fet - handover comparison final
TRANSCRIPT
Handover Comparison between CDMA and UMTS Wireless
Communication Systems – Design Concepts and Performance
Ray Chiang, Ming Yuan Tsai, and ChingYao Huang, Wireless and Information Technology Lab
Department of Electronics Engineering National Chiao Tung University, Taiwan
1001 Ta Hsueh Rd., HsinChu, Taiwan Tel: 886-3-5712121 ext: 54175 Fax:886-3-5714361
[email protected] and [email protected] and [email protected]
Key words: CDMA, UMTS, handoff, handover, connection quality, IS-95A, IS-95B, cdma2000,
Abstract
In wireless communication systems, handoff/handover controls are critical to ensure a seamless
migration from place-to-place. Especially in cdma-type technologies, like IS-95A, IS-95B,
cdma2000, and UMTS, Starting with the pilot-strength only handoff/handover control, to the
quality-based or the relative-strength-based handoff/handover controls, different handoff/handover
design concepts are developed to further optimize the system performance. In this paper, various
handoff/handover design concepts will be discussed. Among different handoff/handover control
algorithms, we will show that UMTS with the relative-strength and multiple timers provides the best
average performance in aggregate Ec/Io, forward-link power saving, channel element (CE) saving,
and processing saving. A common simulation platform is developed to quantify the performance of
different handoff/handover control algorithms.
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1. Introduction
Handoff/handover 1 controls have been developed to provide a seamless migration from
place-to-place. Among cdma-type technologies, IS-95A (also known as cdmaOne) [1] is the first
CDMA (code division multiple access) technology for cellular systems. In IS-95A handoff control
designs, the intent is to make sure all pilots with reasonable strength will be added to the active set
without any delay. This pilot-strength-based handoff control algorithm has been proven successfully
in the field. In IS-95B and cdma2000 (the 3G systems) [2, 3], a so-called quality-based handoff
control algorithm is developed to reduce unnecessary legs (the connections) and the required
resources for each connection. The quality-based handoff control algorithm use the dynamic
thresholds calculated from the existing aggregate Ec/Io of all active-set legs. The same idea of the
dynamic triggering thresholds applies to 3G UMTS (Universal Mobile Telecommunication System)
[4]. But instead of using the aggregate Ec/Io for calculating the triggering thresholds, the best pilot is
used. Besides, in UMTS, multiple timers are used for all handoff/handover triggers.
The following paper is organized as follows: All handoff algorithms are discussed in Section 2. In
Section 3, the comparison of various design concepts is evaluated. Simulation results are provided in
Section 4. Finally, conclusions are included in Section 5.
2. CDMA-Types Handoff Algorithms
In this section, we will describe the handoff/handover control algorithms for IS-95A,
1 Handoff and Handover have the same meaning. In IS-95A and IS-95B/cdma2000, the name of the handoff is used to represent the cell-to-cell migration. In UMTS, the name of handover is used.
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IS95-B/cdma2000, and UMTS.
2.1. IS-95A Handoff Algorithm
IS-95A is the first implemented cdma-type cellular communication system. The pilot channel, used
as a reference channel, is important for the handoff process in IS-95A. Basically, all related handoff
triggers, like the add-leg and drop-leg triggers, are based on pilot Ec/Io measurements. In this
pilot-strength-based handoff control algorithm, there are four important handoff parameters, T_ADD,
T_COMP, T_DROP, and T_TDROP. The meaning of each parameter is listed below:
T_ADD (add-leg threshold): The threshold is used to decide whether to add the new pilot in the
candidate set or the active set.
T_COMP：The relative pilot strength measurement to the pilots in the active set. The threshold
is used to report the new pilot when the pilot strength is stronger than the weakest active-set
pilot by T_COMP (* 0.5) dB. It is a second notice to the base station on the new pilot.
T_DROP (drop-leg threshold)：The threshold is used it to decide whether to move a candidate
pilot and/or an active pilot into the neighbor set.
