Practical Approach to Optimize Paging Success Rate in CDMA Network

Practical approach to optimize Paging Success Rate in CDMA Network

Dr.Jey Veerasamy, John Jubin, and Sanjay Kodali

Wireless System Laboratory Samsung Telecommunications America

1301 E. Lookout Dr. Richardson, Texas 75082 USA {j.veerasamy, j.jubin, s.kodali}@samsung.com

Abstract - Paging success rate is one of the important performance metrics used by wireless service providers. It measures the page response acceptance rate when the MSC/VLR believes that the mobile is in the coverage area and pages the mobile to find the terminating mobile party to complete the incoming call. Obviously, the page message has to reach the mobile, and then the page response message has to reach MSC before the originating party gives up. Note that this paper is not about paging algorithms, rather about achieving the best page response rate for any paging algorithm. We present all the important factors & the parameters that impact the paging success in this paper, based on our experience with commercial operation of Samsung CDMA BSS. We believe that the most details are applicable to other wireless networks too.

1 BACKGROUND

Locating the mobiles to terminate the incoming calls successfully & quickly is one of the challenges of wireless network. Obvious complexity for the wireless network comes from the air-interface and the mobility. Within the wireless network, termination processing is more complex than origination processing since the mobile has to be located first & the page response should be received from the mobile as quickly as possible. Lot of work has been done in the area of designing “effective” paging algorithms. References [2] [3] [4] and [5] are a few recent publications. In this paper, we focus on the practical considerations to get the best paging success rate, independent of any particular paging algorithm.

Mobile users expect all incoming calls to be reliably delivered to them as long as the mobiles show “in-coverage”. Also, callers would prefer the calls to be either answered quickly or rerouted to voicemail quickly. Currently, it is possible for the paging process to take as long as 30 seconds and to see that the incoming call being rerouted to voicemail upon page timeout after 35 seconds or so. This is significantly longer duration for reaching voicemail compared to the landline network, in which it is typically 15 seconds or so. This paper primarily focuses on improving call delivery reliability, which is also called as paging success rate by service providers. Our secondary goal is to reduce the page response delay, to speed up call

setup for terminations and to reroute the calls to voicemail quickly.

2 DEFINITION

Most common definition of Paging Success rate is the percentage of valid page responses received by the system divided by the # of times paging process was initiated by MSC to complete the incoming termination attempts.

Why should the Paging Success rate be maximized? 1. Service provider wants to reliably deliver the

incoming calls to the mobile subscribers who are in service at that moment.

2. Service provider wants to connect the incoming call to the terminating party or voicemail as quickly as possible. Page timeout occurs when the paging process fails to bring in a valid page response. Lengthier the paging process is, more time the originator has to wait for page timeout to get rerouted to voicemail.

3. Service provider uses the paging success rate to measure the effectiveness of the paging process.

We present various factors and parameters involved in improving paging success rate in CDMA networks.

3 TIMER VALUES

Mobile can be in any one of the following three areas at any particular instant: in coverage, out of coverage, or marginal coverage. We will cover the marginal coverage case bit later. Ideally, every time a termination request comes in, MSC should know whether the mobile is in coverage at that moment. If the mobile is known to be out of coverage, MSC will not initiate the paging process. Unfortunately, since the mobile cannot send a message to the system after it has gone out of coverage or has run out of power, MSC’s knowledge is not guaranteed to be accurate. So, the MSC approximates this decision by following a simple rule: If it has not received any message from a mobile for “reasonable” period of time, it assumes that the mobile is not reachable. Such duration or timer is called as Mobile inactivity timer, and it is typically configurable in MSC.

IEEE Communications Society / WCNC 2005 1353 0-7803-8966-2/05/$20.00 © 2005 IEEE

So, the mobiles are required to keep informing the system periodically that they are active & reachable. Another timer called periodic registration timer is used for this purpose. This timer value can be configured in BSS and it is conveyed to mobiles as part of broadcast control messages over the air. Since the periodic registration is used for detecting mobile’s activity, periodic registration timer value has to be smaller than the value of mobile inactivity timer.

