International Journal of Engineering Trends and Technology- Volume3Issue2- 2012
Soft Handoff in Mobile Networks
Megha and Narendra Kumar
CMJ University Shillong
Abstract:
In power controlled CDMA systems soft handoff
is preferred over hard handoff strategies. This is more
pronounced when the IS-95 standard is considered wherein
the transmitter power is adjusted dynamically during the
operation. The previous and the new wide band channels
occupy the same frequency band in order to make an
efficient use of bandwidth, which makes the use of soft
handoff very important. Our main aim is to maintain a
continuous link with the strongest signal base station
otherwise a positive power control feedback would result in
system problems. Soft handoff ensures a continuous link to
the base station from which the strongest signal is issued.
In this paper we are presenting a brief overview of the
subject and some elementary concepts.
Introduction:
Mobility is the most important feature of a
wireless cellular communication system. Usually,
continuous service is achieved by supporting handoff
(or handover) from one cell to another. Handoff is the
process of changing the channel (frequency, time
slot, spreading code, or combination of them)
associated with the current connection while a call is
in progress. It is often initiated either by crossing a
cell boundary or by a deterioration in quality of the
signal in the current channel. Handoff is divided into
two broad categories— hard and soft handoffs. They
are also characterized by “break before make” and
“make before break.” In hard handoffs, current
resources are released before new resources are used;
in soft handoffs, both existing and new resources are
used during the handoff process. Poorly designed
handoff schemes tend to generate very heavy
signaling traffic and, thereby, a dramatic decrease in
quality of service.
The act of transferring support of a mobile
from one base station to another is termed handoff.
Handoff occurs when a call has to be handed off from
one cell to another as the user moves between cells.
In a traditional "hard" handoff, the connection to the
current cell is broken, and then the connection to the
new cell is made. This is known as a “break-beforemake" handoff. Since all cells in CDMA use the
same frequency, it is possible to make the connection
to the new cell before leaving the current cell. This is
known as a "make-before-break" or "soft" handoff.
Soft handoff requires less power, which reduces
interference
and
increases
capacity.
The
ISSN: 2231-5381
implementation of handoff is different between the
narrow band and the CDMA standards. This is an
effort to analyze a soft handoff strategy, taking into
consideration the trade-offs between selecting system
parameters and the performance benefits. We
consider a scenario involving two base stations and a
single mobile, where the changes in the pilot signal
strength are tracked and a conditional decision is
made allowing communication with only one base
station. This results as an outcome of simple
measurements, which determine the strongest signal
and results in handover to the corresponding base
station. In a CDMA system the same frequency band
is shared between all the cells. Thus there is welldefined efficient bandwidth utilization. Though there
is frequency reuse the orthogonal nature of the
waveforms serves to distinguish between the signals
that occupy the same frequency band. A typical
handoff scenario is taken into account wherein the
need for a mobile to alter its frequency or rather to
switch its carrier frequency is negated. The main idea
of a soft handoff scheme is to ensure that there is
connectivity with the old base station while the new
base station has been assigned to take control over
the communication link. This way at a given instant
of time we have the mobile maintaining a constant
communication link with at least one base station
simultaneously ensuring a non disrupted call activity.
