AN EFFICIENT MONITORING STRATEGY FOR INTERSYSTEM HANDOVER FROM TD-SCDMA TO GSM NETWORKS Gianluca Durastante*, Alberto Zanellao *
Siemens Mobile Communications S.p.A.-System department, Via Monfalcone 1, 20092, Cinisello Balsamo (MI), Italy {
[email protected]} o
CSITE-CNR/DEIS-University of Bologna, 40136, Bologna, Italy {
[email protected]}
Abstract - This paper deals with intersystem handover between third and second generation mobile radio systems. Here, we propose an efficient monitoring strategy for acquisition of both Frequency Correction Channels (FCCHs) and Synchronization Channels (SCHs) of a GSM system by an user equipment (UE) connected to a Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) network. The proposed strategy, which is based on a discontinuous search, exploits the properties of periodicity of TD-SCDMA and GSM radio frames. The proposed strategy proves to be very efficient in the utilization of the monitoring windows and allows to acquire a number of FCCH/SCH bursts which is comparable with the one achievable adopting a continuous (unrealistic) monitoring strategy taken as reference. Keywords – Mobile Radio Systems, Radio Management, TD-SCDMA, intersystem handover. I.
Resource
INTRODUCTION
Third Generation Mobile Radio System are required to support intersystem handover [1-3]; that is, handover between networks using different standard. In particular, in the next few years, 2G Systems, i.e. GSM, characterized by a large coverage, will coexist with emerging 3G networks which will be used, at first, for high traffic urban areas. This could cause a large number of handovers from 3G to 2G, and motivates the need for efficient intersystem handovers. The scenario investigated in this work is given by a dual mode (TD-SCDMA, GSM) UE which is connected to a TD-SCDMA system and has to get ready to carry out an intersystem handover towards the GSM one. The TD-SCDMA network here considered is a TSM (TD-SCDMA System for Mobile [4, 5]) compliant one. Given the almost equal structure of the radio frames, the proposed study is directly applicable to an UTRA TDD 1.28 Mcps radio interface too [6]. In order to prepare for the just mentioned intersystem handover, the UE has to regularly monitor on signals coming from surrounding cells. Such a monitoring implies both an evaluation of the received power level of signals coming from surrounding GSM Base Transceiver Stations (BTSs) and a periodical refresh of their identities. At the same time, analogous measurements on TD-SCDMA surrounding BTSs have to be performed too. The UE is required to decode the cell identity of 32 surrounding cells (TSM and/or GSM) as often as possible, and as a minimum at least once every 10 seconds [5]. A monitoring strategy for intersystem handover should be able to manage all these aspects by exploiting the different frame structure of the cellular systems involved. The rest of the paper is organized as follows. In Section II the frame structure of both
0-7803-7589-0/02/$17.00 ©2002 IEEE
TD-SCDMA and GSM is considered and some relations of periodicity between the radio frames are outlined. In Section III, we describe the monitoring strategies: that based on a continuous search, which is taken as reference, and the proposed one, based on a discontinuous search. Numerical results are shown in Section IV and finally, Section V deals with conclusions. II.
FRAME ANALYSIS
During the intersystem handover preparation phase, the evaluation of the received power level of signals coming from surrounding GSM BTSs is done by listening to beacon frequencies of GSM BTSs; moreover, the periodical refresh of the corresponding cell identities is performed by reading the Base Station Identity Codes (BSICs) transmitted on the SCH. Before the SCH acquisition takes place, the FCCH has to be read in order to have a correct reference of frequency. The time scheduling to be kept when doing these measurements is specified in [5]: the UE is required to maintain an average of received signal levels for all adjacent cells as indicated by the Broadcast Control Channel (BCCH) of the serving cell and to report to the network the power values received from the 6 strongest cells of TD-SCDMA and/or GSM type once every 240 ms. To evaluate the GSM monitoring capabilities of an UE being in TD-SCDMA connected mode we have to look at frame structures on the radio interface. The TD-SCDMA frame [4] is subdivided into 7 normal time slots of 675 µs duration each and 3 special time slots DwPTS (Downlink Pilot), GP1 (Guard Period) and UpPTS (Uplink Pilot) lasting for 75, 75 and 125 µs respectively. The total duration of these 3 slots is 275 µs. On the whole, one frame lasts for TSCDMA= 5 ms. In TD-SCDMA connected mode some of the above mentioned 7 normal time slots are used for uplink (UL) and downlink (DL) speech/data transfer. If the UE is assumed to be equipped with one single synthesizer, the intersystem monitoring can be performed only when the UE neither transmits nor receives its user data on the TD-SCDMA frequency. An UE being in TD-SCDMA connected mode uses not utilized time intervals to perform cell identities refresh as well as measurements of received power levels of surrounding GSM BTSs. The GSM control multiframe consists of 51 GSM frames [7] and the FCCH and SCH bursts necessary for the synchronization are located in the slot 0 (first slot in the GSM frame) of frames 0, 10, 20, 30, 40 and 1, 11, 21, 31, 41 respectively. One GSM frame lasts for TGSM=60/13 ms. On the whole, one control multiframe lasts for 51 GSM frames, that is 51*60/13=3060/13 ms.
