6.2 Mapping in frequency of logical channels onto physical channels

05.023GPPMultiplexing and Multiple Access on the Radio PathTS

6.2.1 General

The parameters used in the function which maps TDMA frame number onto radio frequency channel are defined in clause 6.2.2. The definition of the actual mapping function, or as it is termed, hopping sequence generation is given in clause 6.2.3.

In CTS, the specific mapping in frequency depends on the start condition defined by the parameters given in clause 6.2.2. The hopping sequence generation for CTS is given in clause 6.2.3.

6.2.2 Parameters

The following parameters are required in the mapping from TDMA frame number to radio frequency channel for a given assigned channel.

General parameters of the BTS, specific to one BTS, and broadcast in the BCCH and SCH:

i) CA: Cell allocation of radio frequency channels.

ii) FN: TDMA frame number, broadcast in the SCH, in form T1, T2, T3′ (see clause 3.3.2). For COMPACT, FN is broadcast in the CSCH, in form R1, R2 (see clause 3.2.2).

Specific parameters of the channel, defined in the channel assignment message:

i) MA: Mobile allocation of radio frequency channels, defines the set of radio frequency channels to be used in the mobiles hopping sequence. The MA contains N radio frequency channels, where 1  N  64.

For COMPACT, the reduced MA (see GSM 04.60) shall be used for a fixed amount of data blocks, see clause 6.2.4.

ii) MAIO: Mobile allocation index offset.(0 to N‑1, 6 bits).

For COMPACT, MAIO_2 shall be used for the data blocks using the reduced MA.

iii) HSN: Hopping sequence (generator) number (0 to 63, 6 bits).

In CTS, the following parameters are required in the mapping to radio frequency channel for a CTS-FP and CTS-MS pair. They are given by the CTS-FP to the CTS-MS during the non-hopping access procedure :

i) VA: the vector a defines the elements which are used from the shift register to generate the codeword. The vector a shall be randomly chosen upon up to 16 non-repeating integer elements where 0 £ ai < 16 and ai ¹ aj for i ¹ j.

ii) VV: the elements of vector v are added modulo 2 to the codeword from the shift register. For vector v, up to 16 binary elements shall be chosen randomly.

NOTE: The length of the vectors a and v is dependent on the number of frequencies used for the hopping and can be truncated according to the number of frequencies used (see vi) below).

iii) CSR: current shift register contents. In order that a CTS-MS is able to synchronize on a running hopping sequence the CTS-FP transmits the CSR.

iv) TFHC1: value of counter TFHC1.

v) TFHC2: value of counter TFHC2.

vi) TFH carrier list (see GSM 05.56) : ordered list of frequencies, with 1st freq referenced by the frequency index 1, 2nd frequency referenced by the frequency index 2, etc.

The number of frequencies in the TFH carrier list, NF shall be computed. The number of elements to be taken from the vectors a and v shall be determined by the function élog2NFù.

vii) VC: the vector c is the base sequence to map the codeword. It shall be randomly chosen upon NF non-repeating integer elements:
c = {c0, c1, … , cNF-1}, 0 £ ci < NF and ci ¹ cj for i ¹ j.

6.2.3 Hopping sequence generation

For a given set of parameters, the index to an absolute radio frequency channel number (ARFCN) within the mobile allocation (MAI from 0 to N‑1, where MAI=0 represents the lowest absolute radio frequency channel number (ARFCN) in the mobile allocation, ARFCN is in the range 0 to 7023 and the frequency value can be determined according to GSM 05.05 sec 2 with n= ARFCN), is obtained with the following algorithm:

if HSN = 0 (cyclic hopping) then:

MAI, integer (0 .. N‑1) : MAI = (FN + MAIO) modulo N

else:

M, integer (0 .. 152) : M = T2 + RNTABLE((HSN xor T1R) + T3)

S, integer (0 .. N‑1) : M’ = M modulo (2 ^ NBIN)

T’ = T3 modulo (2 ^ NBIN)

if M’ < N then:

S = M’

else:

S = (M’+T’) modulo N

MAI, integer (0 .. N‑1) : MAI = (S + MAIO) modulo N

NOTE: Due to the procedure used by the mobile for measurement reporting when DTX is used, the use of cyclic hopping where (N)mod 13 = 0 should be avoided.

where:

T1R: time parameter T1, reduced modulo 64 (6 bits)

T3: time parameter, from 0 to 50 (6 bits)

T2: time parameter, from 0 to 25 (5 bits)

NBIN: number of bits required to represent N = INTEGER(log2(N)+1)

^: raised to the power of

xor: bit‑wise exclusive or of 8 bit binary operands

RNTABLE: Table of 114 integer numbers, defined below:

Address

Contents

000…009:

48,

98,

63,

1,

36,

95,

78,

102,

94,

73,

010…019:

