44 CHAPTER 3 CHANNEL EFFICIENCY ENHANCEMENT IN WCDMA SYSTEMS USING SOFT HANDOFF 3.1 INTRODUCTION Mobile radio communication has become a rapidly growing market since the GSM standard has been established. Meanwhile, 3G mobile radio systems like UMTS and CDMA 2000 have been standardized and the 4G is currently under investigation. During this development, WCDMA has become a widely accepted multiple access technique. In most cases, it is implemented as Direct-Sequence CDMA (DS-CDMA) in single-carrier systems. An important feature of cellular mobile communication is handoff. It is defined as the transfer of a user's connection from one radio channel to another (can be the same or different cell). As mobile moves towards the boundary of its serving cell, the movement causes dynamic changes in the interference levels and the link quality. This may cause the mobile to transfer communication to or to migrate to a different BS. This change of serving BS is called a handoff. Hard Handoff, also known as a `break-before-make' handoff, is the category of handoff procedures in which the mobile switches to a new radio link after breaking connection with the old radio link [13]. At any time, the active set (set of BSs with which the user is in communication) will have only one BS. SHO on the other hand, is the handoff procedure in which a mobile has connection with more than one radio link simultaneously during handoff. Once the signal from a single radio link is considerably stronger than the others, a decision will be made to communicate with that one only. The addition and
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44
CHAPTER 3
CHANNEL EFFICIENCY ENHANCEMENT IN WCDMA
SYSTEMS USING SOFT HANDOFF
3.1 INTRODUCTION
Mobile radio communication has become a rapidly growing market since
the GSM standard has been established. Meanwhile, 3G mobile radio systems like
UMTS and CDMA 2000 have been standardized and the 4G is currently under
investigation. During this development, WCDMA has become a widely accepted
multiple access technique. In most cases, it is implemented as Direct-Sequence
CDMA (DS-CDMA) in single-carrier systems.
An important feature of cellular mobile communication is handoff. It is
defined as the transfer of a user's connection from one radio channel to another
(can be the same or different cell). As mobile moves towards the boundary of its
serving cell, the movement causes dynamic changes in the interference levels and
the link quality. This may cause the mobile to transfer communication to or to
migrate to a different BS. This change of serving BS is called a handoff. Hard
Handoff, also known as a `break-before-make' handoff, is the category of handoff
procedures in which the mobile switches to a new radio link after breaking
connection with the old radio link [13]. At any time, the active set (set of BSs with
which the user is in communication) will have only one BS. SHO on the other hand,
is the handoff procedure in which a mobile has connection with more than one radio
link simultaneously during handoff.
Once the signal from a single radio link is considerably stronger than the
others, a decision will be made to communicate with that one only. The addition and
45
removal of BSs into and from the active set is dependent on parameters such as the
add threshold (TADD), drop threshold and drop timer. A BS is added to the active set
when its pilot signal strength exceeds the add threshold. A BS is removed from the
active set when its pilot signal strength drops below the drop threshold and
stays below it for the time specified by the drop timer. The process is illustrated in
Figure 3.1.
Figure 3.1 Soft handoff procedure
3.2 CHANNEL CONVERTIBLE SET (CCS)
3.2.1 Cell Geometry of SHO
Accurate geometry of SHO in WCDMA cellular system is hard to depict
due to various factors, such as irregular cell boundaries, traffic conditions and the
movement of mobiles. To simplify the problem, the following reasonable
assumptions are made.
1) The cellular system includes a number of cells of identical size and shape
and the coverage area of each cell can be approximated by a circle.
46
2) Mobiles initiating the calls are uniformly distributed throughout each cell
and one mobile unit may carry at most one call at a time.
3) The cells in the system are symmetrically located and well distributed.
Each cell is surrounded by six other cells.
4) An MS in handoff area occupies at most two channels in its active set,
i.e., there are at most two different sources in diversity reception.
Figure 3.2 Cellular structure of Soft handoff
Figure 3.2 illustrates an example of regions and boundaries based on the
assumption of a circular cell area. With regard to signal strength [102, 103], a cell
can be divided into two areas, namely 1) the normal area and 2) handoff area. In the
SHO area, represented by the intersection of target cell and neighbourhood cell,
each MS holds two channels for transmission in diversity. The ratio ah of the
handoff area to the entire cell area is defined as
hthe area of the handoff regiona
the area of the cell (3.1)
The handoff area is further divided into two regions as shown in
Figure 3.3; target controlling (TC) region and neighbour controlling (NC) region.
