Automatic measurement and detection of GSM interferences

Transcription

1 Auomaic measuremen and deecion of GSM inerferences Poor speech qualiy and dropped calls in GSM neworks may be caused by inerferences as a resul of high raffic load. The radio nework analyzers from Rohde & Schwarz make he deecion of such sources of error quick and efficien. A new algorihm auomaically analyzes measuremen resuls and, in addiion, unambiguously idenifies inerferers. WIRELESS TECHNOLOGIES Coverage measuremen sysems Deecion of GSM inerferences more imporan han ever GSM neworks are sill he workhorses of nework operaors, as he expansion of WCDMA neworks is lagging behind. However, he higher he raffic load in such GSM neworks primarily as a resul of voice elephony and he higher he number of carrier frequencies added per secor, he greaer he risk of problems caused by inerferences. This ranslaes ino poor speech qualiy and dropped calls. GSM inerference measuremen sysems from Rohde & Schwarz suppor users in deecing such sources of inerference: One ousanding example is he R&S TSMU radio nework analyzer, which was launched in 23. Togeher wih he R&S ROMES coverage measuremen sofware, he analyzer helps o quickly and efficienly measure GSM inerferences and o idenify poenial inerferers in he individual nework [1]. While he basic es seup and T&M mehod remain he same, he following innovaions/enhancemens have been inroduced over he pas wo years: The R&S TSMx radio nework analyzers are exremely powerful. The R&S TSMQ radio nework analyzer, for example, (see FIG below) no only suppors neworks of all sandards (WCDMA, GSM, CDMA2 ) bu can also simulaneously perform measuremens in all sandards. The R&S TSMQ radio nework analyzer [2] (see box on lef) is one of hese new producs. Compared o he R&S TSMU, i provides a higher measuremen speed in GSM neworks (1 channels/s insead of 8) and hus maximizes he chance of deecing inerferers. The new R&S ROMES4COI GSM inerference sofware opion unambiguously idenifies inerferers based on power measuremens performed in imeslos, hus eliminaing he ime-consuming esing of poenial inerference frequencies displayed on he lis of inerfering signals. Owing o an improved GUI, operaion has become even easier. New sofware opion analyzes measuremen resuls fully auomaically Previously, he GSM nework scanner wihin he R&S TSMU / R&S TSMQ plaform could only measure BCCH or C channels. I was no able o deec raffic channels (Cx) or individual imeslos. Alhough he R&S ROMES coverage measuremen sofware did provide a selecion of poenial inerferers ogeher wih curren receive levels, he displayed measuremen resuls sill had o be evaluaed by he user. Now, he new algorihm can fully auomaically analyze measuremen resuls and unambiguously idenify inerferers. For deails on he new measuremen mehod, see box on pages 13 and 14. A compleely new mehod developed by Rohde & Schwarz, which has been implemened in he R&S ROMES coverage measuremen sofware and in he firmware of he GSM nework scanners, now enables users o individually measure he power in specific imeslos while using he same hardware configuraion. If he measured signal componen (i. e. he cell causing he inerference) is srong enough, he source of error can be deeced. This is done by idenifying he raining sequence code (TSC), which, using he base saion lis, can unambiguously be assigned o a base saion color code (BCC) and hus o a cell. NEWS 199/9 11