PIMRC 2002
A given time slot within the TD-SCDMA frame is referred to as “idle” with respect to a given measurement if and only if the corresponding time interval can be used to perform that measurement. On the contrary, a not available time slot is referred to as “busy slot”. Obviously these idle windows repeat every TD-SCDMA frame period i.e. every 5 ms. Periods of subsequent idle slots make up a so called “idle window” or “idle period”. The two special time slots GP1 and UpPTS are regarded as a single time slot (indicated as TSx1) being generally available for intersystem measurements, whereas the time interval corresponding to the special time slot DwPTS (TSx2) is either used to perform intersystem monitoring or not depending on the considered monitoring strategy. In the following the term “continuous search” indicates that a well defined pool of idle windows is devoted to the FCCH/SCH search in all the TDSCDMA frames; all the times a continuous search won’t be performed, the search will be indicated as “discontinuous search”. Similarly, a “search period” N1 TD-SCDMA frames long means that a well defined pool of idle windows belonging to N1 subsequent TD-SCDMA frames is devoted to the FCCH/SCH search. A “no-search” period N2 TD-SCDMA frames long means that all the idle windows within N2 subsequent TD-SCDMA frames are not available for the FCCH/SCH search. If a specific monitoring strategy makes provision for a continuous FCCH/SCH search, the time interval corresponding to DwPTS and TS0 should be excluded from the available idle periods; on the contrary, if a discontinuous acquisition is considered, even DwPTS and TS0 can be included among the available idle periods as they will be used to perform TD-SCDMA intrasystem measurements during no-search intervals. Moreover, apart from the considered strategy, the time slot TS0 has not to be used for DL data transfer to allow the recovery of TD-SCDMA (synchronized) BTSs identities [5]. The set of possible traffic busy slots is then given by {TS1, …, TS6}. If a single synthesizer is assumed at the UE side, twice as much the synthesizer switching time (Tsw) has to be subtracted from the idle periods to obtain the useful length of time intervals we can use for the FCCH/SCH search on the GSM beacon frequencies. Power levels measurements on signals coming from surrounding asynchronous GSM BTSs can be performed by simply listening to the corresponding beacon frequencies for a time interval long enough to allow the acquisition of a certain number of samples; the position of this time interval with respect to the GSM frame timing does not influence the outcome of the measurement as all the time slots being on the beacon frequencies are transmitted at the same power level. On the contrary, to get the identity of adjacent asynchronous GSM BTSs the UE has to search for FCCH/SCH bursts which are placed in well defined positions within the GSM control multiframe. Therefore, to evaluate the UE monitoring capabilities, a detailed analysis of the relative positions taken up by the looked for information (FCCH/SCH bursts) with respect to the monitoring frame timing is required. On this subject, let us consider the first FCCH burst within the GSM control multiframe number N. With respect to the TDSCDMA frame timing, this FCCH burst takes a position
depending on the time misalignment existing between the two systems (parameter “Offset” of Fig. 1): without any loss of generality, we can label this position with the number “0” (see Fig. 1). 5 ms
TD-SCDMA Frame
Offset
IDLE WINDOW 577 µs=7.5/13 ms= FCCH burst length
FCCH positions
0 1
5/13 ms
5/13 ms
2 3
5/13 ms
Li
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4
…… … 5/13 ms
FCCH FCCH FCCH position 7 position 10 position 0
FCCH FCCH position 6 position 9
GSM Control Multiframe N
12
GSM Control Multiframe (N+1)
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3060/13 ms
Fig. 1. Positions taken by FCCHs belonging to two subsequent GSM control multiframes with respect to the TD-SCDMA frame timing. To evaluate relative positions assumed by subsequent FCCH bursts, time intervals of 10 and 11 GSM frames have to be expressed in terms of TD-SCDMA frames as follows: 10TGSM=9TSCDMA +(3/13)TSCDMA
(1)
11TGSM=10TSCDMA +(2/13)TSCDMA.
