Soft handoff on the quick paging channel

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The authors are with 3G Standards Development Group, QUALCOMM Incorporated, San Diego, CA 92121, USA. A6stmct - The Quick Paging Channel has been.
Future Wireless Cornmunicotion System

SOFT HANDOFF ON THE QUICK PAGING CHANNEL Sandip Sarkar, Brian Butler and Edward Tiedemann The authors are with 3G Standards Development Group, QUALCOMM Incorporated, San Diego, CA 92121, USA. A6stmct - The Quick Paging Channel has been proposed as a method to significantly improve t h e standby times of t h e phones for the 3 6 systems. This paper looks at a way to p u t this common channel in

soft handoff, thereby improving the reliability, and leading to a better standby time for the cdma2000 phones. Both the physical layer, and network implications are analyzed in detail. Indez Terms- Quick Paging Channel, cdma2000, soft handoff, standby time.

11. QPCH OPERATION The QPCH contains single bit messages to direct slottedmode MS’sto monitor their assigned slot on the Paging Channel or CCCH, whichever is being used. The details of its operation can be found in [3]. In particular, the bit may be repeated twice for reliability in a predefined time slot. These bits are uniformly spaced out in a 80ms period shortly before the associated 80 ms slot of the assigned Paging Channel or CCCH slot. After repitition, the QPCH data rate is 9600 or 4800 bps. As these slots are shared by a number of MS’s, the mobile may find that an 1 has been transmitted in its slot, although it was not paged. This can be exactly quantified [3]. However, note that using Little’s formula (N= AT), if we assume 30 Erlangs of voice traffic per sector, and an average

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A. MODULATION SELECTION The QPCH bit detection is pilot aided. Hence coherent OOK (On-off Keying) and coherent BPSK (Binary Phase Shift Keying) are considered as possible modulation schemes. Define oa -t) erfc(z) = e dt. In AWGN, for coherent maximum likelihood (ML)detection, the symbol error rate is given by: Pr(e) = ;erfc(.”: t ‘z ) , where NO is the noise spectral den-

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I. INTRODUCTION The IS-95Paging Channel is used to communicate from the base station (BS) to the mobile station (MS)when the MS is not assigned to a dedicated channel, i.e. the Traffic Channel. The Paging Channel carries overhead messages, pages, acknowledgements to messages sent by the MS on the Access Channel, and channel assignments. The details of the Paging Channel operation can be found in ([I], [2],[3]). The phone standby time largely depends on the amount of time spent in decoding the paging channel messages, hence it is desirable to wake up for the minimum possible time. On the other hand, missed pages lead to missed calls, so it is extremely important to send these messages reliably. The present paper addresses these issues, and proposes a scheme to improve the standby time of the new 3G phones. The organization of cdma2000 common channels will be a structure consisting of three forward link physical channels: the QPCH (Quick Paging Channel), the BCCH (Broadcast Channel), and the CCCH (Common Control Channel). The QPCH carries indications of pages directed to the MS. The BS transmits on the QPCH whenever the BS needs to contact a MS operating in slotted mode. The BCCH carries overhead information and broadcast short measages. The CCCH carries pages and responses to MS’s. The CCCH is time slotted and the MS monitors its assigned time slot on the CCCH for pages. The MS also monitors the CCCH when waiting for a response from the BS.

‘This research wa8 supported by Qudcornm Inc. {ssarkar,bbutler,etiedQquaIcomm.com)

call duration (T) of 1209, we get X = f call setups per second per sector. Assuming 30% of them to be mobile terminated and 2 pages per call setup, we have 0.15 pages per second, or 1 call every 6 - 7 seconds on an average, excluding data and SMS type service. Thus, the collision rate does not have a major effect on the standby time of cdma2000 MS units even for large paging zones of 5OO.sectors.