T_TDROP (drop-leg timer): The timer is set for the duration such that the pilot needs to fall
below the T_DROP before moving to the neighbor set.
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Active
Candidate
Neighbor
Remaining
Active set is not full and candidate pilot exceeds T_ADD
or Active set is full,
Candidate pilot swap
Pilot strength exceeds T_ADD
Pilot replaced by Candidate pilot
Pilot strength below T_DROP and wait for △T
Pilot is below T_DROP and wait for △T.
Set Size ExceededOr
AGE>AGE_MAX
Figure 1, Various Handoff Sets in IS-95A
In IS-95A, as shown in Figure 1, to differentiate the functions and searching priorities of different
states, four handoff sets are used to describe the handoff process:
Active set: Pilots in this set have RF connections with the mobile
Candidate set: Pilots in this set have the high priority (shorter measurement interval) in the pilot
Ec/Io measurement
Neighbor set: Pilots in this set are the potential pilots for the soft handoff connections. The
measurement interval is longer than the pilot in the active and candidate sets.
Remaining set: Pilots other than the active, candidate and neighbor sets. The measurement
interval is longest.
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2.1.1. Add-Leg Stage
In the add-leg stage, a mobile will detect the pilot strength and uses it as the handoff trigger. When
the neighbor set pilot strength is greater than T_ADD, the mobile will add the pilot to the candidate
set. There are two requirements to move the pilot from the candidate set to the active set: (1) If the
active set is not full, the new pilot will be added to the active set directly. However, if for some
reason (vendor control) that the system does not add the pilot to the active set, the mobile will
inform the system again only when the candidate pilot strength exceeds T_ADD and T_COMP (*0.5)
above the pilot in the active set then the system needs to decide again whether to add the pilot to the
active set or not. (2) If the active set is full, the system will decide whether to swap the existing pilot
in the active set to the new pilot. The swapping control is vendor proprietary.
2.1.2. Drop-Leg Stage
When either the active set pilot or the candidate set pilot is lower than T_DROP, the system will start
a timer. When the timer expires (after T_TDROP seconds), the system will drop the pilot and moves
it to the neighbor set. The timer will be reset, if the pilot strength exceeds T_DROP before the
expiration of the timer.
For the neighbor set to the remaining set transition, as shown in Figure 1, the mobile maintains an
age counter, AGE, for each pilot in the neighbor set. If a pilot moves from the active set or candidate
set to the neighbor set, its counter is initialized to zero. If the pilot moves from the remaining set to
the neighbor set, its counter is set to the maximum age value. The mobile will update the counter
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value under the direction of the serving site. When the number of the neighbor set pilots exceeds the
limit, the pilot with the largest AGE will be removed to the remaining set.
Figure 2, IS-95A Handoff Procedure
To have a better understanding in the handoff procedure, in Figure 2, the original active set is the
pilot from the cell A. Even the pilot A is lower than T_DROP once since the timer is not expired, the
pilot A remains in the active set. In other words, the timer is designed to avoid the ping-pong effect
due to a quick fading channel. When the pilot from the cell B exceeds T_ADD, because the active
set is not full, the system adds the pilot B into the active set directly. Finally, the pilot A is lower than
the T_DROP and the timer exceeds T_TDROP seconds, the system moves the pilot A to the neighbor
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set.
Figure 3, IS-95A Handoff Process with Messages
The Figure 3 explains the complete process of how and when a handoff is executed. At first, the pilot
is in the neighbor set. As shown, the pilot strength is increasing gradually. At time (1) when the pilot
strength exceeds T_ADD, the mobile sends PSMM (pilot strength measurement message) to the base
station and put the pilot into the candidate set. At time (2), the base station received PSMM and
decides to add the pilot. Therefore it sends HDM (handoff direction message) with proper code
information to the mobile to execute the handoff process. After the mobile received HDM, it sends a
response message to the base station to notice the base station that the mobile has received the
message and then adds the new traffic channel to its active set. To complete the handoff process, the
final message is HCM (handoff completion message) at time (3). When at the time (4), the pilot
strength is lower than T_DROP. The drop timer starts. At (5), when the timer is expired (given the
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pilot strength is continuously lower than T_DROP for T_TDROP seconds), the mobile sends PSMM
to the base station requesting for moving the pilot to the neighbor set. At (6), the base station
received PSMM then sends HDM back to the mobile. At (7), the mobile receives HDM and moves
the pilot to the neighbor set and stops the handoff process.