(2)
From (1) and (2) it results: (10TGSM) mod (TSCDMA)=(3/13)TSCDMA
(3)
(11TGSM) mod (TSCDMA)=(2/13)TSCDMA.
(4)
From (3), it comes that the second FCCH burst of the GSM control multiframe N takes, with respect to the TD-SCDMA frame, a position being (3/13)*5 ms forward compared to the one taken by the first FCCH burst of the same control multiframe. The position taken by this second FCCH burst is labelled as “position 3” in Fig. 1. In the same way, (3) allows to calculate the positions of the last three FCCHs of the multiframe N and generally of the last four FCCHs of next control multiframe, whereas (4) allows to calculate the positions of the first FCCH of the control multiframe (N+1) and generally, of the first FCCH of next control multiframes. Fig. 1 shows positions taken by FCCHs belonging to 2 subsequent GSM control multiframes. Using (3) and (4), it is possible to calculate all possible FCCH positions with respect to the TDSCDMA frame timing; it results that once the time misalignment between monitoring and monitored systems has been fixed, the FCCH burst can take only 13 different positions with respect to the TD-SCDMA frame timing and then with respect to the position of idle periods. As far as the SCH is concerned, we know it follows exactly one frame after the FCCH; being
(TGSM) mod (TSCDMA)=(12/13)TSCDMA
(6)
As a consequence, in the case of SCH the set of possible positions is the same as for the FCCH. For this reason, only the FCCH search is considered in the following.
1-st FCCH (Frame 0) 2-nd FCCH (Frame 10) 3-rd FCCH (Frame 20) 4-th FCCH (Frame 30) 5-th FCCH (Frame 40)
GSM Control Multiframe
By applying (3) and (4) to FCCHs of 28 GSM subsequent multiframes we can get values reported in Table 1; this table shows the order by which the 13 positions of Fig. 1 are assumed by the FCCHs.
N 0 3 6 9 12 N+1 1 4 7 10 0 N+2 2 5 8 11 1 N+3 3 6 9 12 2 N+4 4 7 10 0 3 N+5 5 8 11 1 4 N+6 6 9 12 2 5 N+7 7 10 0 3 6 N+8 8 11 1 4 7 N+9 9 12 2 5 8 N+10 10 0 3 6 9 N+11 11 1 4 7 10 N+12 12 2 5 8 11 N+13 0 3 6 9 12 N+14 1 4 7 10 0 N+15 2 5 8 11 1 N+16 3 6 9 12 2 N+17 4 7 10 0 3 N+18 5 8 11 1 4 N+19 6 9 12 2 5 N+20 7 10 0 3 6 N+21 8 11 1 4 7 N+22 9 12 2 5 8 N+23 10 0 3 6 9 N+24 11 1 4 7 10 N+25 12 2 5 8 11 N+26 0 3 6 9 12 N+27 1 4 7 10 0 Table 1. Positions taken by subsequent FCCH bursts with respect to the TD-SCDMA frame; the positions have been labelled using the same naming convention used in Fig. 1. Now, let us consider an idle period having length Li; as depicted in Fig. 1, a FCCH burst lasts for TFCCH=7.5/13 ms ≅0.577ms and two subsequent (relative) positions are 5/13 ms far away among each other. The considered idle period allows to acquire the FCCH if and only if it comprises at least one of these 13 positions. This condition will be satisfied depending on the actual length and position (i.e. intersystems misalignment) of the idle period as follows: first, the idle period length has to be greater than a value Tmin,1 such to include a full FCCH burst; as we have to perform an interfrequency measurement, twice as much Tsw has to be subtracted from the total idle window length; then it results: Li ≥Tmin,1= 2Tsw + TFCCH = 2Tsw+ (7.5/13) ms.