Too large value for mobile inactivity timer will make the MSC predict that a mobile is in-service even though it has gone outside the service area. MSC will unnecessarily page such mobiles for long duration and this will result in lower paging success rate. Too small value for mobile inactivity timer will force the BSS to set small periodic registration timer value too. Mobiles will re-register very often, and use up too much system resources including access channel, paging channel, BSC-BTS backhaul, and BSC-MSC A-interface, and all the processors along the way. If this results in congestion in those elements, paging success and system performance can be seriously impacted. Mobiles also will experience bit more battery drain in idle mode, since it has to send more messages.

Practically, these timer values have to be tuned based on each network’s coverage characteristics. If insignificant % of mobiles leave coverage area and the coverage holes (including in-building issues) are minimal, then these timer values can be as large as several hours, and the paging success will not be impacted significantly. However, to avoid the unnecessary waiting time for originating parties, it makes sense to limit the mobile inactivity timer to 2 to 4 hours or so. If a significant % of mobiles do leave the coverage area, then the mobile inactivity timer can be set to 1 hour or so. Related question is, how small the periodic registration timer should be, compared to the mobile inactivity timer? It can be set a few minutes smaller to avoid race conditions. So, if the periodic registration timer is set to 30 minutes, it would make sense to set the mobile inactive timer to 40 minutes.

4 BALANCING LINK BUDGETS

Link budget is a term used to indicate the maximum distance at which “reasonable” quality signal can be received and the calls can be made and received with reasonable quality. It is good to keep the forward (BTS to mobile direction) and reverse links (mobile to BTS direction) to be reasonably balanced to ensure good performance in the wireless network. Let us consider the extremes: - Forward link reaches lot more distance than the

reverse link, i.e. an area exists that is covered by the forward link, but not by the reverse link. In that area, the mobiles can get the pilot signal and decode the

paging channel messages, but the base station cannot decode the mobile’s messages. Users in that area will believe that they are in coverage, but they will not receive any calls – which can be a frustrating experience. Either the base station transmits power for pilot and paging channels should be reduced or reverse link sensitivity of base station should be enhanced to balance the links.

- The reverse link reaches more distance than the forward link, i.e. an area exists that is covered by the reverse link, but not by the forward link. In that area, occasionally the mobiles may manage to read the system configuration messages from the paging channel and send registration messages to indicate to MSC that they are active in the network. However, since the forward link quality is poor, the mobiles will have trouble in decoding the most paging channel messages. Since the mobiles cannot decode the paging channel messages consistently, they will keep going in and out of coverage. Additionally, since the CDMA standard requires that the system configuration parameters should be current in the mobile (i.e. the mobile should have read the configuration parameters within the past 10 minutes) before transmitting any message in the access channel, it is unlikely that the mobile will be able to send the registration message. So, this imbalance case may not impact the paging success rate compared to the previous case. However, the sensitivity of the access channel can be tuned down to balance the links so that the registrations will not get into the system and impact the paging success rate.

While it is possible to recognize & correct major link

budget differences, it is impossible to achieve the perfect balancing of both links, since the link budgets for each subcell vary throughout the day depending on the traffic load. Additionally, the paging channel in each subcell is required to broadcast the overhead control messages periodically every 1.28 sec once. So, the mobiles will manage to read the control parameters at some point in the link-imbalanced areas. Similarly, the mobile retries the access probes in increasing power levels until the acknowledgement is received from the base station, when sending access channel message. So, as long as the mobiles access the system from the link-imbalanced areas (i.e. areas with marginal coverage), paging success will be impacted. Service providers should ensure that a negligible % of users are present in such link-imbalanced areas to avoid poor performance and the impact to paging success.


5 PAGING CHANNEL OPTIMIZATION

There are several other items related to the paging channel that have to be optimized to ensure that the paging messages received from MSC will go out in the paging channel almost 100% of the time.

5.1 PAGING CHANNEL SCHEDULING

Scheduling messages into the paging channel is quite complex due to the varying requirements that are associated with each type of messages. Inefficient scheduling algorithms can result in delay & loss of messages during scheduling, even at very low paging channel occupancy levels. There are three types of messages that try to get through paging channel (each paging slot contains the messages in this order too): 1. Slotted mode messages: General page messages

(GPM), feature notification messages (FNM), and databurst messages (DBM) are the common slotted mode messages. These messages are very restrictive w.r.t. scheduling, since these messages are directed at the “idle mode” mobiles that are operating in slotted mode. Most commercial CDMA networks use slot cycle index of 2, which allows the mobiles to wake up once in every 5.12 seconds to save the battery. In other words, a slotted mode message directed to a particular mobile has to go in one particular slot within the span of 64 paging slots, otherwise it has to wait for 5.12 seconds more to get another chance.