2.2. Handoff algorithm in IS-95B and cdma2000
In IS-95B and cdma2000 (the 3G), the handoff control algorithms focus on how to minimize the
required resources to support a mobile connection. To achieve that, two extra dynamic handoff
trigger thresholds are used. In IS-95B/cdma2000, there are four important trigger thresholds T1, T2,
T3, and T4 where T1 and T4 are fixed and are similar to T_ADD and T_DROP in IS-95A. T2 and T3
are dynamic thresholds calculated by a specific function defined below.
2.2.1. Add-Leg Stage
When the neighbor set pilot strength is larger than T1, the pilot will be moved to the candidate set. If
the candidate pilot strength is larger than T2 , the pilot will be added to the active set when the active
set is not full. The add-leg threshold, T2, is calculated in Equation (1):
} T , EPT/2ADD_INTERC))(Ec/Io10log(SLOPE/8)max{(SOFT_ T 1i
N
1i2
a
+×= ∑=
(1)
The new pilot Pj will be added to the active set if 10 ㏒(Pj)≧T2. In other words, the dynamic add
threshold is calculated based on the aggregate pilot i
N
1i
)(Ec/Ioa
∑=
but will be lower bounded by T1.
Here, the aggregate Ec/Io is used to represent the connection quality. The mobile will report the
PSMM on the same pilot if the pilot Pj is not added to the active set at the first report but is larger
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than the T2 and has T_COMP*0.5 greater than any pilot in the active set.
2.2.2. Drop-Leg Stage
To drop a pilot from the active or candidate sets to the neighbor set, T3 and T4 are used. The
triggering threshold T3 is calculated by Equation (2).
} T , /2_INTERCEPTD))(Ec/Io10log(SLOPE/8)max{(SOFT_ T 4i
N
1i3
a
ROP+×= ∑=
(2)
When the pilot in the active set is less than T3 for T_TDROP seconds, the pilot will be moved to the
neighbor set. The timer will be reset if the pilot is greater than the T3 before the expiration of the
timer (T_TDROP seconds). Same as T2, T3 is calculated based on the aggregate Ec/Io of all pilots in
the active set.
Figure 4, IS-95B/cdma2000 Handoff Process
The handoff process in IS-95B/cdma2000 systems is depicted in Figure 4. As shown, initially, P1 is
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in the active set and P2 is in the neighbor set. As the increase of P2 strength, at (1), P2 is larger than
T1 therefore the P2 will be added to the candidate set. At (2), when P2 is larger than dynamic
threshold T2, the mobile sends PSMM to the base station to start the handoff process. At (3), the
mobile receives the HDM from the base station and then adds P2 into the active set. Now, there are
P1 and P2 in the active set. Therefore the dynamic thresholds are calculated based on P1 and P2’s
aggregate Ec/Io. At (4), when P1 is lower than the dynamic threshold T3, the drop timer starts. At (5),
the timer expires and P1 does not exceed the T3.within the timer interval. The mobile will send
PSMM and at (6), the base station sends HDM back to the mobile and removes P1 from the active set
to the candidate set. For now, the active set has P2 only. All related dynamic thresholds are calculated
based on P2’s Ec/Io only. At (7), when P1 is lower than T4, the timer starts. At (8), when the timer
expires, the mobile moves the P1 from the candidate set to the neighbor set.
2.3. UMTS Handover Algorithms
Different from IS-95A, IS-95B, and cdma2000, in the UMTS handoff control, there are only three
handover sets to differentiate the functions and priorities for the handover process.
