1、 Rec. ITU-R F.1605 1 RECOMMENDATION ITU-R F.1605 Error performance and availability estimation for synchronous digital hierarchy terrestrial fixed wireless systems (Question ITU-R 122/9) (2003) The ITU Radiocommunication Assembly, considering a) that synchronous digital hierarchy (SDH) and plesiochr
2、onous digital hierarchy (PDH) systems play an important role in current fixed wireless systems; b) that Recommendations ITU-R F.1491 and ITU-R F.1397 established the error performance objectives for real connections; c) that Recommendations ITU-R F.1492 and ITU-R F.1493 established the availability
3、objectives for real connections; d) that Recommendation ITU-R P.530 provides methods to allow the prediction of propagation parameters affecting the planning of terrestrial line-of-sight systems; e) that suitable path planning and systems design methods are needed to minimize the degradation of fixe
4、d radio system operation due to propagation effects, recommends 1 that the error performance prediction methods for SDH radio links (paths and sections) set out in Annex 1 should be adopted for planning terrestrial line-of-sight systems in the respective ranges of parameters indicated. Annex 1 Predi
5、ction of error performance and availability of line-of-sight SDH radio links 1 Introduction SDH networks, or synchronous optical networks (SONET), currently allow radio link systems with capacities in synchronous transfer mode (STM) from 51 Mbit/s (STM-0) to 622 Mbit/s (STM-4). International Recomme
6、ndations and Standards define the error performance and availability objectives for SDH networks. These objectives are media independent, and still have to be met whenever radio forms a part of the network. 2 Rec. ITU-R F.1605 The prediction methods given in this Annex are based on a theoretically-d
7、erived relationship between bit error ratio (BER) and the SDH parameters based on errored blocks (EBs). The methods take into account system characteristics such as burst errors along with the parameters needed for predicting the outage time based on different BER thresholds. The term “outage” in th
8、is Recommendation is defined as the probability that the BER is larger than a given threshold. It should be noted that the outage intensity (OI), defined in 2.1 is a distinctly different parameter. 2 Error performance and availability objectives ITU-T Recommendations G.826 and G.827 give the require
9、ments for error performance and availability, respectively. Since these Recommendations give the end-to-end objectives, other recommendations are needed when dealing with actual radio links. In order to apportion the allowances to individual paths with actual hop lengths, operating radio frequency,
10、etc., a number of additional recommendations have therefore been issued dealing with radio as an SDH network element. 2.1 Error performance and availability parameters ITU-T Recommendations G.826 and G.828 define a set of block-based error performance events and parameters devoted to in-service erro
11、r performance monitoring of an SDH path (see Note 1). An EB is one in which one or more bits are in error. An errored second (ES) occurs if there are one or more errored blocks or at least one defect such as a loss of pointer (LoP) during a one-second period. A severely errored second (SES) occurs i
12、f there are 30% or more EBs or a defect. SES is a subset of ES. Background block error (BBE) is an EB not occurring as part of an SES. Error performance parameters defined are severely errored second ratio (SESR), background block error ratio (BBER), and errored second ratio (ESR). Each direction of
13、 a path can be in one of two states: available time or unavailable time. The criteria determining the transition between the two states are as follows: A period of unavailable time begins at the onset of 10 consecutive SES events. These 10s are considered to be part of unavailable time. A new period
14、 of available time begins at the onset of 10 consecutive non-SES events. These 10s are considered to be part of available time. A path is available if, and only if, both directions are available. The availability parameters defined are: availability ratio (AR) and mean time between digital path outa
15、ge (Mo). The converse of AR is the unavailability ratio (UR). Thus, AR + UR = 1. The reciprocal of Mo is defined as the outage intensity (OI). Thus, OI = 1/Mo. OI is regarded as the number of unavailable periods per year. NOTE 1 An SDH path is a trail carrying an SDH payload and associated overhead
16、through the layered transport network between the path-terminating equipment. A digital path may be bidirectional or unidirectional and may comprise of both customer-owned portions and network-operator-owned portions (for more information see ITU-T Recommendations G.803, G.805 and G.828). Rec. ITU-R
17、 F.1605 3 The definition of block size and error performance events for SDH multiplex section (MS) and regenerator section (RS) are presented in ITU-T Recommendation G.829. The error performance parameters and objectives for SDH MS and RS on individual digital fixed wireless links are given in the I
