primary Common Control Physical Channel

5.8.2.4 Secondary common control physical channel (S-CCPCH)

The Secondary CCPCH is used to carry FACH transport channels.

For MBSFN IMB, there are two types of Secondary CCPCH:

– Secondary CCPCH frame type 1; consists of 15 slots per radio frame

– Secondary CCPCH frame type 2; consists of 3 slots (i.e. one sub-frame) per radio frame.

Both of the Secondary CCPCH frame types may include TFCI in order to support multiple transport format combinations. It is the UTRAN that determines if a TFCI should be transmitted, hence making it mandatory for all UEs to support the use of TFCI. The structures of the Secondary CCPCH frame type 1 and Secondary CCPCH frame type 2 are shown in figure 18vA and figure 18viA, respectively.

Physical channel bits of Secondary CCPCH frame type 1 slots are mapped to a QPSK signal point constellation whereas physical channel bits of Secondary CCPCH frame type 2 can be mapped either to QPSK or 16QAM signal point constellations. In the case of Secondary CCPCH frame type 2, the signal point constellation to be used for the data field is given by higher layer signalling.

Figure 18vA: Frame structure for Secondary Common Control Physical Channel frame type 1

Figure 18viA: Frame structure for Secondary Common Control Physical Channel frame type 2

The parameter m in figure 18viA determines the total number of bits per Secondary CCPCH slot. The parameter m takes the value of 2 for QPSK modulation and 4 for 16-QAM modulation. The sub-frame index i in figure 18viA determines the start position of the sub-frame within the radio frame.

The values for the number of bits per field are given in table 8iA in which the channel bit and symbol rates are the rates immediately before spreading.

A FACH transport channel may be mapped to one Secondary CCPCH of frame type 1 or to one or more Secondary CCPCHs of frame type 2 that reside within the same sub-frame.

Table 8iA: Secondary CCPCH frame type 1 and 2 fields

Slot Format #i	Channel Bit Rate (kbps)	Channel Symbol Rate (kbps)	SF	S-CCPCH frame type	Bits/ Frame	Bits/ Slot	Ndata1	NTFCI
0	30	15	256	1	270	18	18	0
1	30	15	256	1	270	18	16	2
2	480	240	16	2	864	288	288	0
3	480	240	16	2	864	288	272	16
4*	960	240	16	2	1728	576	576	0
5*	960	240	16	2	1728	576	560	16**

* Slot formats applicable to 16QAM.

** This indicates that the number of modulation symbols occupied by TFCI is 4. As described in [7] and [8], QPSK modulation is applied to 8 TFCI bits per slot which results in the same number of 4 TFCI symbols

For slot formats using TFCI, the TFCI value in each radio frame corresponds to a certain transport format combination of the FACHs currently in use. This correspondence is (re-)negotiated at each FACH addition/removal. The mapping of the TFCI bits onto slots for the IMB option is described in [7].

In the case of S-CCPCH frame type 1, when an S-CCPCH CCTrCH carries TFCI, the TFCI field shall be present on all slots of the radio frame. In this case there is only one S-CCPCH in the CCTrCH.

In the case of S-CCPCH frame type 2, when an S-CCPCH CCTrCH carries TFCI, the TFCI field shall be present on all slots of the sub-frame for the S-CCPCH with the lowest channelization code index in the CCTrCH. In this case, the TFCI field shall not be present on the other S-CCPCHs of the same CCTrCH.

5.8.2.5 Synchronisation channel (SCH)

The Synchronisation Channel (SCH) is a downlink signal used for cell search and radio frame synchronisation on the MBSFN carrier. The SCH consists of two sub channels, the Primary and Secondary SCH. The 10 ms radio frames of the Primary and Secondary SCH are divided into 15 slots, each of length 2560 chips. Figure 18viiA illustrates the structure of the SCH radio frame.

Figure 18viiA: Structure of Synchronisation Channel (SCH)

The Primary SCH consists of a modulated code of length 256 chips, the Primary Synchronisation Code (PSC) denoted c_p in figure 18viiA, transmitted once every slot. The PSC is the same for every cell in the system.

