978-1-4244-4522-6/09/$25.00 ©2009 IEEE ISCIT 20091280

Common Wireless CommunicationServices Recognition for GSM, UMTS and LTE

via Synchronization Channels DetectionHendra Setiawan

Computer Science and ElectronicsKyushu Institute of TechnologyFukuoka 820-8502, Japan

Email: hendra@dsp.cse.kyutech.ac.jp

Hiroshi OchiComputer Science and ElectronicsKyushu Institute of TechnologyFukuoka 820-8502, Japan

Email: ochi@dsp.cse.kyutech.ac.jp

Abstract—This paper discusses about design of common wire-less communication services recognition for three different stan-dards, i.e. Global System for Mobile Communications (GSM),Universal Mobile Telecommunications System (UMTS) or Wide-band Code division Access (WCDMA), and Long Term Evolution(LTE), also known well as 2G, 3G, and 3.9G respectively. Torecognize them using a common system, we propose performingwithin synchronization channel detection since the similarity incell search procedures i.e. always detect synchronization signals.To minimize resources, we prefer using autocorrelation to detectGSM and LTE synchronization signals and match filter forUMTS synchronization signal. However, it is not enough todistinguish each others. Therefore, we also propose to recognizethe peak period of the autocorrelation output as a main con-cept to distinguish the standards. We design this system usingsymplify DSP tool and simulate in simulink. The simulations areperformed within AWGN channel and show good performanceeven with 10 dB SNR. Moreover, FPGA implementation fortarget device Virtex 4 XC4VLX200, the proposed design canbe implemented within 24171 look up tables (LUTs).

I. INTRODUCTION

Based on Global mobile Suppliers Association [1] untilearly 2009, there were more than 3.54 billion GSM andWCDMA subscribers in this world. However, the wirelessworld is rapidly moving towards the next generation of thesystems. In this respect, conventional management technol-ogy should be replaced by advanced schemes that considermultidimensional characteristics, increased complexity, andhigh speeds. The era of beyond third generation wirelesscommunications is highly heterogeneous in that it comprisesseveral radio access technologies that need to cooperate witheach other, such as long term evolution (LTE) worldwide in-teroperability for microwave access (WiMAX), mobile secondgeneration (2G)/ third generation (3G), wireless local areanetworks (WLAN), etc. [2].Some investigations to anticipate future compound com-

munication systems have been done such as [2], [3], and[4]. However, the discussions were only in management,application, and policy levels, regardless physical layer imple-mentation. Therefore, we want to design a portion of com-pound communication systems in physical implementation,

i.e. common service recognition systems. We involve threedifferent wireless communication generations i.e. GSM (2G),UMTS (3G) and LTE (3.9G) as the targets of investigation.Commonly initialization procedures, after the mobile station

turned on, consist of three principle steps. First, frequencyscanning is performed in radio frequency (RF) block toidentifies the one with strongest signal strength within RFspectrum. Second, cell search procedure is performed to getframe synchronization and some cell information. Finally,using that information mobile station must perform registrationto base station as well as authentication.We assume the first step already done. However, we still can

not recognize any services. Therefore the second step becomean important key to distinguish one service to the others. Wewill discuss it deeply in the next sections.This paper will be organized as follow. In section II cell

search procedures for GSM, UMTS and LTE are introducedthen followed by synchronization channel detection in sectionIII. The proposed design for common service recognition isserved in section IV. Section V discuss about fixed pointsimulation, and finally conclusions are drawn in section VI.

II. CELL SEARCH PROCEDURES

A. GSM Cell Search Procedure

The cell search procedures in GSM system is shown inFig.1. There are 3 processes, first is search for the frequencycorrelation channel (FCCH), then search the synchronizationchannel (SCH). In synchronization channel there is informa-tion about frame number to identify the position of broadcastcontrol channel (BCCH). Finally, from BCCH, mobile stationcan extract all information needed to the next processes.FCCH and SCH appear every 10 TDMA frames within

51 TDMA frames. SCH always lag one frame after FCCHthen followed by BCCH. See [5] for further details regardingmapping of radio packets onto physical channels.

B. UMTS Cell Search Procedure

According to [6] and [7], cell search procedures in UMTSare performed in three steps as shown in Fig.2. The first step

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Fig. 1. GSM cell search procedures.