In the TC region, the BS of a target cell has stronger power than that of a
neighbouring cell in active sets of MSs. In the NC region, the BS of a neighbouring
47
cell has stronger power than that of a target cell in active sets of MSs. Basically,
since selection diversity is used for uplink interference in WCDMA systems; the BS
that has a higher receiving power in the active set of a call plays an important role in
demodulating the received signal.
Figure 3.3 Cellular system model of Soft handoff
3.2.2 Relative Mobility Estimation
It is assumed that the received pilot strength from a BS decreases when
the MS moves away from the BS and increases when the MS moves towards the BS.
An MS in the handoff area can detect pilot strength from the serving BS and current
time t broadcast by the sync channel which is the forward link channel used to
transmit some system parameters. A mobile uses this information for time
synchronization, which is crucial for the mobile to establish a forward traffic
channel with the BS. Let ps(t,i) be the received pilot strength from the serving BS,
which is measured at time t by MSi and cr_ps(t,i) be the rate of change of ps(t,i)
given as
( , ) ( , )_ ( , ) ps t t i ps t icr ps t it
(3.2)
where, ∆t is the time period of information update in the cellular system.
TC-region NC-region
Cell A MS2
Cell B
MS1
48
With the pilot strength and the rate of changing, the relative mobility of
calls in the handoff area, such as relative position, moving direction and velocity can
be estimated [101, 104]. It is reasonable to assume that the BS with stronger pilot
strength ps(t,i) in the active set of the MSi in the process of handoff should be nearer
to the MS than the BS with weaker pilot strength. In addition, a handoff call
must be moving toward the BS if the rate of change cr_ps(t,i) detected by the BS is
positive. The bigger the value of cr_ps(t,i), the higher the velocity of the MS.
If |cr_ps (t,i)|<ε, where ε is a suitably chosen small number, the MS is considered to
be stationary. The stationary calls in the handoff area request multiple channels,
even though the call is not actually approaching the neighbouring BS from the target
BS.
Therefore, soft handoff can be performed in terms of both the received
signal strength and measured relative mobility of calls [99]. The defined cellular
areas can be identified by measuring ps(t,i) and cr_ps(t,i). The coverage area of a
cell is determined by checking if the pilot strength (ps(t,i)) of the call in the area is
greater than TADD. The SHO area of two cells is determined by checking if more than
two pilot strengths from both the target BS and the neighbouring BS at time t are
greater than TADD. Therefore, the normal area of a target cell is determined by
checking the ps(t,i) from the target BS is greater than TADD and the pilot strengths
from all neighbouring BSs are less than TDROP. Furthermore, in the handoff area the
TC and NC regions can be distinguished by comparing the measured pilot strength.
Let psT(t, i) be the strength of the pilot received from the target BS and let psN(t,i) be
the strength of the pilot received from the neighbouring BS. If psT(t, i)>psN(t,i), the
MS must be in the TC region, in which calls are mainly controlled by the target BS.
If psT(t,i)<psN(t,i), the MS must be in the NC region, in which calls are controlled by
the neighbouring BS.
It should be noted that relative mobility estimation is essentially
different from mobility management which is utilized in current wireless cellular
systems [100]. The main purpose of mobility management is to support the
registration and deregistration of an MS by using a Direct-Transfer Application Part
49
(DTAP) message whereas relative mobility estimation is concerned with the relative
position, speed and moving direction of an MS in a SHO area needed for
constructing the CCS in terms of the measured signal strength [35].
3.2.3 Pseudo Handoff Calls
The set that contains all handoff calls in the NC-region of the target cell,
such that they have channels from both the target BS and neighbouring BS and they
are staying stationary or moving away from the target cell, is defined as the CCS of
the target cell. According to the preceding definition, a CCS includes three types of
SHO calls:
1) The first are new calls that originated in the NC-region of the SHO area and
are moving away from the target cell. These calls request SHO to the
neighbouring BS immediately after their new calls are accepted by the
serving BS. The number of such calls is about 25% of the total number of
new calls in the handoff area [102].
2) The second are handoff calls that stay stationary while talking. In real urban
WCDMA systems, the number of stationary MS calls is about 40%–50% of
the total number of the MS calls.