(7)
Condition (7) does not guarantee the FCCH acquisition be achieved whatever the intersystems misalignment is. Fig. 1
Tmin,2= 2Tsw + TFCCH +5/13 ms = 2Tsw + 12.5/13 ms
(8)
where 5/13 ms represents the minimum relative shift within the idle period of two FCCH positions. As a consequence, an idle window smaller than (12.5/13)=0.961 ms does not allow to satisfy the just mentioned condition. Equations (7) and (8) show that, in case Li ≥ Tmin,2, the FCCH acquisition is guaranteed apart from the value of the time misalignment. Differently, in case Tmin,1 ≤ Li < Tmin,2, the FCCH acquisition is achieved if and only if the misalignment (parameter ‘Offset’ of Fig. 1) is such that the idle period comprises at least one of the 13 different FCCH positions. Table 2 shows the number and the lenght of idle windows in a two (traffic) busy slot case, which will be taken as the reference scenario. All the 15 possible positions of idle slots are considered and particularly, time slots TS0 and TSx2 (DwPTS) are supposed to be unavailable to perform FCCH acquisition. Denoting with Lmax the length of the largest idle window, the monitoring is possible corresponding to values of Tsw satisfying Lmax ≥ Tmin,2. Therefore an increase of Lmax leads to a corresponding increase of the maximum allowed Tsw and allows to use low cost terminals. It is easy to show that, in the scenario depicted in Table 2, we get a maximum allowed Tsw≅0.531 ms, whereas it goes down to about 0.194 ms in a 3 (traffic) busy slots case (not shown in Table 2).
L2
it results:
shows that the FCCH detection is guaranteed apart from the actual value of the time misalignment if and only if the idle period length is greater than a value Tmin,2, given by:
TS0 TSX2 TSX1 TS1 TS2 TS3 TS4 TS5 TS6 # IDLE L1
(5)
CASE
TGSM=(12/13)TSCDMA,
1 1 1 1 1 1 1 1 0 0 1 3.65 2 1 1 1 1 1 1 0 1 0 2 2.97 0.67 3 1 1 1 1 1 0 1 1 0 2 2.3 1.35 4 1 1 1 1 0 1 1 1 0 2 1.62 2.02 5 1 1 1 0 1 1 1 1 0 2 0.95 2.7 6 1 1 1 1 1 1 0 0 1 1 3.65 7 1 1 1 1 1 0 1 0 1 2 0.67 2.97 8 1 1 1 1 0 1 1 0 1 2 1.35 2.3 9 1 1 1 0 1 1 1 0 1 2 2.02 1.62 1 1 1 1 1 1 0 0 1 1 1 3.65 1 1 1 1 1 0 1 0 1 1 2 0.67 2.97 1 1 1 1 0 1 1 0 1 1 2 1.35 2.3 1 1 1 1 1 0 0 1 1 1 1 3.65 1 1 1 1 0 1 0 1 1 1 2 0.67 2.97 1 1 1 1 0 0 1 1 1 1 1 3.65 Table 2. Number and length (Li, i=1,2) of available idle windows corresponding to the 15 different positions of 2 (traffic) busy slots: the value 1 (0) marks an available (unavailable) slot to perform FCCH acquisition. III. MONITORING STRATEGIES The results of the previous section allow us to draw some preliminary considerations: first of all, the whole sequence of FCCH bursts positions shows, as expected, a 13 multiframes (i.e. 3060 ms) periodical structure. So, if there is at least one idle window comprising one of the 13 possible FCCH positions and at the same time a continuous search is performed, the acquisition it is guaranteed in 3060 ms. However, the sequence of values reported in Table 1 shows other peculiarities. First of
all, 14 subsequent FCCHs take all the 13 different positions and only one of the 13 positions is taken twice. This implies that a continuous search guarantees the acquisition within a time interval corresponding to 14 subsequent FCCHs from the beginning of the search. The duration of this interval can take two different values depending on the position of the first of these FCCHs within the GSM control multiframe: either 142 (655.38 ms) or 143 (660 ms) GSM frames. So, the maximum synchronization time achievable through a continuous search goes down to 660 ms; in other words, a continuous search lasting for 660 ms guarantees the FCCH acquisition. III.A.
STRATEGY 1: CONTINUOUS SEARCH
An ideal monitoring strategy should allow to probe the looked for information such that the synchronization time can be optimized. A continuous monitoring strategy well approaches such an ideal situation; in fact, as shown in Table 1, a continuous search lasting for 660 ms, i.e. the time duration corresponding to 14 subsequent FCCHs, allows to get all the 13 different FCCH relative positions. In the continuous monitoring strategy considered here, all idle windows available to perform intersystem measurements are devoted to the FCCH search, and no resources are reserved to intersystem power measurements. As a consequence, intersystem power measurements are not possible. Moreover, the DwPTS field is assumed to be included in the available idle periods; so, even intrasystem measurements on surrounding synchronized TD-SCDMA BTSs cannot be correctly performed given the continuous nature of the search. Although this monitoring strategy is not feasible, it provides a top reference case whose performances concerning GSM cell identities acquisition should be as far as possible approached. III.B.