Ernail:

sity. If E denotes the average energy per bit, for BPSK: d,i, = 2&. Let p denote the probability of a 1 being trans-

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mitted. Then, for OOK, dmin = ‘. Thus, OOK conserves average energy when p < 0.25. As an example, for 8 pages per 80ms (rather high) at 9600 bps, p = & = 0.021. Fig. I compares the performance of OOK vs BPSK in AWGN. Coherent AWGN BER 1.E41

1.E-02

f

E

1.E-03

l.E-04

1.E-05

-2

0

2

4

6

8

10

Awmge E b M O In dB

Fig. 1: Performance in AWGN

Thus OOK saves about 5% power over BPSK. Further, the peak to average ratio was plotted (Fig. 2) for a load of the BS supporting 25 voice calls (with QPSK modulation) for 8 pages/80ms (rather high). It plots the inverse cumulative distribution function against the peak to average ratio in dB for the instanteneous envelope going into the power amplifier. There is hardly any noticable difference, and hence OOK is preferred.

111. DETECTION OF THE QPCH BIT The QPCH bit is decoded coherently. The pilot is used

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Future Wireless Communication System to the estimated pilot energy. So, let T = [ p a . Note that 0 5 4 5 1. F o r t > 1, Pm2 $. Next, let?! , be a random variable with a Rayleigh probability density function @(A)). Then, p2 has a chi-square pdf The miss with two degrees of freedom. Define X = probability can now be found as: ..................................... I.::::..::'.::.; ............................... :...........................................

Prn(7)

=

Lrn

Pm(X)p(X)dX.

Substitution of r = [p&? makes the integral a function of and the SNR of the QPCH bit 7 = Let E[7] = I'. Evaluating the integral,

e.

The correspondingfalse alarm probability is found as: Fig. 2: P e e to Average Ratio with QPCH in OOK ,

.

to provide the phase reference for doing so. Multiple paths may be combined using the maximal ratio combining as described in [4]. To obtain a reliable estimate, the decision is tri-valued (Erasure, 1, or 0). If the pilot level falls below a certain threshold, the bit is declared an erasure, else the bit is detected in the regular way. This threshold is tuned to keep the miss probability within acceptable limits. The performance in AWGN is the same as outlined in section A Next, the channel is considered in a fading scenario.

Thus, the design reduces to a proper choice of the calculation of the false alarm probability.

B. PERFORMANCE CURVES The system design here is similar to a Neyman-Pearson detection problem in the sense that we fix P, and try to minimize Pf.For the present system, we target P, = 5 x The simulation results closely matches the theoretical predictions.

A. RAYLEIGH FADING CHANNELS

QPCH Performance in Rayleigh Fading Channel 1

For each bit, the received signal can be written as: r(t) = P(t)e-j+%(t)

-

>.

+ z(t),o 5 t 5 T

Here, the transmitted signal is u ( t ) , ~ ( t is) the complex valued Gaussian noise corrupting the signal, and P ( t ) is the time varying channel gain. Let us assume that the signal fades slowly enough to estimate the phase $(t) perfectly. For a fixed t, the channel is fixed, and the error statistics can be calculated as a function of PI the value of P ( t ) at time t. Then, we can average over the pdfof P ( t ) to get the final error statistics. Let n N ( O , F )be a zero mean Gaussian random variable and E be the signal energy. The detection with variance problem can be set up as:

E, followed by

0.1

8

f

0.m

o.mt

9,

Fig. 3: Rayleigh Fading Channel

HI: r = m+n

Ho:r=n

For a threshold T , declare HO if r < r , H I otherwise. Let Pm(P,T ) and P f ( p ,T ) denote the miss and false alarm probabilities respectively. In terms of the complementary error function (erfc()), this is written as (see [4]):

The channel is estimated using the pilot transmitted from the BS. It is assumed that the channel fading is slow enough such that the QPCH bit signal to noise ratio (SNR) is proportional

In the figures, the numbers in parentheses refers to the value of 4. One can reverse the roles of the Pf and P, curves to obtain the curves for thresholds between 0.5 and 1. The curves for Pj and P, overlap for 4 = 0.5. The cost of a false alarm for the phone is to lose some battery life by monitoring the paging channel. However, a miss could mean a lost call. Since the pilot is used to determine the fading condition, the bit is declared an erasure when the pilot is in a deep fade, and the phone monitors the CCCH. T y g ically, the pilot is sent at 3dB higer power than the QPCH. We seek a threshold such that the miss probabilities will still meet our design criteria in a deep fade. This is governed by the receiver noise floor. When the received pilot SNR is within the required confidence interval, the bit is called an erasure.