2 FIG 4 This window defines he display of idenified raining sequences in he imeslo view. FIG 3 If he defined limis for RxLev / RxQual are reached, an error message will be oupu in he even lis. Finding inerferers As in he pas, he es mobile phone is operaed in he endless call mode while he measuremen is performed. If a defined minimum/maximum value of C/I or RxLev / RxQual is exceeded, an error message will be oupu in he even lis. For example, in FIG 3, he values for RxLev / RxQual in he Time 11:46:46,29 column violae he limis se for channel 98. The upper secion of FIG 5 shows he resuls of he measuremens ha are performed in he imeslos: he impaired channel 98 (Cx) is highlighed in blue a he beginning of he imeslos and he associaed C on channel 48 in yellow a he beginning of he imeslos. These highlighed areas represen he raining sequences of he measured imeslos. The degree of sauraion highlighed in orange corresponds o he relaive receive power in he corresponding imeslo in his channel (o provide a beer overview, he power display was normalized). The imeslo view conrol panel (FIG 4) defines how idenified raining sequences are displayed in he imeslo view. The imeslo view hus displays he raining sequences of differen base saions in differen colors. Since each cell broadcass he same color code (BCC) on all carriers, a second color in he imeslo view indicaes ha a second base saion was received. The example in FIG 5 shows imeslos ha are highlighed in yellow (BCC 4) and in ligh blue (BCC 6): This reveals ha hey originae from differen base saions. If he black verical line is posiioned on he impairing imeslo using he cursor, he loaded BTS lis of he operaor allows he received cell o be idenified unambiguously and displayed in he lower par of he window shown in FIG 5 (M- Seinhausen, BCC 6 (ligh blue), power, disance, ec.). In addiion, wo differen imeslo offses beween he channels are presen, as displayed in he d(slo) column. This indicaes wo differen inerferers (wih idenical BCC bu differen iming offse) as all carriers of a cell are synchronized. FIG 5 The new power measuremen and analysis on a imeslo basis. 12

3 The new measuremen mehod in deail The ever increasing demand for ransmission channels in GSM mobile radio neworks and limied frequency resources call for he muliple re-use of frequencies. As a consequence, ransmission channels are ofen impaired by a high number of co-channel and adjacen-channel inerferences. As he srucure of waned and unwaned signals is basically he same, i is quie difficul o deermine and discriminae beween waned and unwaned signal componens. While mobile phones canno be used for such asks, he R&S TSMx radio nework analyzers from Rohde & Schwarz are ideal for such inerference measuremens. Which signal srucures are bes suied for performing inerference analysis? Every base ransceiver saion (BTS) uses specific auxiliary signals o suppor he synchronizaion of he mobile phones wih he nework. Each cell broadcass hese auxiliary signals via a radio channel, which is usually referred o as he BCCH of he cell (or even more precisely, as he BCCH carrier or C). The auxiliary signals are referred o as a frequency correcion burs (FB) and synchronizaion burs (SB). All oher frequencies used in he cell serve as raffic channels (TCHs or Cx) o ransmi voice or daa. The FB is a frequency-shifed sinusoidal signal sen periodically on he BCCH carrier. This signal is a reference signal ha suppors he synchronizaion of he mobile phone in he frequency domain. The SB conains informaion ha is used for synchronizaion in he ime domain. Such informaion includes a characerisic bi sequence, also referred o as he exended raining sequence (ETS), frame numbers, and he base saion ideniy code (BSIC). This code offers one of he firs means of differeniaing beween differen signal sources. The ETS is sored in each GSM mobile phone and is a unique parameer in he GSM world. Is lengh and uniqueness opimally suppor he synchronizaion process of a mobile phone. An impaired ETS on he ransmission pah enables GSM equipmen o draw conclusions abou poenial losses in useful daa. Acually, recognizing such an impaired ETS makes i possible o correcly decode he BSIC and he frame number (equalizer principle). Each cell broadcass he FB and SB in a regular predefined ime paern (51 muliframe srucure, FIG 1). This paern is he key o deermining or disinguishing beween differen signal componens wihin a channel. How do signal componens from differen sources differ in he receive channel? Depending on how far various ransmiing saions are locaed from he measuremen sysem, signals using an idenical paern ha are broadcas from differen cells are received by he measuremen sysem wih differen iming offses and receive levels. In case of a fixed measuremen locaion, he saring ime of his characerisic 51 muliframe Frame srucures / imeslos in he BCCH (carrier) of a base saion TS TS TS7 TS7 1 frame ~ 4.62 ms F S B B B FB Sync burss Normal burss Normal burss (BCCH / CCCH) (raffic channels or conrol channels) F S B B B B C C C C F S C C C C C C C C F S C C C C I F S B B B B C C C C F S C (Downlink) 51 muliframe srucure 51 muliframe srucure (repeiion of paern) ~ 235 ms Channel C (BCCH carrier), all imeslos (Downlink) Channel C (BCCH carrier), only imeslo F: Frequency correcion burs S: Synchronizaion burs B: Broadcas conrol channel C: Common conrol channel I: Idle TS: Timeslo (577 μs) FIG 1 BCCH carriers ransmi he frequency correcion and synchronizaion burs as well as addiional auxiliary signals in imeslo TS. NEWS 199/9 13