3) The third calls that move from the TC-region to the NC-region of the target
cell while talking and continue to move towards a neighbouring cell. In this
case, the BS of the neighbouring cell has been changed as the controlling BS
of the call during the handoff process.
The calls in the first two cases are defined as pseudo handoff calls
because those calls do not really carry out handoff. In each BS, a CCS is set up to
identify pseudo handoff calls and serve for incoming handoff calls.
50
3.2.4 Construction and Updating of CCS
The CCS is constructed and managed as shown in Figure 3.4(a). For each
accepted handoff call in the target cell, the relative mobility in the SHO area
is periodically measured and estimated, as described previously. Once the relative
(a) (b)
Figure 3.4 Flow diagram of CCS construction and update
mobility is updated, the call will be checked if it stays in the NC-region of the target
cell such that psT(t,i)<psN(t,i). Then, all handoff calls in the NC-region should be
tested in the MSC to make sure that each of them holds at least two channels in its
Yes
Yes
Yes
No
No
No
Each accepted handoff call
Estimation of mobility
No. of channels in active set >1?
In the NC?
Pseudo handoff
call?
|CCS| = |CCS| + 1
End
No
No
Yes
No
Each handoff call in CCS
No. of channels in active set >1?
Call out or call completion?
Pseudo handoff call?
End
Estimation of mobility
|CCS| = |CCS| - 1
Yes
Yes
51
active set. cr_ps(t,i) is evaluated to check whether the call is stationary or moving
away from the target cell such that cr_ps(t,i)<0 or |cr_ps(t,i)|<ε. The call will be
added to the CCS of the target cell if all the constraint conditions in the CCS
definition are met.
On the other hand, Figure 3.4(b) illustrates how handoff calls are
removed from the CCS. Each handoff call in the CCS is also periodically checked.
The call will be taken out of the CCS of the target cell if one of following three
situations takes place: if it is out of coverage of the target cell, if it completes the
call, or if no longer satisfies the conditions for a pseudo handoff call.
3.3 CHANNEL CONVERSION WITH DYNAMIC GUARD CHANNEL
RESERVATION (CCDG)
3.3.1 Channel Allocation for SHO Requests
In order to improve the efficiency of channel utilization and reduce both
dropping and blocking probabilities, a new handoff scheme based on the relative
mobility of users is proposed. Calls in the NC-region of the target cell periodically
report their pilot strength to the BS and the MSC where the CCS is constructed and
updated as discussed in section 3.2.4.
Figure 3.5 shows the proposed handoff scheme for new handoff calls.
When a new handoff call arrives, the BS at the target cell first checks if there exists
any free channel. If so, the channel is allocated to the handoff call. If no free channel
is available and the new handoff call is a real handoff call, the noncontrolling
channel (or the channel with weaker pilot strength) of a call in the CCS is converted
to the new handoff request as long as the CCS is not empty. Otherwise, the new
handoff call is placed in a queue to wait for a free channel or a channel from a call in
the CCS. The call will be refused if the queue is full [94]. However, refusing a
handoff call request does not mean that it is dropped. A handoff call in the handoff
52
queue would be dropped only after the call moves out of the handoff area without
getting any channel from the target cell.
Figure 3.5 Flow diagram of the channel allocation in the CCDG scheme
3.3.2 Dynamic Allocation of Guard Channels
Since the dropping of a handoff call is considered more severe than the
blocking of a new call, a fixed number of channels are often reserved exclusively for
handoff calls. These exclusively reserved channels are referred to as guard channels.
It is not efficient for such guard channels to be reserved even when handoff traffic is
not heavy.
Thus in this scheme, the number of soft guard channels for handoff is
dynamically adjusted according to the variation of the CCS. The number of guard
channels is updated every time that the CCS is changed. The bigger the CCS, the
less the number of guard channels. Figure 3.6 shows a flow diagram of dynamic
channel reservation. At first, the number of guard channels is initialized as g0. Then,
No
No
No
Yes
Handoff call request HO
Free CH?
Estimation of mobility
|CCS|>0
Place HO into Queue Allocate CH to HO
The call dropped Convert a CH to HO
End
Pseudo call? Out of
handoff area?