STRATEGY 2: DISCONTINUOUS SEARCH
The monitoring strategy proposed here (strategy 2) is characterized by a discontinuous search of GSM cell identities; in particular all idle windows available to perform intersystem measurements are devoted to the GSM cells identity acquisition during search intervals and to intersystem power measurements during no-search intervals. Since we are considering a discontinuous search, time slots TS0 and TSx2 (DwPTS) can be included into the available idle periods; as a consequence, the time interval corresponding to TS0 and TSx2 will be used to perform GSM cells identity acquisition during search intervals whereas it will allow both intersystem power measurements and intrasystem measurements on surrounding synchronized BTSs during no-search intervals. The idea of the proposed strategy is that of managing a discontinuous search of the wanted data signal (FCCH or SCH bursts) such that it looks like a continuous search in terms of efficiency for the signal detection, but still leaves some idle periods to perform alternative measurements which may be required by the serving system without the need for the monitoring system to interrupt the active connection to the detriment of the perceived quality. The selection procedure of search and no-search intervals concerning the monitoring strategy 2 takes its origin from the following observation: by looking in detail at the Table 1, it can be deduced that the sequence of FCCH positions has a “nearly period” equal to 14 FCCH time intervals. In practice, moving
forward by 14 positions (from left to right and from top to down in Table 1) we find sequences of values almost equal among each other. For instance, let us start from the second position of the first row; moving forward from 14 to 14 positions, the corresponding values are those highlighted in Table 1: First 14 FCCHs-interval: 3-6-9-12-1-4-7-10-0-2-5-8-11-1 Second 14 FCCHs-interval: 3-6-9-12-2-4-7-10-0-3-5-8-11-1 And so on. It can be seen how adjacent sequences of 14 values are almost the same: going from a 14 FCCHs-interval to the next one at least 11 of the 14 values stay unchanged. Obviously, this behavior disappears on a long time interval; for instance, the sequences corresponding to the first and the sixth 14 FCCHsintervals are quite different among each other. However, on a short time interval a “nearly periodical” behavior is evident. Therefore, by suspending the search for a time interval corresponding to the above mentioned “nearly period” and by resuming it immediately after, we can actually perform a discontinuous search which is quite similar to a continuous one. So, the no search intervals will last for a time interval corresponding to the reception of 14 subsequent FCCHs. If the search interval includes the reception of M subsequent FCCHs, the search algorithm can be expressed as follows: 1) “Search” lasting for a time interval which guarantees the arrival at the receiver of M subsequent FCCHs; 2) “No Search” corresponding to the arrival at the receiver of 14 subsequent FCCHs (1 “false” period); 3) “Search” lasting for a time interval which guarantees the arrival at the receiver of other M subsequent FCCHs; this new search starts after the jump of step 2; because of the “nearly” periodical structure of the FCCH positions (see Table 1), this new search is almost a continuation of the previous one as if the search would not never be interrupted. Because of the duration of the search period, the time interval being between the beginning of two subsequent searches corresponds to (14+M) FCCH-FCCH time intervals; 4) Steps indicated in 2) and 3) have to be repeated until the acquisition took place. Now, we have to express the length of these “Search” and “No Search” intervals in terms of number of GSM frames, and translate these values in terms of TD-SCDMA frames. As the frame duration of the two systems is different, these values will have to be rightly rounded. On this subject it should be also noticed that the time intervals corresponding to the reception of 4 FCCHs and (14+M) FCCHs do not take a fixed value because of the time variant distance on the time axis being between subsequent FCCHs. However, before doing the above mentioned calculations, an evaluation of the length M of the search intervals has to be carried out. We know that other measurements which do not deal with the FCCH acquisition have to be performed by the UE every 240 ms using the same idle periods. Assuming that a time interval of about 40 ms is necessary to do these additional measurements, it follows that the length of a search interval cannot be longer than about 200 ms. Let us consider a search period comprising 4 subsequent FCCHs, that is M=4, though