18、TU-R F-Series Recommendations. NOTE 2 The threshold of 30% EBs is defined for SDH path in ITU-T Recommendations G.826 and G.828, while for SDH MS and RS the threshold value is defined in ITU-T Recommendation G.829. 2.2 Objectives for digital fixed wireless digital paths The error performance objecti
19、ves given for digital paths at or above the primary rate (2 048 or 1 544 Mbit/s) are different for national and international portions. The specified objectives must be met for any month. ITU-R has adopted objectives for both national and international paths. For the national portion there is a subd
20、ivision into three sections. These are: long haul, short haul and access network sections. The allocation of the error performance objectives for the short haul and access sections shall each make use of a fixed block allocation in the range of 7.5% to 8.5% of the end-to-end objectives. The allocati
21、on of the objectives for the long haul section shall make use of a distance-based allocation per 100 km and a fixed block allocation in the range of 1% to 2% of the end-to-end objectives. For apportioning these objectives to real digital fixed wireless links consisting of one or more hops, Recommend
22、ations ITU-R F.1397 and ITU-R F.1491 shall be used for the international and national portion respectively. The availability objectives are defined in ITU-T Recommendation G.827. For real digital fixed wireless links consisting of one or more hops the availability objectives for a link forming part
23、of an SDH path at or above the primary rate are defined in Recommendations ITU-R F.1492 and ITU-R F.1493 for the international and national portions, respectively. 3 Predicting error performance and availability The relationship between the error performance parameters and BER is evaluated employing
24、 both random (Poisson) and burst error (Neyman-A) distributions. Modern radio transmission systems, which implement complex modulation schemes, error correction codes, equalizers, etc., tend to produce clusters or bursts of errors (see Note 1). Typical values for high level modulation (e.g. 128-trel
25、lis code modulation (128-TCM) are 10 to 20 errors per burst. NOTE 1 A burst of errors is defined as a sequence of errors which starts and ends with an errored bit such that the time between two errors is less than the memory of the system (e.g. the constraint length of the convolutional code, the co
26、de size for block code, etc.). The SDH prediction methods are based on theoretical assumptions relating BER, which is the fundamental parameter of existing prediction methods, to the error performance events ES, SES and BBE. Since the methods are based on BER, they automatically cover both multipath
27、 and rain attenuation. In the following paragraphs, some background information is given followed by step-by-step calculation procedures. 4 Rec. ITU-R F.1605 The prediction methods for SDH can also be used for PDH with the following choices: use the BERSESclosest to the transmission rate (Mbit/s), e
28、.g. a VC-12 for a 2 Mbit/s PDH radio; use the BERSESas given in Table 1 (the BERSESis under study for PDH, but only minor differences are expected). TABLE 1 BERSESfor various SDH paths and MS sections 3.1 Prediction of SESR The procedure for predicting SESR is based on the relationship between SES a
29、nd BER, where the methods for predicting outage defined by a specified BER are given in Annex 1 of Recommendation ITU-R P.530. Generally the SESR is predicted by: xxgxfSESRBERSESBERBERBAd)()(= (1) where: :)(xfSESrelationship between SES and the BER :)(xgBERprobability density function (PDF) of BER B
30、ERAand BERB : integration limits (typical values are 1 109and 1 103). Path type Bit rate supported (Mbit/s) BERSES(1),(2)Blocks/s, n(2)Bits/block, NB(2)VC-11 1.5 5.4 104 2 000 832 VC-12 2 4.0 104 2 000 1 120 VC-2 6 1.3 104 2 000 3 424 VC-3 34 6.5 105 8 000 6 120 VC-4 140 2.1 105 8 000 18 792 STM-1 1
31、55 2.3 105 1.3 105 2.2 1048 000 192 000 9 940 801 (1) = 1 indicates a Poisson distribution of errors. (2)The blocks/s are defined in ITU-T Recommendations ITU-T G.826 and G.828 for SDH path, in ITU-T Recommendation G.829 for SDH sections. Some STM-1 equipment might be designed with 8 000 blocks/s (1
32、9 940 bits/block), but ITU-T Recommendation G.829 defines the block rate and size to be 192 000 blocks/s and 801 bits/block, respectively. Rec. ITU-R F.1605 5 3.1.1 Prediction of SESR due to multipath propagation effects The SESR parameter is given by equation (1) where the second term is the PDF of
33、 BER. In the case of multipath propagation, the BER is strictly connected to the outage probability Pt, evaluated by the methods in Annex 1 of Recommendation ITU-R P.530. Using appropriate methods for the transmission system considered, the outage probability Ptversus BER can be derived (see Fig. 1