Fig. 2. UMTS cell search procedures.

is slot synchronisation. During the first step the mobile stationuses the primary Synchronization Channel (SCH) to acquireslot synchronisation to a cell. The primary synchronisationcode is common to all cells. The second step is frame syn-chronisation and code-group identification. During this step,the mobile station uses the secondary SCH to find framesynchronisation and identify the code group of the cell. Thelast step is scrambling-code identification. During the last step,the mobile station determines the exact primary scramblingcode used by the found cell. After the primary scrambling codehas been identified, the Common Control Physical Channel(CCPCH) can be detected, and the cell Broadcast Channel(BCH) information can be read.

C. LTE Cell Search Procedure

In LTE, cell search procedure consists of four steps asshown in Fig.3. First, mobile station must detect primarysynchronization channel (P-SCH) that always exists each 5 ms.The results of this process are 5 ms cyclic timing reference[8]. Second is detect secondary synchronization channel (S-SCH) to get slot numbering and physical layer cell identitygroup. To get full cell identity, mobile station must detectdownlink reference signals and extract Physical BroadcastChannel (PBCH).

D. Common Service Recognition Concept

Based on previous discussion, we can integrate all cellsearch steps for all standards in Table I. Now we roughly seethe intersection of the cell search processes. GSM, UMTS andLTE always detect frame synchronization channel and broad-cast channel in their cell search procedures. Since broadcast

Fig. 3. LTE cell search procedures.

TABLE ICELL SEARCH PROCESSES COMPARISON

Processes GSM UMTS LTE

Detect Frequency Correction Channel Yes No No

Detect Synchronization Channel Yes Yes Yes

Detect Downlink Reference Signal No No Yes

Extract Broadcast Channel Yes Yes Yes

Fig. 4. Synchronization burst structures in GSM

extraction performed after synchronization channel detection,synchronization channel become easier to detect comparedwith broadcast channel. Therefore, we propose synchroniza-tion channel detection to represent GSM, UMTS and LTEservices recognition.Moreover, synchronization signals have characteristics such

as they are transmitted using highest power and periodicallytransmitted using a constant pattern within an active cell.These properties assure us that synchronization signals can beused as an identity of cell existence. Synchronization signalformat and detection for each standard will explain in the nextsection.

III. SYNCHRONIZATION SIGNALS DETECTIONA. Synchronization Channel in GSM

GSM access scheme is Time Division Access (TDMA) witheight basic physical channels per carrier. The carrier separationis 200 kHz. A physical channel is therefore defined as asequence of TDMA frames [9].According to [9], one TDMA frame consists of 8 time slot

with interval 15/26 ms. Its physical content is called a burst.Five different types of bursts exist in the system, normal burst(NB), frequency correction burst (FB), synchronization burst(SB), access burst (AB) and higher symbol rate burst (HB). Wefocus one frequency correction burst (FB) and synchronizationburst (SB) since they are the most important bursts in cellsearch process.Fig.4 shows synchronization burst structure. Synchroniza-

tion burst (SB) contains a long training sequence (64 bits)

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Fig. 5. Training sequences in SB for normal format

Fig. 6. Block diagram of synchronization signal detection

and carries the information of the TDMA frame number (FN)and base station identity code (BSIC) (78 bits). The repetitionof FBs is also named frequency correction channel (FCCH).According to [5], there are three types of training sequences,

i.e. normal, cordless telephony system (CTS), and Compact.Fig. 5 shows an example of training sequences after differ-ential encoding process for normal format. These sequenceshave symmetrical patterns that can be easily detected usingcorrelation function, slide the received GSM sample data andcorrelate it with the next received data, then find the correlationpeak, as shown in Fig. 6.

B. Synchronization Channel in UMTS

UMTS system physical channels are arranged into 10 msradio frames which consist of 15 slots of 0.67 ms each. Eachslot corresponding to 2560 chips or chip rate 3.84 Mcps.Synchronization channel (SCH) is one of common downlinkphysical channels. The SCH consists of two sub channels, thePrimary and Secondary SCH. The 10 ms radio frames of thePrimary and Secondary SCH are divided into 15 slots. Fig. 7illustrates the structure of the SCH radio frame.The Primary SCH consists of a modulated code of length

256 chips, the Primary Synchronisation Code (PSC) denotedcp, transmitted once every slot. PSC is generated by repeatinga constant sequence modulated by a Golay complementarysequence, and creating a complex-valued sequence with iden-

Fig. 7. UMTS synchronization channel(SCH) structure.