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IV. SOFT HANDOFFON QPCH A MS at a distance of r from a BS suffers a signairattenuation proportional to a ( r , x) = rmlOX/lO.Here x is the shadowing effect, which is often modelled as a lognormal process with standard deviation 8dB. Experimental data shows that a choice of m = 4 models the realistic scenario pretty well [5]. Due to the susceptibility of call drops at cell boundaries due to too much signal attenuation and other cell interference, it is often advantageous to do soft handoff (SHO)over hard handoff. This section analyzes such a condition in detail. In-order to motivate the discussion,we define the following: Let I,, denote the received signal strength, NO the thermal noise, and I,, to be the interference due to pther cells. The The utility geometry G of a MS is defined to be G = of this parameter stems from the fact that a higker geometry implies that the MS is well within the cell of a BS. If we assume that NOis dominated by I,,, a OdB geometry implies that the MS is at a cell boundary (in reality a slightly negative geometry due to NO).A very negative geometry usually means a deep fade, or that the MS is not listening to the optimal BS, or is going out of coverage. Assume the same conditions as in the single path case, except that we have P independently faded paths, each having statistics similar to the single path case. Let path i have a Gaussian noise ni N(O,%). Let the energy in path i be Ei and the channel gain pi. Then, the hypothesis problem is set up as follows:

P-1

pf(e)-- ( i+ ) e~ ~ ~ - 1 + k c ~ ( l + e ) k 2

k=O

For two paths the performance is plotted in Fig. 4. Performanceof QPCH In SHO

e.

-

The detector with threshold T has the following performance:

Fig. 4: Peformance of QPCH in SHO at cell boundary

6,

Note that this plots the combined and hence the 3dB combining gain is already included in the graphs. It is observed that the diversity gain due to combining is almost 1.7 dB for Pm and Pf. Thus, in a practical system, where the combining gain is not quite 3 dB, one can expect a diversity gain of about 1dB for the miss probabilities. This is confirmed by actual simulations. While this is great from a physical layer perspective, it leads to certain issues of network management. These. are addressed next.

B. UNEQUAL STRENGTH BASESTATIONS Set the threshold as a function of the sum of received pilot T =6 pi2&.Then, we have:

SNFb,i.e.

ELl

The general solution is found from calculating the pdf of the SNR for the particular configuration. As an example, consider the case when the BS receives signals from two BS's, one arriving with energy 2E, and two weaker paths with energy E each. If they are combined as described above with each path being faded identically and independently, the pdf of the total SNR can be similarly computed to obtain Fig. 5. It shows some diversity gain over the previous case.

e,

Let the received SNR per bit for path k be: r k = and the 7k.k.It is assumed that pc i E {I, 2 . . .P } total SNR 7 = are iid random variables with Ftayleigh distribution.

xi'=,

A. EQUALSTRENGTH BASESTATIONS If P equally strong BS's with energy P2E per path are received, the average S N R per channel is: 5 = &Ep2].Then, the total

S N R has pdf: p(r) = ~ p - ~ , - q p - l e - ~ , 7 2 0. Let

p= J B , a n d e =

are given by:

Jz

The error probabilities

Fig. 5: Unequal Strength Signals

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V. CONTROLLING SOFT HANDOFF In the overhead messages the BS lists the BS’s which are permitted to transmit to the MS in SHO on the CCCH and QPCH. It should be noted that the IS-95-B Access Handoff and Access Probe Handoff may be performed with som-e BS’s and soft handoff performed with other BS’s. In particu‘lar, IS95 has a flag for every member of the neighbor list for which Access Handoff and Access Probe Handoff are permitted. A flag is introduced for every member of the neighbor list in which soft handoff is permitted on the CCCH. A separate flag is introduced for every member of the QPCH. This is illustrated for five cells with three sectors each in Table 1.