Active set: Pilots in this set have RF connections with the mobile
Monitored set: Pilots in this set are the potential pilots for the soft handover connections. The
measurement interval is longer than the pilot in the active sets.
Remaining set: Pilots other than the active and monitor sets. The measurement interval is
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longest.
The monitored set in the UMTS is similar to the neighbor set in the IS-95A, IS-95B, and cdma2000
systems.
In UMTS, to trigger the handover process, following parameters are used:
As_Th：The reporting range from the best pilot
As_Th_Hyst： The hysteresis to prevent the ping-pong effect
As_Rep_Hyst: The threshold used for replacement/swap
∆T：The timer to ensure the handover triggers are effective
As_Max_Size： maximum allowable number of pilots for the active set
The CPICH (common pilot channel) strength (Ec/Io) is used as the measurement to decide whether
to trigger various handover processes and will be explained as follows.
2.3.1. Add-leg Stage
A leg (the pilot) is added directly when the active set is not full (less than As_Max_Size). The trigger
is when the pilot is larger than the value of Best_Ss2-As_Th+As_Th_Hyst continuously for ∆T
seconds. After ∆T, the pilot will be added directly to the active set. The ∆T is designed to avoid the
ping-pong effect and the condition of Best_Ss-As_Th+As_Th_Hyst is to make sure that the pilot can
actually contribute the connection quality.
2.3.2. Drop-leg Stage
2 Best_Ss means the strongest pilot in the active set.
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A leg is dropped when the pilot in the active set is lower than Best_Ss-As_Th-As_Th_Hyst
continuously for ∆T seconds. After ∆T, the pilot will be moved from the active set to the monitored
set.
2.3.3. Swap-Leg Stage
When the active set is full, a new pilot will replace the existing pilot in the active set if the
Best_Cand_Ss (the strongest pilot strength in the monitored set) is larger than Worst_Old_Ss (the
weakest pilot strength in the active set) by As_Rep_Hyst and for ∆T seconds.
Figure 5, UMTS Handover Process
Figure 5 shows an example of UMTS handover procedure. In the figure, pilot 1 is in the active set
where pilot 2 and pilot 3 are in the monitored set. Suppose, there is only one vacancy in the active
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set. When pilot 2’s CPICH strength exceeds Best_Ss -As_Th+As_Th_Hyst for ∆T seconds, then the
pilot 2 will be added to the active set at 1A. After adding the pilot 2, the As_Max_Size is reached.
When pilot 3’s pilot strength exceeds the weakest active pilot (assume pilot 1 is the weakest) by
As_Rep_Hyst for ∆T seconds, then the pilot 1 is swapped by pilot 3 (at 1C). Finally, suppose the
pilot 2 is the strongest pilot, when the pilot 3 is less than pilot 2’s by As_Th+As_Th_Hyst (satisfies
Best_Ss -As_Th-As_Th_Hyst), the pilot 3 will be removed from the active set (at 1B). We can see
that the UMTS handover procedure is similar to IS-95B and cdma2000 in which both algorithms use
the pilot strength to decide the add-leg trigger and the drop-leg trigger. In UMTS, the standard
further defines the Swap-leg criterion while in other systems the swap control is vendor proprietary.
3. Handoff/Handover Comparison
Based on the discussion of various handoff/handover controls for IS-95A, IS-95B/cdma2000, and
UMTS, in this section, we will compare different algorithms qualitatively. In order to have a better
understanding of the differences, we will first compare the differences between the IS-95A and
IS-95B/cdma2000 and then the differences between the cdma2000 and UMTS.
3.1. Comparison between IS-95A and IS-95B/cdma2000
As stated, in IS-95A handoff control designs, the intent is to include all reasonable good pilots to
ensure the connection quality. But, in IS-95B and cdma2000 system, besides the connection quality,
the handoff control algorithms focus on how to reduce unnecessary legs in order to reduce the
required resources for each mobile connection.
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3.1.1. Common Designs:
Since the IS-95B/cdma2000 are developed based on the IS-95A handoff control experience, besides
the dynamic thresholds, basically, IS-95B/cdma2000 inherits the fundamental design concepts from
IS-95A. The common parts are listed as follows:
Pilot is used as a basic triggering measurement for all handoff controls.