The Secondary SCH consists of repeatedly transmitting a length 15 sequence of modulated codes of length 256 chips, the Secondary Synchronisation Codes (SSC), transmitted in parallel with the Primary SCH. The SSC is denoted c_s^i,k in figure 18viiA, where i = 0, 1, …, 7 is the number of the scrambling code group, and k = 0, 1, …, 14 is the slot number. Each SSC is chosen from a set of 16 different codes of length 256. This sequence on the Secondary SCH indicates which of the code groups the cell’s downlink scrambling code belongs to.

The primary and secondary synchronization codes for the MBSFN IMB option, defined in [8], are modulated by the symbol a = -1.

5.8.2.6 The MBMS indicator channel (MICH)

The MBMS Indicator Channel (MICH) is a fixed rate (SF=256) physical channel used to carry the MBMS notification indicators. The MICH is always associated with an S-CCPCH frame type 1 to which a FACH transport channel carrying MBMS control data is mapped. MICH uses QPSK modulation.

Figure 18viiiA illustrates the frame structure of the MICH where the 10 ms radio frames of the MICH are divided into 15 slots, each of length 2560 chips. One MICH radio frame of length 10 ms consists of 270 bits (b₀, b₁, …, b₂₆₉). Of these, 256 bits (b₀, b₁, …, b₂₅₅) are used to carry notification indicators. The remaining 14 bits are not formally part of the MICH and shall not be transmitted (DTX). This implies that the transmitter is turned off during the last 2048 chips of slot #14 in every radio frame.

Figure 18viiiA: Frame structure for the MBMS Indicator Channel (MICH)

In each MICH frame, Nn notification indicators {N₀, …, N_Nn-1} are transmitted, where Nn=16, 32, 64, or 128.

The NI calculated by higher layers is associated to the index q of the notification indicator N_q, where q is computed as a function of the NI computed by higher layers, the SFN of the P-CCPCH radio frame during which the start of the MICH radio frame occurs, and the number of notification indicators per frame (Nn):

where G = 2¹⁶ , C = 25033 and NI is the 16 bit Notification Indicator calculated by higher layers.

The set of NI signalled over Iub indicates all higher layer NI values for which the associated notification indicator on MICH shall be set to 1 during the corresponding modification period. Hence, the calculation in the formula above shall be performed in the Node B every MICH frame for each NI signalled over Iub to make the association between NI and q and set the related N_q to 1. All other notification indicators on MICH shall be set to 0.

The mapping from {N₀, …, N_Nn-1} to the MICH bits {b₀, …, b₂₅₅} are according to table 8iiA.

Table 8iiA: Mapping of notification indicators N_q to MICH bits

Number of notification indicators per frame (Nn)	Nq = 1	Nq = 0
Nn=16	{b16q, …, b16q+15} = {1, 1,…, 1}	{b16q, …, b16q+15} = {0, 0,…, 0}
Nn=32	{b8q, …, b8q+7} = {1, 1,…, 1}	{b8q, …, b8q+7} = {0, 0,…, 0}
Nn=64	{b4q, …, b4q+3} = {1, 1,…, 1}	{b4q, …, b4q+3} = {0, 0,…, 0}
Nn=128	{b2q, b2q+1} = {1, 1}	{b2q, b2q+1} = {0, 0}

5.8.3 Timing relationship between physical channels

Timing between the common physical channels is summarized in figure 18ixA. The P-CCPCH, on which the cell SFN is transmitted, is used as timing reference for all the physical channels. The SCH, P-CPICH, T-CPICH, P-CCPCH and S-CCPCH frame types 1 and 2 have identical radio frame timings. The sub-frame number i of an S-CCPCH frame type 2 radio frame is signalled by higher layers. The start position of an S-CCPCH frame type 2 sub-frame is then given by, (), chips after the start of the radio frame.

The frame timing of MICH is advanced by _MICH = 3 slots (7680 chips) with respect to the timings of the other physical channels.

Figure 18ixA: Radio frame and sub-frame timing of downlink physical channels