Fig. 8. PSC sequence in UMTS

tical real and imaginary components [10]. The Secondary SCHconsists of repeatedly transmitting a length 15 sequences ofmodulated codes of length 256 chips, the Secondary Syn-chronisation Codes (SSC), transmitted in parallel with theprimary SCH. The SSC is denoted ci,k

s , where i=0,1,c,63 is thenumber of the scrambling code group, and k=0,1,c,14 is theslot number. This sequence on the Secondary SCH indicateswhich of the code groups the cell’s downlink scramblingcode belongs to. The primary synchronisation code (PSC) isconstructed to be orthogonal to both the primary and secondarySCH codes, detailed in [10] and [11].Actually PSC detection, regardless SSC detection, is enough

to recognize UMTS service availability, and it will savemany computation resources. Since PSC is constructed byrepeating a constant sequence, see Fig.8, and fixed to allcells, it can be detected using correlation functions. Hence, thesimplest architecture to recognize UMTS is same with GSMrecognition system in Fig.6.

C. Synchronization Channel in LTE

The multiple access schemes for the LTE physical layeris based on Orthogonal Frequency Division Multiple Access(OFDMA) with a cyclic prefix (CP) in the downlink, andon Single-Carrier Frequency Division Multiple Access (SC-FDMA) with a cyclic prefix in the uplink. To support transmis-sion in paired and unpaired spectrum, two duplex modes aresupported: Frequency Division Duplex (FDD), supporting fullduplex and half duplex operation, and Time Division Duplex(TDD), see [12] and [13] for further details.Similar with UMTS, LTE also has primary and secondary

synchronization channel, P-SCH and S-SCH respectively. P-SCH is used for 5 ms timing synchronization and representphysical-layer identity. There are three physical-layer identitiesrelated to three difference primary synchronization sequences.

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Fig. 9. A primary synchronization sequence in LTE

Fig. 10. Block diagram of P-SCH detection in LTE

The sequences are generated from a frequency-domain Zadoff-Chu sequences according to [13]. Fortunately, they havesymmetrical patterns, one of them is shown in Fig.9. Hence,we can use correlation function to detect it, regardless anyphysical-layer identity. Since P-SCH detection is enough torecognize LTE service availability, we do not discuss S-SCHanymore.P-SCH detection in LTE is more difficult compare with

GSM and UMTS because of frequency domain term. More-over, the spectrum flexibility in LTE makes P-SCH detectionseem more difficult. However, P-SCH is transmitted only inthe central part of the overall transmission band of the cell, i.e.the constant bandwidth of 1.25 MHz, regardless of the overalltransmission bandwidth of the cell [8]. Therefore, we onlyfocus on some sub carriers around the central of bandwidthand detected by a low pass finite impulse response (FIR) filtereliminating the sub carrier other than P-SCH. Since the P-SCH is symmetric in frequency domain, in time domain alsosymmetric and easily to detect using correlator, Fig.10.

IV. PROPOSED DESIGN

A. Common Clock Requirement

Since GSM, UMTS and LTE use different data rate and wewant to combine them in one system, common clock systembecome an important issue. A GSM burs period is 15/16 ms,

Fig. 11. Common service recognition system proposed design

Fig. 12. FIR response for LTE’s P-SCH

consist of 156.25 bits, therefore it need 1.083 MHz clock. AUMTS slot period is 2/3 ms consist of 2560 chips, thus at least3.84 MHz clock is required. In LTE, there are 15360 symbolswithin 0.5 ms, need 30.72 MHz clock rate. To combine allsystem, we propose to use highest clock, i.e. 30.72 MHz. Itmean eight times higher than UMTS and more than 28 timesof GSM clock.

B. Common Architecture

A common recognition system for GSM, UMTS and LTEmust support synchronization detection for all of them. Itmust include design in Fig.6 and Fig.10, consist of a FIR,a correlator, a peak detector and a decision maker. Unfortu-nately, the structures of synchronization signals are differentfor GSM, UMTS, and LTE. GSM and LTE have symmetricsynchronization signal, while UMTS has repetition pattern.Hence, we need two different correlations. Since real systembecomes an important issue, we can not share peak detectorfor both correlation. However, decision maker need peak inputfrom both of them. Thus, the proposed design is shown inFig.11, consist of a FIR, a down sampling, two correlations,a peak detector and a decision maker.