No
Yes
Yes
Yes
53
the number of guard channels is dynamically adjusted. If the number of calls in the
CCS is less than or equal to 1, g remains the assigned value g0. If |CCS| > g0, which
means there are enough channels in the CCS that are available for new handoff calls,
g is set to 0. Otherwise, if 0 < |CCS| < g0, g is set to g0 − |CCS| + 1. This scheme
avoids unnecessary guard channel assignment when adequate channel resources are
available for handoff requirement.
Figure 3.6 Flow diagram of dynamic guard channel reservation
in the CCDG scheme
When channel conversion is performed, it is important not to influence
the quality of voice and increase the total interference. Since selection diversity is
used for uplink interference in a WCDMA system where one BS (called controlling
BS) that has a higher receiving power than another BS demodulates the received
signal, the transmitting power of the MS will remain almost the same. In the
proposed scheme, the noncontrolling channel of a call in the CCS is converted to an
incoming handoff call without remarkably degrading voice quality, handoff process
and interference requirement as the channel conversion goes on.
Yes
Allocate initial guard g0
|CCS| changes?
|CCS|>1?
|CCS|>g0?
g = g0 g = 0 g = g0 − |CCS|+ 1
End
No
Yes
No
Yes
No
54
3.4 RESULTS AND DISCUSSION
3.4.1 Simulation Parameters
The main parameters used in the simulation are given in Table 3.1.
Table 3.1 Simulation parameters for CCDG scheme
Td Number of channels in a cell
le Maximum handoff queue length
ah Ratio of the handoff area to the entire cell area
λn New call arrival rate in a cell
λh Handoff arrival rate in a cell
New call arrival rate in normal area
New call arrival rate in handoff area
Transferring rate of a call from the normal area to the handoff area
Transferring rate of a call from the handoff area to the normal area
Call departure rate from handoff area
λt Call departure rate from normal area
Moving rate to an adjacent cell
Moving rate from TC-region to NC-region of the target cell
Call departure rate from CCS
Call departure rate from handoff queue
Tc Channel holding time
µc Mean of channel holding time
c Probability that a handoff call is a pseudo handoff call
g Current number of guard channels
gO Predefined initial number of guard channels
Under the condition that all the neighbouring cells are statistically
identical and behave independently. The characteristics of the overall system can be
55
captured by focusing on a single cell [97]. In this analysis, atmost two different
sources in diversity reception are considered. Each cell will reserve g channels out
of a total of Td available channels exclusively for handoff calls (which are the guard
channels) [98]. Every handoff request is assumed to be perfectly detected in analysis
and the assignment of the channel is instantaneous if the channel is available. It is
also assumed that the allowable maximum handoff queue length is equal to le. In
addition, assuming that calls initiated within the cell arrive at a Poisson rate of ,
handoff request arrivals also form the Poisson process with rate and channel
holding time Tc follows an exponential distribution with mean μc-1. Assuming that
the location of a newly generated call is uniformly distributed over a cell and the
new call arrival rates in the normal and handoff regions are = (1 – ah) and
sλn = ah /2 respectively.
Consider that the new calls in the TC-region of the target cell as new
calls in the handoff region, from the viewpoint of the target cell, whereas the new
calls in the NC-region of the target cell are taken as handoff calls to the target cell.
The dwelling times of a call in the two distinct regions are assumed to be
exponentially distributed. The transferring rate of a call from the normal area to the
handoff area is and the transferring rate of a call from the handoff area to the
normal area is aλc . The rate that a call is terminated is denoted as λt. Numerical
results are reported in this section. Considering a system with parameters Td = 24,
ah = 0.3, g0 = 2 (channels), λt = 0.01 (calls/s), le = 4 and all call arrival rates are
assumed to be Poisson distributed. The simulation is done for the proposed scheme
and is compared with the conventional IS-95 SHO scheme [95].
3.4.2 New Call Blocking Probability
The new calls arising in a cell are blocked when all the available
channels are busy and it degrades the service offered by the network. In the
conventional IS-95 SHO scheme [96], the SHO calls occupy multiple channels and
are responsible for the blocking of new calls. In the proposed scheme, those extra
56
channels (weaker channels in the active set) are grouped into CCS and are available
for channel conversion for the new calls. These available channels in CCS reduce
the new call blocking probability which is evident from the Figure 3.7. The scheme
accommodates more handoff calls by channel conversion and gets more channel
resources for a new call due to dynamic guard channel adjustment.