The algorithm may be designed in such a way as to
ensure that as soon as the mobile is within the range
of the new cell, the old base station releases the
Related work:
In summary, even though the capacity and
outage of forward link has been studied [1] [27], the
current research on the CDMA forward link is far
from enough due to the neglecting of the soft handoff
threshold and multipath fading. Furthermore, the 3G
CDMA systems are designed to support both the high
data rate and voice services after the successfully
deployment of the 2G CDMA systems, such as IS-95,
to support mainly the voice service.The high data rate
services embody the property of asymmetry, that is,
the forward link is capacity-limited due to its highvolume traffic. The forward link performance of a
CDMA system has been analyzed by treating the SIR
Eb/I0 as a random parameter taking all possible values
in [1]. However, the SIR is considered to be affixed
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International Journal of Engineering Trends and Technology- Volume3Issue2- 2012
value in a perfect power-controlled CDMA system or
a random variable with a small variance in the
presence of the power-control error [24]. Since the
power allocated from the base station (BS) connected
to the mobile station (MS) decides the amount of SIR
achieved at the MS, it is more appropriate to evaluate
the system performance from the viewpoint of the
fraction of BS power allocated to a MS, which is the
essence of the SIR-based fast power control in the
forward link. The SIR-based fast power control in the
forward link is quite di®erent from that in the reverse
link. In the reverse link, the other cell interference
can be well approximated by a Gaussian variable [19]
[31]. However, in the forward link, the interference at
the output of each branch of the Rake receiver was
shown to be approximated by a Gamma variable by
using the channel-gain-matched combining and
assuming a perfect slow power control [27]. But the
statistical property of the interference in a SIR-based
fast power control is still unavailable for the the
maximum ratio combining (MRC). The MRC is
optimal in the sense of maximizing the total SIR at
the output of the Rake receiver and the total SIR is
the summation of the individual SIR's at the output of
each branch of the Rake receiver. The analytical
expression of the total SIR is complicated by using
the MRC, since the signal term in one branch of the
Rake receiver is treated as the interference in another
branch. It was reported in the literature that when the
MS is connected to only a single BS in the non-soft
handoff mode (NSHM), the concept of the
orthogonality factor (OF) can be used to simplify the
total SIR expression into one term [30]. However, it
is very complex to simplify the total SIR expression
into one term for the MS communicating with
multiple BS's in the soft handoff mode (SHM), since
the soft handoff threshold (SHT) truncates the
strengths of both the signal and interference as
discussed later. However, an exact closed-form
expression of the blocking probability is still not
available. In addition, for simplicity, majority of the
previous work studying the forward link performance
assumes equal BS powers throughout the system [1],
[24] and a static capacity is obtained without
considering the time-variant property of the
interference caused by other cells.
Handoff Architecture for 4G Technology: In this
paper, an IP -based handoff architecture using mobile
IP is used. The mobile host has a multi -mode card
that can access the WLAN and cellular networks.
Their hierarchical foreign agents and multi-path
structure used. For conventional handoff techniques,
the criteria that select the initial mode in mobile host
are the radio link quality, data rate, service type,
ISSN: 2231-5381
speed of mobile host, and capacity of cellular
network. If its data rate is low and fast moving, then
the mobile host can select the CDMA network. For
high data rates, then the WLAN is selected. For
accessing the internet, we know that the uplink and
downlink traffics are not balance. Normally, user
prefers a wider downlink frequency than the uplink.
Here our goal is to use the combination of cellular
network for uplink traffic services and WLAN
network for downlink traffic services to provide an
efficient application for mobile user to access the
internet. Structured mobility anchor point can offer a
mobile node seamless mobility when it moves from
MAP2 to MAP3 while communicating with a
corresponding node. In this approach, different
mechanisms and protocols can handle authentication,
billing and mobility management in the cellular and
802.11 portions of the network [4 -7]. When an MN
enters a new foreign sub network, it first acquires a
new physical care-of address (PCoA) by means of
address auto configuration, in which the MN uses it
as the source address for all datagrams that it sends.
The MN will also register a unique virtual care-of
address with a home agent and CN for each level of
the hierarchy. It all starts when the MN receives a
router advertisement with the mobility information
option that contains a new hierarchy, in which it will
send a binding update. That binding update binds its
PCoA to its lowest VCoA. After that the lowest
MAP will send a surrogate binding update to the next
higher MAP. That binding forms a binding between
the VCoAs of the mobile node in the MAPs
hierarchy. This continues until the highest MAP
receives a surrogate binding update when it will
check whether that MN is allowed to use the network
and finally sends a binding acknowledgement to the
next
lower
MAP.
These
surrogate
acknowledgements are sent until the lowest MAP
receives one. Then the lowest MAP sends
acknowledgement to the MN. In figure 2, mobile
node sends binding update to the MAP2, which is the
lowest MAP. MAP2 sends a surrogate binding
update to MAP1. MAP1 is the highest MAP and it
processes the authentication header of the original
binding update and authenticates the mobile node.
Multiple paths are maintained while mobile node
transits the overlapping area of two adjacent cells,
keeping connections for both cells. To avoid drastic
quality degradation and stream disruptions, [9]
proposed a scheme that reduces packet loss and
maintains high throughput during handoffs by
transmitting packets on multiple paths. Meanwhile,
high throughput is maintained by exploiting all the
available bandwidth on multiple paths. To allow a
source node to be able to maintain multiple paths
simultaneously, mobile IP simultaneous binding [10,
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International Journal of Engineering Trends and Technology- Volume3Issue2- 2012
11] and route optimization option [12] are used.
Simultaneous binding option allows a mobile node to
simultaneously register multiple CoAs, and route
optimization option allows the sender to be always
informed of the CoA registration directly form the
receiver.
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