T1 plays almost the same role as T_ADD in IS-95A to decide whether a sector has exceeded the
lowest requirement to improve the connection quality. In other words, only the pilot stronger
than the lowest requirement can be added to the candidate set.
T_COMP are used to decide whether the connection quality is necessary to be further improved.
T4 and T_DROP are used to decide the lowest pilot strength before declaring the pilot is not a
valid.
T_TDROP is the timer for removing the pilot from the candidate or active set to the neighbor set.
The timer used in for dropping a leg is to avoid the ping-pong effect.
3.1.2. Different Designs:
Since IS-95B/cdma2000 consider the actual effect to the connection quality, the dynamic thresholds
based on the aggregate Ec/Io can effectively prevent (remove) the unnecessary legs added to (from)
the active set.
The add-leg and drop-leg thresholds are dynamic in the IS-95B and cdma2000 systems, where
in IS-95A, fixed thresholds are used. To control the effective legs, IS-95B and cdma2000
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provide a stringent control on the threshold T2 and T3 and a delay timer for the transition
between the active set and the candidate set.
IS-95B and cdma2000 use the aggregate Ec/Io, the connection quality, to calculate the
dynamic triggering thresholds
Figure 6, Different Adding Triggers in IS-95A and IS-95B/cdma2000
Figure 6 is plotted to explain the differences. The figure shows the fixed adding threshold in IS-95A
and the dynamic adding thresholds (depending on the aggregate Ec/Io) in IS-95B/cdam2000. The
add-threshold line will merge with IS-95A’s fixed threshold if the pilot is less than or equal to the T1
(or T_ADD).
3.2. Comparison between UMTS and IS-95B/cdma2000
UMTS and cdma2000 have been defined as 3G wireless technologies. To provide an advanced
handoff/handover control algorithms to optimize the use of resources while maintaining the
connection quality, both technologies support dynamic thresholds for adding/dropping leg criteria. In
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this section, we will discuss the common and differences in the design concepts for UMTS and
cdma2000.
3.2.1. Common Designs
Both cdma2000 and UMTS consider the effectiveness of connection legs. Dynamic thresholds are
used to decide the actual effect by adding (drooping) legs to (from) the active set. The common parts
are listed as:
A common pilot is used to as the measurement for handover triggers
Dynamic add/drop thresholds are used
Timer is used for drop-leg trigger (to avoid the ping-pong effect)
3.2.2. Different Designs
In cdma2000, the aggregate Ec/Io, considered as the connection quality, is used to decide whether the
new legs can contribute to the connection quality. In UMTS, the best pilot is used instead to decide
whether the new legs can be effective. The differences are listed as:
In cdma2000, the aggregate Ec/Io is used to calculate the add/drop thresholds. In UMTS, the
best pilot is used to calculate the add/drop thresholds
Besides the timer for the drop-leg trigger, in UMTS, the timer is also used for the add-leg trigger
and swap-leg trigger
In UMTS, between the active set stage and the remaining set stage, there is only the monitored
set stage which is similar to the neighbor set stage. In cdma2000, the candidate set stage is
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inserted between the active set stage and the neighbor (monitored) set stage. The pilots in the
candidate set stage have the same search priority as the pilots in the active set stage.
3.2.3. Discussion
Basically, we can see IS-95A handoff control algorithm is a generic algorithm to ensure a seamless
migration. While in IS-95B/cdma2000 and UMTS, more sophisticated control algorithms are
considered to decide whether the new leg or the existing connections can actually contribute to the
connection quality. Whether the pilot can contribute to the connection quality or not depends on (1)
the existing quality and (2) the relative strength to the other pilots. The IS-95B/cdma2000 and
UMTS handoff algorithms should maintain the similar connection quality as the IS-95A by utilizing
either (1) or (2) (or both) for handoff/handover control designs. To that extend, superior
handoff/handover controls on the required resources are expected in the 3G handoff/handover
control algorithms. In our simulation, based on the same simulation platform, we will quantity the
performance for different handoff/handover control algorithms.