34、for an example). The function gBERis derived from the applicable figure, similar to Fig. 1 i.e. gBER= dPt /dBER. 1605-011051041061051041032525 25 25PtFIGURE 1An example of outage probability, Pt , versus BER,due to multipath propagationBERIn general, the values of outage probability are obtained for
35、 only a few values of BER where equipment signature and threshold are known. If an extended set of BER values are available, a more precise curve can be obtained. The SESR evaluation is made in two steps, first to determine the amount of SES due to EB, and the second to obtain the usually negligible
36、 part corresponding to LoP. The curve of SES due to EB can be approximated by a step function. The BER value, where the SES probability changes from 0 to 1 is denoted BERSES. =SESSESSESBERBERBERBERxffor1for0)( (2) The value of BERSESnormalized to the mean number of errors per bursts ( = 1 for Poisso
37、n distribution) is given in Table 1. 6 Rec. ITU-R F.1605 Considering equation (2), in the case of SES due to EB, equation (1) becomes: )(d)()(SEStBERSESBERBERBERPxxgxfSESRBA=(3) Upper limit for SES due to LoP To evaluate the upper limit, SESRLoPu, it is assumed that the probability of SES for BER va
38、lues less than BERSESis a constant. Then, )(1()(SEStSESLoPuBERPBERBERSESPSESR = (4) NOTE 1 The contribution from LoP has been found to be small, and can therefore be ignored. 3.1.1.1 Step-by-step procedure for calculation of SESR due to multipath Step 1: Calculate the outage probability PtSESfor BER
39、 = BERSESfor the appropriate system, using the outage prediction methods described in Annex 1 of Recommendation ITU-R P.530. )(SESttSESBERPPSESR = (5) where BERSESis given in Table 1. 3.1.2 Prediction of SESR due to rain attenuation Rain may introduce severe attenuation. Most of the time X % when ra
40、in attenuation exceeds the threshold ASES, an unavailability condition will occur. The remaining time 100 X % = Y % is considered as available time giving rise to SES. The division between available and unavailable time for any climatic region has to be found from experimental measurements. The proc
41、ess of obtaining the SESR due to rain implies finding the attenuation margin, ASES, of the radio link for a BER, BERSES, where all seconds are SES. Then it is possible to evaluate the percentage of time in the worst month that the rain attenuation exceeds this margin, and finally to evaluate the cor
42、responding annual percentage of time. The SESR value due to rain will be Y % of this probability. 3.1.2.1 Step-by-step procedure for calculation of SESR due to rain attenuation In the following, a step-by-step procedure for evaluating SESR due to rain is given. The input parameters are: frequency, h
43、op length, polarization, rain zone, transmission power, gain of transmitting antenna, gain of receiving antenna, relationship between received power and BER, SDH path type, and error burst length. Step 1: Calculate the rain attenuation exceeded for 0.01% of time, A0.01, using the method in Annex 1 o
44、f Recommendation ITU-R P.530. Step 2: Calculate the nominal received power without rain attenuation, PRXnominal. Step 3: Using the relationship between the received power and BER (usually obtained from equipment manufacturers) obtain the value of ASES, where ASESis the attenuation margin of the radi
45、o link for BER = BERSES(see Table 1). Rec. ITU-R F.1605 7 Step 4: Calculate the annual time percentage, paSES, that the rain attenuation is larger than ASES, using the method in Annex 1 of Recommendation ITU-R P.530. Step 5: Translate the annual time percentage, paSES, to worst-month percentage, pwS
46、ES, using the method in Recommendation ITU-R P.841. Step 6: Calculate SESR from: wSESPYSESR (%)= (6) where PwSESis the worst-month probability (PwSES= pwSES / 100). NOTE 1 The value of X is under study; for the time being Y = 0% is suggested, that is, rain attenuation results in an unavailability of
47、 PwSES in the worst month. 3.2 Prediction of BBER The BBER is evaluated for multipath and rain conditions from: xxgxfBBERBERBBERBERBERBAd)()(= (7) where: )(xfBBER: relationship between BBE and BER )(xgBER: PDF of BER BERAand BERB : integration limits (typical values are 1 1012and 1 103). 3.2.1 Predi
48、ction of BBER due to multipath Two alternative methods can be used: a complete one based on the analytical solution of the integral in equation (7), where gBER(x) is derived from the outage prediction model (the applicable figure, similar to Fig. 1) and fBBER(x) is the theoretical relationship for B
49、BE due to BER, and a simplified method based on approximation of fBBER(x) and of gBER(x). 3.2.1.1 Step-by-step procedure for calculation of BBER due to multipath In this section, a step-by-step procedure based on the simplified prediction model is given. Step 1: Calculate the outage probability, PtR, for the residual BER (RBER) (typically in the range from 1 1010and 1 1013 for the bit rates of 2 to 155 Mbit/s, respectively), using the outage prediction methods described in Annex 1 of Recommendation I
