GPRS for Mobile Internet

2017-07-07 02:10:07

4.2 RF Physical Layer

This section first describes the concept of multislot classes, which allows for transmission in several time slots in the TDMA frame. It then focuses on the transmitter and receiver requirements. For the former, the constraints introduced by multislot transmissions are presented. For the latter, the sensitivity and interference performance specifications are discussed, as well as GSM circuit-switched requirements such as blockings, intermodulation, and AM suppression, which also apply in GPRS Release 99. The transmitter and receiver requirements are defined in [1].

4.2.1 Multislot Classes

One of the most important GPRS characteristics on the air interface is the possibility of increasing the achievable bit rate by grouping together several channels. In order to do so with a reasonable impact on the MS in terms of implementation complexity, it has been decided to allow the RF transmission on several slots of a TDMA frame, with a number of restrictions, as listed below.

Several bursts can be transmitted within a TDMA frame, but should all be on the same ARFC number (ARFCN; i.e., the same carrier frequency).

Depending on the mobile capability, delay constraints are needed between the transmission and reception of bursts, and between reception and transmission, to allow the mobile to perform the adjacent cell measurements, or monitoring, as discussed in Chapter 1.

If there are m time slots allocated to an MS for reception and n time slots allocated for transmission, the system requires that min ( m,n ) reception and transmission time slots have the same time slot number (TN; see Section 1.5.1 for the definition) within the TDMA frame.

Two types of MSs are defined: type 1 mobiles are not able to transmit and to receive at the same time, while type 2 mobiles are. For these two types, there exist different classes, depending on the capability of the MS in terms of complexity. The classes are called multislot classes since they refer to the ability of the mobile to support a communication on several time slots of the TDMA frame. For a given multislot class, the mobile is able to transmit on a maximum of Tx time slots, and to receive on a maximum of Rx time slots within a TDMA frame, but the sum Tx + Rx is limited. This means that the maximum of Tx slots and the maximum of Rx slots are not active at the same time.

The definition of the type 1 multislot classes relies on the following time constraints:

T _ta is the maximum number of time slots allowed to the MS to measure an adjacent cell received signal and to get ready to transmit. This parameter is therefore used to set the minimum allowed delay between the end of a transmit or receive time slot and the next transmit time slot, with an adjacent cell measurement to be performed in between.

T _rb relates to the number of TS needed by the MS, prior to a receive time slot, when no adjacent cell measurement is performed. It is the minimum delay between the end of a transmit or receive TS and the first next receive TS.

T _ra is the minimum allowed delay in number of TS, between the end of a transmit or receive time slot and the next receive time slot, when an adjacent cell measurement is to be performed in between.

T _tb is the minimum number of TS between the end of a receive or transmit TS and the first next transmit TS, without adjacent cell measurement in between.

These constraints have been chosen to give the mobile enough time for the frequency change between a receive or transmit slot, and the next receive or transmit slot. It also allows some time to measure an adjacent cell received signal level, which requires a measurement window (the window size is usually in the order of one time slot or less) and two frequency changes (from the transmit or receive frequency to the adjacent cell beacon frequency, and then back to the next receive or transmit time slot). Note that the time constraints T _ta and T _tb may be reduced by the amount of the TA, to derive the allowed time duration before transmission.

The existing multislot classes are given in Tables 4.7 (type 1 MSs) and 4.8 (type 2 MSs), with the Tx and Rx parameters defining the maximum uplink and downlink slots of the TDMA frame, the sum being the maximum value for Rx + Tx. The time constraints T _ta , T _rb , T _ra , T _tb corresponding to each class are also given in these tables.