3.4.3 Handoff Refused Probability
An incoming handoff request to a cell may be refused under either of the
following circumstances:
1) The handoff queue is already full or
2) It does not obtain a handoff channel until either call completion or
departure of the caller from the cell takes place.
Figure 3.8 depicts the handoff refused probability of the conventional
and the proposed scheme. The improvement on handoff refused probability by
CCDG handoff scheme is steady and significant and much greater than the
improvement made by channel borrowing [39] alone, since the CCDG scheme
Figure 3.7 New call blocking probability
57
distinguishes handoff calls in the CCS from ordinary handoff calls and serves more
handoff calls by channel conversion.
3.4.4 Average Number of Guard Channels
The average number of guard channels, which reflects the variation of
guard channel reservation for handoff call is equal to g0 for IS-95 handoff schemes.
Figure 3.9 shows the average number of guard channels for both the schemes. It is
constant for IS-95 handoff scheme because the number of guard channels in the
system is fixed throughout the channel allocation process. However, for the CCDG
handoff scheme, it decreases when a cell is in higher load which implies that the
channel conversion policy and dynamic channel reservation increase capacity for
new calls in the normal area. Since it is not efficient for guard channels to be
reserved even when handoff traffic is not heavy, the number of guard channels is
varied according to the variation of CCS. Number of guard channels is updated
every time that the CCS is changed. The bigger the CCS, the less is the number of
guard channels.
Figure 3.8 Handoff Refused Probability
58
3.4.5 Carried Traffic
The carried handoff traffic per cell is defined as the average number of
occupied channels in the handoff area of the target cell. The scheme makes more
handoff calls being served by the system. Total carried traffic increases as more
number of channels is available with increase in load. The increase in the traffic is
due to the fact that the proposed scheme is serving more channel resources (channels
in the CCS) when the load increases. Figure 3.10 depicts the carried handoff traffic
and Figure 3.11 illustrates the total carried traffic offered by both the schemes. It is
observed that there is significant increase in the traffic offered by the system in the
proposed scheme compared with that of the IS-95 scheme.
Figure 3.9 Average number of guard channels
59
Figure 3.10 Carried handoff traffic
Figure 3.11 Total Carried traffic
60
3.4.6 Channel Conversion Probability
Figure 3.12 shows the probability of channel conversion during the
handoff process for the CCDG scheme which increases with the call arrival rate.
However, as traffic is getting heavy, the increase of conversion probability gradually
turns slower till it finally becomes a constant. This is because more channels
occupied by handoff calls in the CCS may contribute to alleviating possible handoff
dropping as traffic is getting heavier and until the channel pool in the CCS is out of
supply.
3.4.7 Channel Efficiency
It is the ratio of the mean number of calls served in a cell to the total
carried traffic. The proposed scheme is more efficient than the conventional scheme
because of the increased channel resources (CCS and dynamic allocation of guard
channels). As discussed earlier, the proposed scheme offers more channel resources
Figure 3.12 Channel Conversion Probability
61
from the CCS hence when the load is getting heavier, those channels in CCS are
effectively converted for new requests. Further, the dynamic reservation of the
number of guard channels still enhances the performance of the system. Figure 3.13
shows the channel efficiency for both the schemes. It is found that the proposed
scheme is more efficient than the conventional IS-95 handoff scheme.
3.5 CONCLUSION
In this chapter, a new mobility-based SHO scheme is proposed and its
performance is analyzed with that of conventional IS-95/CDMA SHO scheme. It is
observed that there exist SHO calls that move away from the target cell to the
neighbouring cell or in stationary status which unnecessarily occupy multiple
channels. The proposed scheme discriminates such pseudo handoff calls from real
handoff calls by measuring and estimating their relative mobility, thus setting up a
CCS. When there are free channels, the handoff process operates in the same way as
Figure 3.13 Channel Efficiency
62
that in conventional IS-95/CDMA2000 cellular systems. However, when all
channels are occupied and a handoff request occurs, a weaker (or non controlling)
channel used by a call in the CCS is converted to a new handoff request.
The numerical results show that the proposed scheme outperforms the
conventional handoff scheme. Compared with the SHO schemes proposed in patents
and related publications, the advantages of the proposed scheme for SHO are that
1) The scheme can significantly reduce both new call blocking