-3(-s

j

j ! I

BS ACS A2 1 A3 1 B1 1 B2 1 B3 1 c11 e2 1

c3

D1 D2 D3 El E2 E3

1 1 1 1 0 0 0

CCS 1 1 1 1 1 0

o

0 0 0 0 0 0 0

QCS 1 1 1 1 1

1 1 1

0 0 0 0 0 0

Comments AcHO, CCCH/QPCH AcHO, CCCH/QPCH AcHO, CCCH/QPCH AcHO, CCCH/QPCH AcHO, CCCH/QPCH AcHO. QPCH SHO ACHO~QPCHSHO AcHO, QPCH SHO AcHO AcHO AcHO NoHO NoHO NoHO

..........i!

..........................

Tab. 1: Illustration of the SHO Flags

SHO SHO I SHO ‘ - -!-c c’c n s m m SHO SHO Fig. 6 : SHO on QpCH

In the table, ACS refers to the Access Handoff flag, CCS to the CCCH SHO allowed flag, QCS to the QPCH SHO allowed flag, and AcHO implies that Access Handoff is allowed. The MS is located in sector Al. New MS’s are permitted to perform SHO on the CCCH with all sectors in cells A and B. The MS is not permitted to perform SHO on the CCCH with sectors from other cells. It should be noted that it may be desirable to restrict SHO on the CCCH to only sectors of the same BS. In this case, SHO of the CCCH would be only to A2 and A3. This is because it is easier to synchronize and control the SHO from a cell where only single processor is involved. It also permits layer 2 to be fully run from the BTS. The example also shows that SHO on the QPCH may be done in cells A, B, and C. A wider number of cells may be used for SHO on the QPCH since paging can be more readily handled from a central controller. However, this is not required and there is total flexibility in set of cells that permit SHO on the CCCH. Note that SHO on the QPCH is not permitted for cell I).This may occur since cell D is in a different registration zone. No forms of handoff are permitted to cell E. This may be because it is controlled by a different BSC (see Fig. 6 ) . To show the operation of the CCCH, we use a call origination as shown in Fig. 7. It uses the BS’s and the various flags previously described. It is assumed that soft handoff is permitted on the CCCH between BTS’s. The MS sends the Origination Message while located in sector A1 of the corresponding BTS. Two additional BS’s have pilots strong enough to combine: B1 and C1. The flags that the MS receives in the overhead messages indicate that soft handoff is permitted with

I--.

@ -

mwsmrnm3

B1 and that Access Handoff is permitted with C1. The Origination Message is received at a BTS and forwarded to the BSC. At the BSC, the message is processed. At the BSC, an acknowledgement is generated and sent to the BTS’s for transmission to the MS on the CCCH. The BTS’s selected to be sent the message are those corresponding to pilots reported to be strong by the MS and have CCS set to 1. The BSC send the layer 2 acknowledgement to these BTSs and these BTSs send the layer 2 acknowledgement to the MS in soft handoff. In the example, the MS reports B1 and C1 to have strong pilots, and since CCS set to 1 in B1, the BSC sends the acknowledgement to BTS B and Al.

Fig. 7: Illustration of Call Origination

After setting up the channel, the BSC then sends the Channel Assignment Message (or information to determine the Channel Assignment Message) to BTS’s A, B, and C. The BSC includes BTS B since B1 was reported by the MS, and CCS was set to 1. The BSC includes BTS C since C1 was reported and ACS was set to 1. The Channel Assignment Message is transmitted in soft handoff mode from BS’s A1 and B1. As in IS-95-B, the Channel Assignment Message is also transmitted from BS C1. This transmission does not have

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Future Wireless Communication System to be in a soft handoff mode since the M% not combining the transmission with the transmissions from other BS’s. While this has been shown for an origination, the same methods work for all other exchanges which are begun by the MS.