C. FIR Filter

FIR is used in LTE synchronization signal detection. SinceP-SCH in LTE is mapped in 62 sub-carrier around DC subcarrier, which sub carrier spacing is 15 kHz, the maximumfrequency passed by FIR filter (fpass) is 930 kHz. Fivesub carriers in both side of P-SCH are zero, that can beused as (fstop), i.e. 1.08 MHz. Using Filter Design andAnalysis (FDA) Tool available in simplify DSP, for the givenparameters; sampling rate 30.72 MHz, (fpass) = 0.93 MHz,and (fstop) = 1.08 MHz, can be achieved by FIR order 695.Since FIR order 695 need big area and long latency, we change(fstop) = 1.86 MHz or twice of (fpass). As a result, it can be

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Fig. 13. Autocorrelation implementation for GSM and LTE detection

Fig. 14. Match filter structure for UMTS detection

achieved for FIR order 113. Since (fpass) > GSM frequency,all GSM signal can easily pass through this filter.When LTE’s OFDM symbol where P-SCH exist passed

through FIR, the filter will give same response, shown inFig.12, regardless the number of IFFT point. It is a symmetricsignal with low frequency band, that can be detect easily usingautocorrelation after perform down sampling 1/16.

D. Correlator

Correlation can be performed using match filter or auto-correlation. Since GSM’s and LTE’s synchronization signalshave symmetrical pattern, we propose autocorrelation ratherthan match filter due to ignoring the patterns. Suppose r(k) isreceived signal at sample k autocorrelation function A withinduration N can be expressed by,

A =(N/2)−1∑

k=0

r(k) · r∗(N − k) (1)

Fig.13 shown autocorrelation implementation for N = 128.For UMTS recognition, we propose match filter due to better

performance compare with autocorrelation. The match filtercan be described as,

MF =N∑

k=0

p(k) · r(k − N) (2)

where p(k) is the PSC pattern at k-th tap, and N = 256 denoteUMTS’s synchronization signal chip length. The match filterstructure is shown in Fig.14, which multipliers are imple-mented using multiplexer and second complement converter,Fig.15.The output of autocorrelation functions are always fed to a

peak detector. Since our aim is synchronization signal detec-tion only, we just implement peak detector using comparator.There is a threshold used to determine the peak.

Fig. 15. Multiplier implementation for match filter

Fig. 16. Fixed point modeling

E. Decision maker

Proposed autocorrelation can detect any symmetrical signalseven it is not synchronization signal. To solve this problem,we suggest using peak period detection. In GSM, the intervalbetween synchronization signals is 10×8×15/26 ms = 46.15ms, and 5 ms for LTE. Thus, in 1 second there are 21, and200 peaks for GSM, and LTE respectively. Therefore, we puta timer in the proposed design to calculate number of peak inone time period.Since UMTS synchronization signal detect by match filter,

a peak is enough to recognize UMTS service without peakperiod calculation. However, a method might be required torecognize number of UMTS cell in one area due to they usesame frequency band. The UMTS’s P-SCH period is 10/15 msor 15 peaks will appear in 10 ms. If 45 peaks are detected in10 ms, it means 3 cells exist around us.

V. FIXED POINT SIMULATION

The proposed design in fixed point modelling is shown inFig. 16. The simulations performed in AWGN channel areshown in Fig. 17, Fig. 18 and Fig. 19, denote GSM, UMTSand LTE synchronization signals detection respectively, takenat the output of autocorrelation and match filter.As a result, GSM and UMTS synchronization signals can

be detected in AWGN channel case even for SNR 10 dB.However, LTE synchronization signal detection need betterchannel condition. I might be caused by imperfection of FIRand down sample offset before autocorrelation. However, atSNR 20dB, it can be detected easily.From FPGA implementation point of view, proposed design

does not require much resource. It is expressed from compilerreport for implementation in virtex 4 xc4vlx200, that needaround 24171 LUT as shown in Fig.20.

VI. CONCLUSION

The reconfigurable radio system has been issued as a newconcept toward the era of beyond third generation wirelesscommunications. Regarding to this issue, we have developeda common cell search procedures for GSM, UMTS andLTE based on synchronization channel detection. We proposeautocorrelation rather than match filter for GSM and LTE

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Fig. 17. Autocorrelation output for GSM service detection

Fig. 18. Match filter output for UMTS service detection

Fig. 19. Autocorrelation output for LTE service detection

Fig. 20. FPGA implementation resource usage

detection in same time due to they have symmetrical pattern insynchronization signal. To distinguish them, we suggest usingpeak period detection that can be easily implementing usinga timer. Since UMTS does not have symmetrical pattern, wepropose match filter to distinguish it from other standards.In this paper we also have done fixed point simulations in

AWGN channel. As a result, GSM and UMTS synchronizationsignals can be detected easily even in SNR 10 dB. However,LTE synchronization signal detection need better channelcondition.As additional, for FPGA implementation, proposed design

requires around 24171 LUT in virtex 4 xc4vlx200.

ACKNOWLEDGMENT

This work was supported by NEC Corporation Japan.

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