0,

64,

25,

81,

76,

59,

124,

23,

104,

100,

020…029:

101,

47,

118,

85,

18,

56,

96,

86,

54,

2,

030…039:

80,

34,

127,

13,

6,

89,

57,

103,

12,

74,

040…049:

55,

111,

75,

38,

109,

71,

112,

29,

11,

88,

050…059:

87,

19,

3,

68,

110,

26,

33,

31,

8,

45,

060…069:

82,

58,

40,

107,

32,

5,

106,

92,

62,

67,

070…079:

77,

108,

122,

37,

60,

66,

121,

42,

51,

126,

080…089:

117,

114,

4,

90,

43,

52,

53,

113,

120,

72,

090…099:

16,

49,

7,

79,

119,

61,

22,

84,

9,

97,

100…109:

91,

15,

21,

24,

46,

39,

93,

105,

65,

70,

110…113:

125,

99,

17,

123,

The hopping sequence generation algorithm is represented diagrammatically in figure 6.

This algorithm applies also to COMPACT, whereby the parameters T1, T2 and T3 shall be calculated from FN.

In CTS, the general structure of the hopping sequence generation algorithm is shown in figure 6a, with the example of vector a = (a0, a1, a2, a3) = (5, 8, 2, 11) and NF = 9. It consists of a 16 bit linear feedback shift register and two counters. The shift register in the CTS-FP shall be initialized with a random number which shall not be zero. The counter TFHC1 counts modulo NF the number of TDMA frames. The overflow of this counter causes a shift in the shift register. The counter TFHC2 counts modulo NF the number of shifts.

The elements which are used from the shift register to generate the codeword are defined by the vector a. The codeword is built using a modulo 2 addition of these elements and the elements of vector v . Before mapping the codeword into a sequence, the value of the counter TFHC2 is added modulo NF. The mapping is done by a modulo NF addition to the base sequence c. This results in a sequence containing NF elements, each element representing one frequency index in the TFH list. The value of counter TFHC1 points to the current frequency index to use.

6.2.4 Specific cases

On the RFCH carrying a BCCH (C0), frequency hopping is not permitted on any timeslot supporting a BCCH according to table 3 of clause 7. A non‑hopping radio frequency channel sequence is characterized by a mobile allocation consisting of only one radio frequency channel, i.e. with N=1, MAIO=0. In this instance sequence generation is unaffected by the value of the value HSN.

For COMPACT, frequency hopping is not permitted on CPBCCH or CPCCCH for a specific amount of N_CCCH_NH blocks according to the ordered list described in clause 6.3.2.1. If CPCCCH is defined as frequency hopping, those blocks use MAI = MAIO.

For COMPACT, on other frequency hopping channels, the reduced MA and MAIO_2 shall be used for a specific amount of N_CCCH_NH blocks according to the ordered list described in clause 6.3.2.1.

For COMPACT, in case the optional information elements reduced MA and MAIO_2 are not present in the assignment message and the MA and MAIO information elements are present in the assignment message, then the MS shall hop in all allocated time slots according to the MA and MAIO.

6.2.5 Change in the frequency allocation of a base transceiver station

The consequence of adding or removing a number of radio frequency channels in a base transceiver station is a modification of the cell allocation (CA) and the mobile allocation (MA). In order to achieve this without disruption to mobile stations with currently assigned channels it is necessary to send a message to all mobiles with assigned channels. The message, as defined in 04.08, will contain a new cell allocation (if necessary), mobile allocation and a time (in the form of a TDMA frame number) at which the change is to occur. A new cell allocation may not be necessary if channels are only being removed, and not added.

6.2.6 Frequency assignment in CTS

The CTSBCH (CTSBCH-FB and CTSBCH-SB) shall always be mapped on the CTSBCH RF channel (designated as C0 in clause 7 table 8).

The CTSPCH, CTSARCH and CTSAGCH shall be mapped on the predefined set of carriers called TFH carrier list (designated by C0… Cn in Clause 7 Table 8) by the CTS frequency hopping algorithm specified in clauses 6.2.2 and 6.2.3. However, the CTSARCH and CTSAGCH shall be mapped on the CTSBCH RF channel for the specific case of the non-hopping access procedure specified in GSM 04.56 ; the block TDMA frame mapping for these exceptions is specified in clause 7 table 8. The methods for the determination of the CTSBCH RF channel and the TFH carrier list are defined in GSM 05.56.

The TCH, FACCH and SACCH used for a CTS dedicated connection shall always be mapped on the TFH carrier list (C0..Cn) by the CTS frequency hopping algorithm. However, one exception is specified in the case of the CTS enrolment and attachment of a CTS-MS (see GSM 04.56), where a non-hopping access procedure is used; in these particular cases, the dedicated connection shall be used in non-hopping mode and the TCH, FACCH and SACCH shall be mapped on the CTSBCH RF channel (C0).