2. Overhead configuration messages: CDMA air standards mandate that all overhead channel messages have to go out in the paging channel every 1.28 seconds once. This requirement alone takes away ~25% of the paging channel space with the paging channel operating at full rate (i.e. 9600 bps) complicates the paging channel scheduling algorithm.

3. Non-slotted messages: Channel assignment messages (CAM/ECAM), base station acknowledgement order (BS Ack), and registration accept order are a few non-slotted messages. Since these messages can go out in

any paging slot, they are quite flexible w.r.t. scheduling. However, it is important to ensure that these messages are not delayed more than 0.5 second or so, to avoid noticeable delay in call setup.

Goal for paging channel scheduling algorithm is simple: send as many pages as possible, but without violating overhead message transmission requirement, and without starving non-slotted messages. One improvement is to use the best-fit algorithm for sending overhead configuration messages in each slot, instead of trying to accommodate the overhead messages in same order in every cycle. Filed data shows that this reduces the probability of violating the overhead message transmission requirement, while everything else remains the same.

5.2 GPM MESSAGE SIZE

There are two parameters associated with each paging channel message that affects the probability of its successful receipt by the mobile: the starting position and the message size. When the message starts in a new 10msec half-slot (in other words, whether the Synchronization Capsule Indicator (SCI bit) bit is set before the message), then the mobiles can synchronize and try to read the message & error in reading previous 10msec slots will not affect decoding of this message. Each paging channel message has message type, length, pay-load, and CRC. Since there is SCI bit for every 10ms half-frame boundary, paging messages can be started at the boundary to ensure that they will be decoded properly. Fig. 1 shows the packed structure with and without synchronization. However, paging channel capacity will be impacted due to the fragmentation. Also, we can limit # of page records in each GPM to bring down the message error rate. In other words, we can send multiple GPMs with smaller # of page records, instead of one big GPM. Of course, this also reduces the paging capacity due to the fragmentation and the header & CRC overhead for each GPM. Table 1 shows the message error rate for short and long GPMs.

Table 1 Probability for successful decoding of GPM # of page records in GPM: 1 2 3 4 5 6 7 8 9 10 # of bytes in corresponding GPM: 19 29 39 49 59 69 79 89 99 109 Message success rate (BER 0.01%) 98.1% 97.3% 96.6% 95.8% 95.0% 94.3% 93.5% 92.8% 92.0% 91.3% Message error rate (BER 0.01%) 1.9% 2.7% 3.4% 4.2% 5.0% 5.7% 6.5% 7.2% 8.0% 8.7% Message success rate (BER 0.02%) 96.3% 94.7% 93.2% 91.8% 90.3% 88.9% 87.5% 86.1% 84.7% 83.4% Message error rate (BER 0.03%) 3.7% 5.3% 6.8% 8.2% 9.7% 11.1% 12.5% 13.9% 15.3% 16.6% Message success rate (BER 0.05%) 90.9% 87.4% 83.9% 80.6% 77.5% 74.4% 71.5% 68.7% 66.0% 63.4% Message error rate (BER 0.05%) 9.1% 12.6% 16.1% 19.4% 22.5% 25.6% 28.5% 31.3% 34.0% 36.6%


Fig. 1. Packing messages into paging channel

5.3 PAGE RETRANSMISSIONS FROM BTS

Most commercial CDMA networks use the slot cycle index of 2, and this enables the mobiles to wake up just for 1 paging slot (i.e. 80 msec duration) in each cycle of 5.12 seconds. So, to improve the paging success rate, BTS can retransmit the page message in the hashed slot in the next slot cycle. However, this will double the impact of the paging messages on the paging channel occupancy. Unless this mechanism is carefully implemented, non-slotted messages may be starved or delayed too much. One safe mechanism will be to retransmit the message only if there is free space in the paging slot.

Note that the majority of the mobiles do receive the first page message, and respond with the page response right away. So, the page retransmission is not necessary in those cases. Unfortunately, since we are considering the retransmission from each site, valid page response receipt by one BTS has to be informed to all other sites immediately to avoid unnecessary re-transmission of that page. Software complexity of propagating that information to all sites and pulling the message from the paging channel queue can be quite high. Also, there are enough doubts whether “repaging after 5.12 seconds” is really worth the effort since RF conditions may not change much within that duration.