4 srucure always has he same iming offse provided ha i originaes from he same source. A differen iming offse hus indicaes ha he received signal was broadcas from a differen source (FIG 2). If he measuremen posiion and hus signal delays and levels change, he sofware compensaes for such changes by processing posiion informaion, e. g. originaing from a GPS receiver. Analysis is relaively easy if he sources are BCCH carriers. Wha is differen wih inerferences caused by raffic channels? If he waned / unwaned signals are raffic channels, characerisic recogniion srucures such as FB and SB are no presen. The only way o idenify and disinguish beween signal componens in raffic channels is o evaluae he normal raining sequences (TSC) conained in every imeslo (burs). However, an analysis based on normal raining sequences requires more effor han ha based on FB and SB. To perform a correc measuremen, all TSCs mus be analyzed and considered on a imeslo basis. Due o he high measuremen speed of he R&S TSMx family of radio nework analyzers and he new R&S ROMES4COI sofware opion, his is no a problem. These sysems now enable users o measure and idenify differen signal componens in he impaired channel also on a imeslo basis. The same principle also applies o raffic channels in he uplink, hus allowing uplink imeslos o be analyzed, oo. By using he R&S ROMES coverage measuremen sofware and a base saion lis, users can quickly and efficienly idenify which unwaned GSM signal componens occur in a receive channel, how srong hey are, and which saion is broadcasing he signal. Kur Gsaenbauer Differen periodic iming offse a measuremen locaion for BCCH (carrier) from differen saions BTS 3 BTS 1 BTS F S B B B B C C C C F S C C C C C C C C F S C C C C C C C C F S C C C C C C C C F S C C C C C C C C I F S B B B B C C C C F S C C C C C C C C F S C C C C C C C C F S C C C C C C C C F S C C C C C C C C I 1 2 F S B B B B C C C C F S C C C C C C C C F 3 4 S C C C C C C C C F S C C C C C C C C F S C C C C C C C C 5 I ~ 235 ms BTS 1 BTS 1 BTS 2 BTS 2 BTS 3 BTS 3 FIG 2 The differen iming offse of he 51 muliframe srucures shows ha he received signals have a differen source. 14

5 If he black verical line is moved o anoher imeslo, he second inerferer will also be displayed (FIG 6). In his example, he C of he M-Moosfeld- cell impairs raffic channel 98 of he es mobile phone. Summary As in he pas, he laes mehod also enables users o measure and analyze inerferences. And i allows Wha if ways of looking a deailed problems by modifying corresponding rigger hresholds. The sysem records iniial measuremen daa, hus enabling users o run he algorihm a any ime wih modified limis. The new implemenaion provides imporan addiional informaion: Alhough he measuremen ime is he same, he new implemenaion considerably reduces he effor o analyze and verify he inerfering cells and also reduces roubleshooing imes. Since he ime-consuming esing of muliple frequencies displayed on he lis of inerfering signals is no longer required, he saff in he operaion and mainenance cener also have a lo less work o do. Abbreviaions BCCH Broadcas conrol channel (also C) BSIC Base ransceiver saion ideniy code (= NCC + BCC) NCC nework color code BCC BTS color code BTS Base ransceiver saion CCCH Common conrol channel C/I Carrier-o-inerference performance ETS Exended raining sequence FB Frequency correcion burs FCCH Frequency correcion channel RxLev Recepion level RxQual Recepion qualiy SB Synchronizaion burs SCH Synchronizaion channel TCH Traffic channel (also Cx) TSC Training sequence code Curren R&S ROMES users merely need o perform a simple sofware upgrade. GSM nework scanners ha are already available simply need o updae heir firmware o he laes version. Chrisian Fischer REFERENCES [1] Radio Nework Analyzer R&S TSMU: Auomaic deecion of inerferences in GSM neworks. News from Rohde & Schwarz (26) No. 19, pp 4 9 [2] R&S TSMx Radio Nework Analyzers: Radio nework analyzers for all asks and any budge. News from Rohde & Schwarz (27) No. 192, pp 5 8 FIG 6 A second inerferer: In his example, he C of he cell named M-Moosfeld- also impairs raffic channel 98 of he es mobile phone. NEWS 199/9 15