Table 4.7: Type 1 MS Multislot Classes
Type 1 MSs
	Maximum Number of Slots			Minimum Number of Slots
Multislot Class	Rx	Tx	Sum Rx+Tx	T _ta	T _tb	T _ra	T _rb
1	1	1	2	3	2	4	2
2	2	1	3	3	2	3	1
3	2	2	3	3	2	3	1
4	3	1	4	3	1	3	1
5	2	2	4	3	1	3	1
6	3	2	4	3	1	3	1
7	3	3	4	3	1	3	1
8	4	1	5	3	1	2	1
9	3	2	5	3	1	2	1
10	4	2	5	3	1	2	1
11	4	3	5	3	1	2	1
12	4	4	5	2	1	2	1
19	6	2	N/A	3	^[a]	2	^[b]
20	6	3	N/A	3	^[a]	2	^[b]
21	6	4	N/A	3	^[a]	2	^[b]
22	6	4	N/A	2	^[a]	2	^[b]
23	6	6	N/A	2	^[a]	2	^[b]
24	8	2	N/A	3	^[a]	2	^[b]
25	8	3	N/A	3	^[a]	2	^[b]
26	8	4	N/A	3	^[a]	2	^[b]
27	8	4	N/A	2	^[a]	2	^[b]
28	8	6	N/A	2	^[a]	2	^[b]
29	8	8	N/A	2	^[a]	2	^[b]
N/A: not applicable .
^[a] = 1 with frequency hopping or change from Rx to Tx; = 0 without frequency hopping and no change from Rx to Tx; ^[b] =1 with frequency hopping or change from Tx to Rx; = 0 without frequency hopping and no change from Tx to Rx;

Table 4.8: Type 2 MS Multislot Classes
Type 2 MS
	Maximum Number of Slots			Minimum Number of Slots
Multislot Class	Rx	Tx	Sum Rx+Tx	T _ta	T _tb	T _ra	T _rb
13	3	3	N/A	N/A	( ^[*] )	3	( ^[*] )
14	4	4	N/A	N/A	( ^[*] )	3	( ^[*] )
15	5	5	N/A	N/A	( ^[*] )	3	( ^[*] )
16	6	6	N/A	N/A	( ^[*] )	2	( ^[*] )
17	7	7	N/A	N/A	( ^[*] )	1
18	8	8	N/A	N/A
^[*] () 1 with FH, 0 without FH; N/A: not applicable.

Note that only one monitoring window (i.e., an adjacent cell power measurement window) is needed in a TDMA frame, so that only a couple (T _ra ,T _tb ) or (T _ta ,T _rb ) is needed to define a valid configuration of a given multislot class. Figure 4.11 further illustrates the different possible configurations allowed for a class 12 mobile.

Figure 4.11: Allowed configurations for multislot class 12. (a) 4 RX + 1 TX, (b) 3 RX + 2 TX, (c) 2 RX + 3 TX, and (d) 1 RX + 4 TX.

One can notice that for classes 1 to 12 the sum of Tx + Rx slots, added to T _ra + T _tb , is always less than or equal to 8. This comes from the fact that the mobile, for these classes, is not full duplex capable (i.e., it is not able to transmit and to receive at the same time). The total sum of these constraints cannot be more than 8 time slots, which is the duration of a TDMA frame.

For classes 13 to 18, this constraint is not in use, since the mobile has either the ability to receive and transmit at the same time, or to receive or transmit and perform an adjacent cell measurement at the same time.

4.2.2 Transmitter Path Characteristics

The GPRS standard was designed to minimize the changes on the RF layer of standard GSM equipment. On the transmitter part, for both the MS and BTS, it has therefore been decided to keep the existing power classes, as well as the power control ranges and steps. The changes due to GPRS on Tx are limited to the constraint of the multislot transmission, with a new power ramping template. Indeed, as seen in Section 4.1.3, power control is used on uplink, and independent transmit power on adjacent time slots, and therefore any combination of power control steps may be applied on several time slots of the same TDMA frame. The single slot power versus time mask is shown in Chapter 1, in Figure 1.11, for the NB.

For multislot transmissions, the constraint is slightly modified, as follows :

The same template as for the single slot case is to be respected during the useful part of each burst and at the beginning and the end of the series of consecutive bursts.