Implementing these enhancements will increase the paging channel occupancy, which may necessitate additional paging channel. Additional paging channel will take away a portion of site’s transmit power & reduce the

available power for the traffic channels. So, it makes sense to consider other mechanisms like zone based paging, before considering additional paging channel. Additional paging channel will require additional channel element too.

6 ACCESS CHANNEL OCCUPANCY

Optimizing the access channel slot size will maximize the access channel efficiency. Duration of each access channel slot is determined by the following configuration parameters: PAM_SZ (# of preamble frames) and MAX_CAP_SZ (# of max. message related frames). Each frame spans 20 msec. If the access channel sensitivity is improved, # of frames for the preamble can be reduced. Similarly, MAX_CAP_SZ should be set to the lowest value that accommodates the biggest access channel message.

Unlike additional paging channel, additional access channel does not have the negative impact on RF resources of the site. It simply brings down the collision probability in the access channels. It does require additional channel element though. Keeping the access channel collision at reasonable level (~25%) will ensure that all page responses do get into BSS within a reasonable time, as shown in Fig. 2. Also, less collision means that mobiles will spend less time in access attempts and users’ perceived “call setup time” might be reduced.

…

10ms half-frames start with SCI bit to indicate whether the message is aligned to SCI.

1 0 0 0 0 0 0 0

…1 0 1 0 1 0 0 1 0

When paging channel messages are NOT aligned with 10ms half-frames

When paging channel messages are aligned with 10ms half-frames

…

10ms half-frames start with SCI bit to indicate whether the message is aligned to SCI.

1 0 0 0 0 0 0 0

…1 0 1 0 1 0 0 1 0

When paging channel messages are NOT aligned with 10ms half-frames

When paging channel messages are aligned with 10ms half-frames


0.00%

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5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% 80% 85% 90% 95%

Access channel busy%

Prob

abili

tyProbability for exactly ONE transmission

Probability for collison

Probability for BS to detect ONE transmission

Probability for BS to successfully decode accesschannel message (Access Channel Occupancy)

Fig.2. Access Channel Occupancy

7 BSC-BTS BACKHAUL LINK CAPACITY

With the introduction of 3G packet data, managing traffic in BSC-BTS backhaul links has become complicated. There are at least 4 types of data that flow through the links: packet data frames, voice frames, call processing related control messages, and non-call processing related messages including alarms & statistics. Samsung’s approach is to guarantee % of T1 bandwidth for each data type. When one type of data exceeds its allocated %, it will be carried through T1 as long as other types of data together do not occupy their guaranteed %. Page message is considered as part of non-call processing, and appropriate % of T1 should be guaranteed to ensure that the page messages are not discarded during congestion in T1.

While these percentages can be determined using traffic modeling, following approach can be used to ensure that they are working properly under live traffic: Discard statistics for T1 can be collected for each site for each type of traffic periodically, and allocation percentages can be tuned based on that statistics. Real-time discard statistics can also be utilized to do adaptive resource allocation for voice and packet data calls.

8 BSC-MSC SIGNALING LINK CAPACITY

Managing BSC-MSC signaling link occupancy is relatively straightforward, since it carries only the signaling messages. It is easier to compute the occupancy, and add additional signaling links, if necessary. Also, typically minimum 2 signaling links are equipped even for a low-traffic BSC, for redundancy purposes. If signaling link occupancy reaches close to 100%, A-interface messages including page messages will be discarded. It will result in both lower paging success rate and lower call success rate. While the page messages add significant load to signaling links on forward direction (MSC BSC), the location update messages contribute to the loading in reverse direction.

9 PAGE RETRY FROM MSC

Due to the issues related to BTS-initiated page retransmission, it makes sense to retry page from MSC. MSC can intelligently initiate retry only if a valid page response was not received. However, there are a few practical challenges: Since slot cycle index 2 is used by most service providers, mobiles wake up only once in 5.12 seconds, receive the page message, and send the page response. Also, getting the page response through access channel via access attempt can take as long as 8 seconds in


the worst case. So, MSC has to settle for a longer page retry timer.