other choices are possible. Table 1 shows that, in order to comprise within the search intervals all the 13 different FCCH positions, up to a maximum of 6 subsequent search intervals have to be considered. Owing to the GSM control multiframe structure, it can be shown that the shortest time interval comprising at least 4 subsequent FCCHs lasts for 41 GSM frames plus 1 GSM time slot that is 41*(60/13)+(1/8)*(60/13)=189.8076 ms. This value has to be translated and rounded into an integer number of TD-SCDMA frames. The smallest value we can consider as search interval length is then equal to 190 ms, that is 38 TD-SCDMA frames. Since the search interval cannot be longer than 200 ms, other acceptable values are 195 and 200 ms. Given the time variant distance on the time axis being between subsequent FCCHs, the no search periods should have a time variant length. Moreover, the sequence of subsequent no search periods to be considered depends on the position of the first search interval in comparison with that of the GSM control multiframe. Unfortunately, when we start an FCCH acquisition, we do not know the timing of our first search interval with respect to the monitored control multiframe and then we have necessarily to use one single search-no search sequence which is the same apart from the actual position of the first search intervals with respect to the control multiframe. In presence of search intervals having among each other distances which do not exactly correspond to the distance of groups of 4 subsequent FCCHs, a continuous time slip will take place. As the FCCH acquisition will take place in 6 subsequent search intervals or it will never take place, the evaluation of the amount of the maximum time slip can be bounded to the corresponding subsequent 5 no search intervals. By taking a search period longer than that strictly necessary (the just above calculated value of 189.8076 ms) of an amount equal to the maximum time slip, the effect of the slip will be cancelled. Several strategies can be derived; it can be shown that the following management of the idle periods is an embodiment of the search algorithm corresponding to M=4: 200 ms search, 645 ms of no search, 200 ms of search, 650 of no search, 200 ms of search, 645 ms of no search, 200 ms of no search, 650 ms of no search, 200 ms of search and finally 650 ms of no search. From this instant onwards the search-no search pattern is the same. So, the search-no search pattern shows a periodical behavior having period equal to 4240 ms which corresponds to 848 TDSCDMA frames. We know that, due to the measurements scheduling, the search could be resumed at the 240-th ms; on the contrary, the reported algorithm makes provision for resuming the search at the 845-th ms. The time interval from the 240-th to the 845-th ms can then be used for the monitoring using a strategy equal to that just above considered. Beside a “first search pattern” we can consider a parallel “second search pattern”; as depicted in Fig. 2 we can then set up to 3 parallel and independent search patterns in which the UE is generally searching for signals coming from different transmitting units. The second and the third search pattern are then shifted versions of the first one by 48 and 96 TD-SCDMA frames respectively.
FIRST SEARCH PATTERN 129
40
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SECOND SEARCH PATTERN A
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THIRD SEARCH PATTERN 40
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848 TD-SCDMA Frames
OTHER MEASUREMENTS 8
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GSM cells identity measurements
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TD-SCDMA cells identity and TD-SCDMA/GSM power measurements
8
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RESULTING IDLE WINDOWS MANAGEMENT
Fig. 2. Strategy 2: Idle windows management using three logically independent search patterns. Time intervals are given in terms of number of TD-SCDMA frames (A+B=C+D=130). Immediately after a FCCH has been acquired, the search of another FCCH begins on the same search interval within the same search pattern. Note that the periodical search pattern sequence shown in Fig. 2 does not start every time a FCCH has been acquired but it continues unchanged whatever is the time instant in which the looked for FCCHs are actually captured. IV. SIMULATION RESULTS To assess the behavior of the monitoring strategies, a given number, Ns, of sequences, each one characterized by 2000 TDSCDMA frames (10 s), were considered. A random offset between TD-SCDMA and GSM frames, uniformly distributed between 0 to 3060/13 ms, was generated. The number Ns of sequences was chosen large enough to provide a sufficient statistical basis. Several performance metrics were defined: -synchronization time, tsync: it is defined as the time interval between two FCCH acquisitions (or between the starting point and the first acquisition). During the simulations, once a FCCH burst was found, a new offset was generated and a new search was carried out. Both the average and the maximum tsync were considered. -averaged number of FCCH acquisitions in 10 s, NFA: it is given by the total number of FCCH acquisitions averaged over all the Ns sequences and over the 15 possible cases indicated in Table 2. -minimum number of FCCH acquisitions in 10 s, NFm: it is given by the minimum number of FCCH acquisitions among all the Ns sequences and 15 cases. -averaged searching efficiency, ηFA: it is given by the ratio between the total number of FCCH acquisitions and time actually spent to perform the monitoring. This number is averaged over all the Ns sequences and over the 15 possible cases indicated in Table 2. We use the term “time actually spent” since, depending on Tsw, some idle windows cannot be used to perform the measurements (see L1 and L2 in Table 2); moreover, in case of discontinuous search, many TD-SCDMA frames are not used for the monitoring.
-minimum searching efficiency, ηFm, the definition is similar to that of NFm. Fig. 3 shows Ps defined as Prob{tsync