The output power during the guard period between every two consecutive active time slots cannot be greater than the level allowed for the useful part of the first time slot, or the level allowed for the useful part of the second time slot plus 3 dB, whichever is the highest.

Between the active bursts, the output power must be lowered to a specified limit.

Figure 4.12 shows the power-versus-time-mask example for a two-slot transmitter, for different power configurations.

Figure 4.12: Multislot power versus time mask for the NB and for the AB- (a) power level is higher on first time slot; and (b) power level is higher on second time slot.

4.2.3 Receiver Path Characteristics

4.2.3.1 Reference Sensitivity Performance

In the case of GSM circuit-switched services (see Section 1.5.6.2), a minimum input level at the receiver is defined, at which the MS and the BTS are required to reach a certain level of performance.

For GSM voice service, for instance, we have seen that GSM900 small MSs (see definition in Section 1.5.6.1) are required to reach a given BER, FER, and RBER, for an input signal level (ISL) of -102 dBm. The performances (BER, FER, RBER) are different depending upon the channel profile (static, TU50, RA250, HT100), but the level is -102 dBm for all profiles.

The performance to be met in GPRS is the block error rate (BLER), referring to all erroneously decoded data blocks including any headers, SFs, data, and parity bits. Once a radio block, comprising four bursts, is received on a PDTCH, the mobile performs a de-interleaving and a decoding of the convolutional code, for coding schemes CS-1 to CS-3 (no convolutional code is used for the coding scheme CS-4). The parity bits from the block code are then calculated, based on the received bits. If the calculated parity sequence is different from the received sequence, the block is declared erroneous. The ratio of these erroneous blocks to the number of received blocks is therefore an estimation of the BLER. The BLER is also defined for the USF and it refers to the ratio of erroneously decoded USF words over the number of received blocks.

The principle of the sensitivity performance definition is somewhat different in GPRS than in GSM circuit, in the sense that the BLER level to be reached for the MS and the BTS is not dependent on the channel profile. Indeed, for all the channel profiles:

A BLER of 10% will be achieved on the PDTCHs, for all the coding schemes CS-1 to CS-4.

A BLER of 1% will be achieved on the USF field.

A BLER of 15% will be achieved on the PRACHs.

These requirements are valid for both the BTS and MS, and for different propagation conditions. The idea behind this is to ensure a constant quality of the data link (constant BLER) in all the propagation conditions. Nevertheless, the level for which these performance levels are to be reached are different according to the equipment type (MS or BTS), the channel profile, and the coding scheme.

Table 4.9 gives the ISL for the sensitivity performance, for the different coding schemes and propagation profiles, for a GSM-900 or GSM-850 normal BTS. The figures are slightly different for DCS-1800 and PCS-1900. For the MS, all the ISLs for the sensitivity case can be derived from the BTS performance, by adding an offset of 0, +2, or +4 dB (according to the MS power class and its band of operation) to the ISL figures.

Table 4.9: ISL at Reference Performance for Normal BTS
GSM-900 and GSM-850 Normal BTS
	Propagation Conditions
Type of Channel	Static	TU50 (No FH)	TU50 (Ideal FH)	RA250 (No FH)	HT100 (No FH)
PDTCH/CS-1 (dBm)	-104	-104	-104	-104	-103
PDTCH/CS-2 (dBm)	-104	-100	-101	-101	-99
PDTCH/CS-3 (dBm)	-104	-98	-99	-98	-96
PDTCH/CS-4(dBm)	-101	-90	-90	No requirement
USF/CS-1 (dBm)	-104	-101	-103	-103	-101
USF/CS-2 to 4 (dBm)	-104	-103	-104	-104	-104
PRACH/11 bits (dBm)	-104	-104	-104	-103	-103
PRACH/8 bits (dBm)	-104	-104	-104	-103	-103

Note that the specification for PDTCH/CS-1 applies also for PACCH, PBCCH, PAGCH, PPCH, and PTCCH/D.