Since the mobiles are distributed throughput 5.12 seconds to receive the page message, assuming 80% probability for the access probe to reach the base station and 1 access probe per second from the mobile, following can be derived (see Table 2): 98% of the page responses are received by MSC within 7 seconds after sending the page message to BSS. MSC-BSS interface data collected in the commercial network confirms that this computation is reasonably accurate. So, the retry page can be sent 7 seconds after the first page. Note that MSCs are typically smart enough to accept the page responses to the first page, even after sending the retry page. Giving additional 3 seconds for the retry page and the internal system delays, the page timeout can be declared after 7 + 7 + 3 = 17 seconds into the paging process. Current CDMA operators seem to be using values of 20 to 35 seconds to declare the page timeout, in turn, make the originating parties wait for a long time before rerouting to voicemails. Note that, the higher the paging success rate is, lower the importance of timer value associated with the page timeout, since smaller % of terminations will end up with the page timeout.

10 PAGING SUCCESS FOR PACKET DATA CALLS

Field data shows that the paging success rate for the packet data calls is much better compared to the voice calls. Since a typical packet data session switches between active mode and dormant mode every few seconds once, mobiles tend to be in-coverage when network initiated page requests to re-activate the packet data call. In other words, this is equivalent to paging the mobiles within 1 to 2 minutes after they registered – which translates into higher probability that mobiles are still in coverage and have not moved far away.

11 “PUSH TO TALK” CALLS

Insufficient coverage area and coverage holes cause “service unavailability” to users, and cause longer call wait times to the originating parties. Recently, there has been a lot of interest in bringing ‘push to talk’ to all wireless networks. One reason cited for the popularity of ‘push to talk’ calls is the long call setup delay for traditional calls.

Field data shows that current paging process takes 5 seconds on average – which means the termination call setup will take min. 5 seconds on average. BTW, this does not include the time that the terminating party takes to answer the call after hearing the ring.

Currently, most mobiles use slot cycle index of 2, i.e. they wake up once per every 5.12 seconds to check whether there is an incoming call or message. This makes the paging process slow and inflexible. Reducing the slot cycle index to 0 (i.e. mobiles will wake up every second once) or even negative slot cycle index will bring down the delay to reach the mobile. Refer to IS2000 Release D standard [1] for the details. Lower slot cycle index has not been used till now due to the battery drainage concerns. Reduced slot cycle index should make M-M call setup time almost same as M-L call setup time, effectively bringing down the “user-perceived” call setup time.

12 CONCLUSION

We presented several important parameters and issues that impact the paging success rate in a CDMA wireless network. While all of these parameters have an impact the paging success rate, optimization in commercial Samsung BSS suggests that, unless there are serious bottleneck issues within the network elements, coverage plays the major role in lowering the paging success rate.

REFERENCES

[1] IS-2000 CDMA air-interface standards, http://www.3gpp2.org/Public_html/specs/index.cfm [2] B. Krishnamachari, R.H. Gau, S.B. Wicker, and Z.J. Haas, “Optimal Sequential Paging in Cellular Wireless Networks”, Wireless Networks, Vol. 10, 2004, pp. 121–131. [3] A. Pal and D.S. Khati, “Dynamic Location Management with Variable Size Location Areas”, Inter. Conf. on Computer Net. and Mobile Computing, 2001, pp. 73-78. [4] J. Li, Y. Pan, and X. Jia, “Analysis of Dynamic Location Management for PCS Networks”, IEEE Trans. on Vehicular Tech., Vol. 51, No. 5, Sep. 2002, pp. 1109-1119. [5] H.I. Liu and C.C. Yeh, “Time-Varying Population based Location Management Schemes”, 8th Inter. Conf. on Comm. Systems, Nov. 2002, pp. 839-845.

Table 2 Page Response% distribution over time Time (sec) 1.024 2.048 3.072 4.096 5.12 6.144 7.168 8.192 9.216

Paging Completed 20% 40% 60% 80% 100% PgRsp% (computed) 16.0% 35.2% 55.0% 75.0% 95.0% 98.9% 99.7% 99.8%

PgRsp% (field data #1) 16.7% 37.2% 53.2% 71.2% 90.4% 98.7% 99.4% 100.0%


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Practical Approach to Optimize Paging Success Rate in CDMA Network