4. Simulation Results
In this section, the performance will be evaluated on the same simulation platform.
4.1. System model
Figure 7 shows the block diagram for the simulation model. With the propagation model, the
lognormal fading channel, and various loadings, the pilot Ec/Io of each mobile is calculated. On
every 200msec, the number of connection paths, mobile locations, the pilot Ec/Io, transmitted power,
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and hardware usage are recalculated.
Figure 7, Simulation Model
In the simulation, we use the following assumptions:
1. Propagation model: Cellular Band: Hata Model
2. Lognormal fading with the correlation distance of 400 meters and the variance of 8dB
3. Two-tier 19 3-sector cells are considered.
Center cell is used to study the performance statistics.
4. Traffic loads: 4~8 user3 cases are used.
The parameters in each system are listed below:
UMTS IS95-A IS95-B
Active Set Maximum size= 3 T_ADD=-13dB Soft Slope=1
3 Assuming all users are transmitted continuously. The simulation is to quantify various handoff/handover control algorithms (instead of calculating the number of users can be supported by different systems. A proper operating range based on 6-user case is chosen for IS-95A [6].
Handoff Decision
AggregateEc/Io
Power Budget
ChannelElements
Handoff Frequency
cdmaOneHandoff Algorithm
Multi-slope Handoff Algorithm
Fading
Propagation
Interference
Mobility
Pilotm
easurements
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AS_Th=3dB T_DROP=-15dB Add Intercept=-3dB
AS_Th_Hyst=1dB T_COMP=2.5dB Drop Intercept=-5dB
AS_Rep_Hyst=1dB T_TDROP=1sec T_TDROP=1sec
Time to Trigger Handoff=1sec
Table 1, Algorithm parameters setting
4.2. Simulation results between IS-95-A and IS-95-B
In this section, to quantify the performance differences, IS-95A and IS-95B/cdma2000 are first
compared. Four performance indexes are used to quantify the performance (1) connection quality, (2)
forward-link power, (3) channel elements, and (4) handoff frequency.
4.2.1. Connection quality
CDMA system uses the diversity gain provided by each active leg to support the communication.
Therefore aggregate Ec/Io is used to represent the call quality. If the aggregate Ec/Io is large, it
means the call quality is good and may not need new legs to improve the call quality. In contrary,
when the aggregate Ec/Io is low, new legs might be needed to improve the quality. In Figure 8, we
find that the values of the average aggregate Ec/Io are about the same. It means the average user
connection quality are similar regardless of which handoff control algorithm is used.
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Figure 8, Average aggregate Ec/Io at 6-user case
4.2.2. Forward-link power
With similar connection quality, here we like to examine the impacts on the forward-link power
budget. As shown in Figure 9, to support the same number of users and connection quality, the
forward-link power budget is less in IS-95B/cdma2000 handoff control algorithms. Reducing the
required power budget can further improve the forward-link capacity. The main reason for reducing
the power budget on the forward link is the choice of the better and effective legs to the active set.
Since in a CDMA systems, even the power is controlled by the best base station but all legs in the
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active set will transmit at the same power. If the leg can not contribute the connection quality, the
power is wasted. In this case, IS-95B/cdma200 handoff control algorithms provide a better control
by dynamically changing the add/drop thresholds to reflect the needs in the connection quality.
Figure 9, Base station transmit power at 6-user case
4.2.3. Channel elements
Consistent with the argument in the forward-link power budget, in Figure 10, the saving in the
required channel elements to support the same traffic load is about 10% to 25% if IS-95B/cdma2000
handoff control algorithm is used. Again, the dynamic thresholds eliminate the ineffective legs from
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the active set. This saving not only impacts on the initial capital cost but also the operating cost on
the backhaul to support the traffic volume.