To avoid the blinding of the receiver, the sensitivity specification is not required if the received level on either the time slot immediately before or immediately after a received time slot is greater than the desired time slot level by more than 50 dB for the BTS, or 20 dB for the MS. Also, for the MS, these specifications are not to be fulfilled if the received level on any of the time slots belonging to the multislot configuration is greater than the desired (that is the one on which the BLER is actually measured) time slot level by more than 6 dB.

4.2.3.2 Interference Performance

The same BLER performance (10% for PDTCH, 15% for PRACH, and 1% for the USF) is required when interference is added to the wanted signal. The interference specifications are defined for a desired signal input level depending on the channel profile, and for a random, continuous, GMSK-modulated interfering signal.

For a BTS, the ISL is -93 dBm + C/Ic, where C/Ic is the cochannel interference ratio, given in Table 4.10 for the example of GSM-900 and GSM-850. The BLER performance is to be met for the three following cases:

Cochannel interferer -the interfering signal is on the same RF channel as that desired (same ARFCN). The carrier to interference ratio is referred to as C/Ic.

First adjacent channel -the interfering carrier is 200 kHz away from the desired signal RF channel. The carrier to interference is given by C/Ia ₁ = C/Ic - 18 dB.

Second adjacent channel -the interference signal is transmitted at 400 kHz from the desired carrier. The carrier to interference ratio is C/Ia ₂ = C/Ic - 50 dB

Table 4.10: C/I Ratio for Cochannel Performance for Normal BTS
GSM-900 and GSM-850 Normal BTS
	Propagation Conditions
Type of Channel	TU3 (No FH)	TU3 (Ideal FH)	TU50 (No FH)	TU50 (Ideal FH)	RA250 (No FH)
PDTCH/CS-1 (dB)	13	9	10	9	9
PDTCH/CS-2 (dB)	15	13	14	13	13
PDTCH/CS-3 (dB)	16	15	16	15	16
PDTCH/CS-4(dB)	21	23	24	24	No requirement
USF/CS-1 (dB)	19	10	12	10	10
USF/CS-2 to 4 (dB)	18	9	10	9	8
PRACH/11 bits (dB)	8	8	8	8	10
PRACH/8 bits (dB)	8	8	8	8	9

If we take the example of CS-4 in GSM900, with the TU50/no FH propagation profile, we have the following conditions for a normal BTS:

The desired ISL is -93 + C/Ic with C/Ic = 24 dB, so ISL = -69 dBm, for the cochannel, first adjacent, and second adjacent specifications.

To fulfill the cochannel interference performance, the C/Ic is fixed to 24 dB, so the BLER performance is to be met for a signal level of -69 dBm, and a cochannel level of -93 dBm.

In the first adjacent channel interference performance case, the wanted signal level is also equal to -69 dBm, and since C/Ial = C/Ic - 18 dB = 24 - 18 = 6 dB, the interferer level is -69 - 6 = -75 dBm.

For the second adjacent channel interference specification, the wanted signal level is still equal to -69 dBm, with C/Ia2 = C/Ic - 50 dB = 24 - 50 = -26 dB, so the interferer level is equal to -69 - (-26) = -43 dBm.

For the MS, the C/I requirements are the same as for the BTS, but the ISL of the desired carrier for which the performance will be met is higher (by 0, 2, or 4 dB according to the type of MS).

4.2.3.3 Blocking Characteristics

A blocking is an interfering signal at a high power level as compared with the desired signal, either in-band (which means situated in the receiver band), or out-of-band (out from the receiver band). Of course, the frequency bands corresponding to the in-band and out-of-band parts are dependent on the system (GSM-900, DCS-1800, and PCS-1900).