A. QPCH OPERATION:

CALL TERMINATION

To show the operation of the F-QPCH, we use a call termination as shown in Fig. 8. It uses the BS’s and the various flags that have been previously described. As was previously discussed, soft handoff may be restricted to a single BTS. In what follows, it is assumed that soft handoff is permitted on the QPCH between BTS’s. m w m s r e t

Lcs

or* --tu.m.c,,

Fig. 8 Illustration of Call Tarmination

Bsc

will then send a page message, similar to a normal page, on the CCCH. This message contains the full address of the MS. It should be noted that full page step is not required and the MS could respond directly with the Page Response Message &er receiving the quick page. However, the false alarm rate on the quick paging channel may be sufficiently high so that it is preferable for the MS to wait for the page message before sending the Page Response Message.

VI. CONCLUSION

The design allows for seamless interoperability between all combinationsof :BSs and MS’s that do and do not support this new feature. It tdoes not significantly impact call setup time, or increase the missed page probability. However, it greatly reduces the amount of time the phone spends monitoring the slots of the paging channel. The introduction of soft handoff allows the message to be transmitted at lower power levels. This significantly reduces the interfernce to the other mobiles, and specially leads to a far better performance on the cell boundaries, where the problem of call drops are most acute. To provide a comparison, consider 8 pages per 80 ms slot. Assume 5 ms warm-up time, and a cost of 54 ms to monitor full paging slot. Using the present design at 4800 bps, the amount of improvementdepends on hardware platform. Define a to be the ratio of the current drawn when the phone is awake to that when the phone is asleep.Presently, for many commercially available phones, a value of a = 10 to 40 is common. This may go up a little in the near future. Fig. 9 summarizes the results. In the figure, the term SCI refers to Slot Cycle Index. The length of the paging channel slot cycle is 1.28 x 2’O‘ sec.

The BS sends a quick page at a particular time that is given by the IMSI of the MS and the configuration of the BS. Typically, the configuration of all BS’s in a paging region are configured the same so all BS’s would be transmitting the quick page at the same time. For those base stations that transmit the quick page at the same time, the MS can combine these BS’s in soft handoff. For a particular BS, the overhead measages would have the QCS flag set to l for a neighboring BS when the neighboring BS sends the same quick pages and at the same time. If the above example, BS A1 would set QCS to 1 for BS’s A2, A3, B1, B2, B3, Cl, C2, and C3. Base stas to i s n a an as 40 45 so 5s M I IO 18 M tion D1, D2, and D3 do not have QCS set to 1. This is since de*^^^ these base stations are not in the same paging area or are otherwise unable to transmit the quick page at the same time. Fig. 9 Standby Time Improvement When monitoring the appropriate slot on the QPCH, the MS combines the transmissions from multiple BS’sin a soft hand- Thus,the QPCH scheme causes considerableimprovementsin off mode. The MS then determines whether it has received a standby time at very little cost. The cdma2000 proposal thus quick page. It should be noted that the MS is not required includes this novel feature to aid in the phone battery life. to combine signals from other cells in the same slot that are not in soft handoff. This is because the MS would pick up the REFERENCES strongest BS when receiving the quick page. However, there 111 TR45 TIA/’EIA-95-B, Mobile Stotion-Bose Stoion Compotibility Stondord for DuoCMcde Widebond S p w d Spectrum Celluwould be some benefit if the MS knew the configuration of lor System# TIA/EIA 1995. the neighboring BS and thus could also process the quick page transmitted by a neighboring BS. However, the MS may not [2] TR45.5 Working Group 111, Dmft Tezt for cdm02000 Physical Loyer (Revision 5) TIA, 1999. have received the configuration information from the neighboring cell and thus may not know the time that the quick [3] S. Sarkar and B. Butler, Phone Standby Time and the Quick Poging Channel submitted to PIMRC’99. page is being transmitted. It should be noted that the neigh[4] John G. Proakis, Digitol Communicotions, 2nd Ed. McGrawboring BS would typically use the soft handoff mode with a Hill, Sydney, 1989. neighboring BS if the neighboring BS were sending the power control bits in the same slot. After the MS has been alerted by [5] Andrew J. Viterbi, CDMA Principles of Spnxd Spectrum Communication Addison-Wesley, Reading, MA, 1985. the quick page, the MS begins to monitor the CCCH. The BS

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