Figure 10, Channel element usage at 6-user case
4.2.4. Handoff frequency
Figure 11 shows the average handoff events per call. In any designs, reducing the processing load is
always positive. As shown, IS-95B/cdma2000 handoff control algorithms reduce the handoff
frequency by more than 30%.
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Figure 11, Handoff events per 60-sec call
To summary, with the dynamic thresholds, IS-95B/cdma2000 handoff control algorithms provide
superior management for required resources in forward-link power budget, channel elements, and
handoff frequency while maintaining the similar connection quality as compared to the IS-95A
handoff control algorithm.
4.3. Simulation results between UMTS, IS-95A, SI-95B/cdma2000
In this section, the comparison will further be extended to include UMTS on the same performance
metric of: (1) connection quality, (2) forward-link/downlink power, (3) channel elements, and (4)
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handoff/hanover frequency.
4.3.1. Connection quality
Figure 12, Average aggregate Ec/Io at the 6-user case
Based on the average Ec/Io of all users, UMTS handover control algorithm achieves the best
aggregate Ec/Io. As discuss, the major differences between the UMTS and the cdma2000 handover
control algorithms are (1) the extra timers for adding a leg and (2) the relative strength to the best
pilot is used to calculate the dynamic thresholds (3) a standard-defined swap criterion. Obviously,
from the average aggregate Ec/Io points of view, UMTS handover control algorithm performs the
best. In here, we can conclude that the trigger thresholds calculated in the UMTS handover control
algorithm have better control in the average aggregate Ec/Io performance. There might exist a
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problem of delay in adding new leg (delay by the timer) if a corner effect4 occurs. Since it is hard to
capture the effect in our simulation, the studies are beyond the scope of this paper.
4.3.2. Forward-link power
Figure 13, Base station transmit power at 6-user case
Besides of achieving a better average aggregate Ec/Io, UMTS handover control algorithm also
required less forward-link (downlink) power budget to support the same traffic load. As shown in
Figure 13, in some cases the saving is significant.
4.3.3. Channel elements
4 The corner effect happens when the pilot change quickly. In this case, any delay in adding new pilot might cause the failure of the connection.
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Figure 14, Channel element usage at 6-user case
As shown in Figure 14, without a surprise, the UMTS handover control algorithm also have a better
saving in the required number of channel elements for the same traffic load. The handover algorithm
can further reduce the number of required channel elements as compared to the cdma2000 handoff
control algorithm. Besides the differences in the trigger thresholds, the delay in adding new legs
should have the most effect on this saving.
4.3.4. Handoff frequency
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Figure 15, Handoff events per 60-sec call
With similar reasons, from Figure 15, we can see that the UMTS handover control algorithm needs
less handover triggers as compared to other algorithms.
To summary, in general, UMTS handover control algorithm performance the best average
performance in connection quality and the best saving in the forward-link/downlink power budget,
required channel elements, and the processing load in handling the handoff triggers. In other words,
the composite effect from the relative strength to the best pilot trigger, the extra timer in the add-leg
stage, and the easy swap-leg trigger makes the UMTS handover control algorithm achieve the best
performance than others.
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5. Conclusions
In this paper, the handoff/handover control algorithms implemented by the current cdma-type
technologies are discussed qualitatively and are evaluated quantitatively by using the same
simulation platform. Instead of quantifying the actual performance, this study provides a relative
comparison for different handoff/handover control algorithms. Results show that selective dynamic
add/drop thresholds are basically better in saving the required resources for the same traffic load.
Among those algorithms, UMTS handover control algorithm not only improves the average
connection quality but also has the best saving in the forward-link/downlink power budget, required
channel elements, and the processing load in handling the handoff triggers. The extended work on
individual instant quality will be investigated in the future and is beyond the scope of this paper. .
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5. Vijay K. Garg, 2002, “Wireless Network Evolution – 2G to 3G” Prentice Hall, 2002
6. C.Y. Huang, M. Y. Tsai, and J. Huang, “Multi-Slope Quality based Handoff Controls in Third
Generation CDMA Wireless Systems”, submitted to IEEE WCNC 2004