The blocking characteristics of the receiver refer to its capability of achieving a certain performance in the presence of a strong interferer in the in-band or one of the out-of-band frequency bands (see Table 4.11). This requirement exists in the case of the GSM circuit-switched service, and has also been defined for the GPRS system, from Release 99. No blocking characteristics were defined in Release 97/98 GPRS equipment.

Table 4.11: In-Band and Out-of-Band Definitions for the Blocking Characteristics
Frequency Band	MS	BTS
In band	915-980	860-925
Out-of-band (a)	0.1 to 915 MH	0.1 to 860 MHz
Out-of-band (b)	980 to 12,750 MHz	925 to 12,750 MHz

If we take the example of GSM900, this requirement states that the sensitivity reference level performance (BLER requirements) will be met in the conditions here:

A desired signal at frequency f , with a power level 3 dB higher than in the sensitivity requirement (see Section 4.2.3.1);

A continuous sine wave blocking signal at a frequency f , which is an integer multiple of 200 kHz, and with a power level in dBm as specified in Table 4.12.

Table 4.12: In-Band and Out-of-Band Blocking Signal Level (GSM-900)
	EGSM-900 Level of the Blocking Signal in dBm
Frequency Band	Small MS	Other MS	BTS
In-band
600 kHz = f - f < 800 kHz	-43	-38	-26
800 kHz = f - f < 1.6 MHz	-43	-33	-16
1.6 MHz = f - f < 3 MHz	-33	-23	-16
3 MHz = f - f	-23	-23	-13
Out-of-band
(a)			8
(b)			8

4.2.3.4 Intermodulation Characteristics

This performance requirement is valid for the GSM voice services, and has also been introduced in the GPRS requirements from Release 99. Release 97/98 transceivers are not requested to fulfill this requirement. The requirement states that the sensitivity performance will be reached in the following conditions:

A desired signal 3 dB above the sensitivity level requirement, at the frequency f ;

A continuous sine wave at the frequency f ₁ added to this signal, with a level between -43 and -49 dBm according to the frequency band (see Table 4.13);

A modulated GMSK signal on the frequency f ₂ , with the same level as the sine wave.

Table 4.13: Power Levels of Signals on Frequencies f ₁ and f ₂ for Intermodulation Requirement
GSM-400, GSM-850, and GSM-900 small MSs	-49 dBm
DCS-1800 other than class 3 MSs, and PCS-1900 MS
DCS-1800, PCS-1900 BTS
DCS-1800 class 3 MS	-45 dBm
All other cases	-43 dBm

The frequencies are chosen such that f = 2 · f ₁ - f ₂ and f ₂ - f ₁ = 800 kHz.

Due to the third-order nonlinearity of the receiver, the intermodulation product of frequencies f ₁ and f ₂ generates a signal at frequency f . This results in a modulated interference signal that is added to the signal of interest. The effect is therefore similar to a cochannel interference on the received baseband signal. This point is further developed in Section 4.3.5.2.

4.2.3.5 AM Suppression

The AM suppression characteristic is a receiver requirement [1] that concerns the GSM circuit-switched voice and data services, but also the GPRS services from Release 99 (Releases 97/98 GPRS equipment is not needed to fulfill this requirement).

In the AM suppression specification, the recommendations request fulfillment of the sensitivity performance with the following signals at the input of the reception chain:

The desired signal, which is GMSK modulated with a power 3dB higher than the sensitivity level.

A GMSK-modulated carrier situated in the receive band, at least 6 MHz away from the desired signal, with a power level as given in Table 4.14. Bursts are transmitted on this carrier, with a time delay between 61 and 86 bit periods relative to the bursts of the desired signal.

Table 4.14: AM Suppression Requirement
	MS (dBm)	BTS (dBm)
GSM-400	-31	-31
GSM-900	-31	-31
GSM-850	-31	-31
DCS-1800	-29 class 3 MS -31 class1 and 2 MS	-35
PCS-1900	-31	-35

The reason for this specification and the problems that arise in the receiver due to this requirement are discussed in a case study (see Section 4.3.4.2).