International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013
Revocation of interference for Mobile Cellular
Systems
Mandava Bindu Sai Sri* , M.Anil Kumar**
*(IV/IV B.Tech, Department of Electronics and communication engineering,
K.L.University, Andhra Pradesh)
**(Assistant Professor, Department of Electronics and communication engineering,
K.L.University, Andhra Pradesh)
I. INTRODUCTION
Abstract-Wireless network widely used today include, cellular
Recently, major efforts have been spent on the
network, wireless mesh network (WMNs), wireless local area
development of Third Generation Partnership Project (3GPP)
network and personal area network. The increasing demand
Long Term Evolution (LTE) for higher data rate and system
for these networks has turned spectrum into a precious
capacity.Showing substantial performance improvements in
resource. For this reason, there is always a need for methods
throughput and spectral efficiency, LTE is foreseen to be a
to pack more bits/Hz. In this paper, we enumerate the reasons for
solid ground for the future IMT-Advanced (IMT-A)
this widespread scepticism, and discuss how current and future trends
technology [1]. IMT-A will offer a high bandwidth up to
will increase the need and viability of multiuser receivers for both the
100MHz for higher data rates, global operation and economy
uplink, where many asynchronous users will be simultaneously
of scale supporting a wider range of services. Many candidate
detected, and the downlink, where users will be scheduled and largely
radio interface technologies have been submitted to the
orthogonalized, but the mobile handset will still need to cope with a
International Telecommunications Union (ITU) to prepare
few dominant interfering base stations.New results for interference
new technology components for LTE to meet IMT-A
cancelling receivers that use conventional front ends are shown to
requirements.
alleviate many of the shortcomings of prior techniques, particularly
communication has received increasing attentions as a
for the challenging uplink. This paper gives an overview of key recent
promising
research breakthroughs on interference cancellation, and highlights
[1−3].Unlike the infrastructure based cellular network, D2D
system-level considerations for future multiuser receivers.
users (user equipments or mobile terminals) do not
Keywords: interference,spectrum,widespread,orthogonalized,
communicate viathe central coordinator (base station, NodeB
Multiuser.
or
evolved
Among
component
NodeB)but
which,
to
device-to-device
improve
spectral
operate as an
(D2D)
efficiency
underlay and
communicate directly with each other or more hops. D2D
communication shares the same resources with the cellular
system whereas under the control of the evolved NodeB
(eNB) of the cellular network. In such a case, eNB can still
control the resource and power assigned for D2D transmission
to limit the interference to the primary . In the downlink each
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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013
receiver only needs to decode a single desired signal from K
Figure 1: In the downlink scenario, each receiver only needs to
intracell signals, while suppressing other cell interference from
decode its own signal, while suppressing other cell interference
a few dominant sources as shown in Figure 1. On the other
from just a few dominant neighboring cells. Because all K
hand, in the uplink the base station receiver must decode all K
users’ signals originate at the base station, the link is
desired users while suppressing other cell interference from
synchronous and the K ¡ 1 intracell interferers can be
many independent sources, as shown in Figure.
orthogonalized at the base station transmitter. Typically though,
some orthogonality is lost in the channel.
For these reasons, downlink receivers at the user
terminals will employ relatively simple multiuser receivers that
attempt to restore the orthogonality of the intracell users via
either a chip-level equalizer (CDMA) or inter-carrier
interference suppression (multiuser OFDM), while handling at
most a few dominant and unknown other-cell interferers. And
although multiuser scheduling may increase throughput and
decrease the number of interfering users, at lower spreading
factors interference suppression will become even more
crucial.
II. MULTIUSER DETECTION
In the uplink, since all users are at different distances from the
base station Future cellular systems will employ sophisticated
Historical
Overview
and
Shortcomings
The
idea
of
scheduling algorithms in the downlink so the primary function
simultaneously receiving multiple interfering users is not
of the mobile unit will be to decode the desired signal in the
particularly new. Most current wireless communication
presence of interference from the neighboring cells. This is
systems already have to cope with a large degree of multiple
fortunate, since the mobile units will still be highly power
access interference.
limited and hence have limited processing power. It is difficult
to
coordinate
and
accurately
synchronize
scheduling
algorithms for and have rapidly changing multipath channels.
Although this is a more challenging task, the base station
receivers will generally have a much higher complexity
allowance than their mobile counterparts.
Figure 2: In the uplink scenario, the base station receiver must
decode all K desired users, while suppressing other cell
interference from many independent users. Because it is
challenging to dynamically synchronize all K desired users,
they generally transmit synchronously with respect to each
other, making orthogonal spreading codes unviable.
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In the important special case of CDMA systems like
original BLAST system and industry adaptations of spatial
IS-95 and 3G cellular, an interference-limited system is
interference cancellation receivers for multi antenna systems
willfully created in order to achieve capacity benefits deriving
can be used to separate spatially multiplexed streams of
from voice activity and universal frequency reuse. While
data.These
simple, proven, and robust, this technique is decidedly
significantly outperform stand-alone linear receivers such as
suboptimal in most circumstances from an information theory
MMSE or zero-forcing in noisy environments. Since the
perspective, particularly when the number of interferers is
cellular environment will invariably have a high level of noise
large.For synchronous systems like the cellular downlink, this
and background interference (from other cells, for example),
approach doesn’t make sense – the better approach would
these linear interference suppression techniques are not viable
simply be to assign the users orthogonal codes at the transmitter
as they amplify this noise when inverting the spatial matrix
and maintain a simple, single user receiver
channel.Analogous logic can be applied to multiuser systems,
types
of
post
processing
receivers
often
and it is well established that linear (dimensional) multiuser
III. INTERFERENCE CANCELLATION
detectors have a noise enhancement penalty.
Although the application of interference cancellation to
multiuser systems is relatively new and unproven,other forms
IV. SYNTHESIS OF NEAR-FAR INTERFERENCE
of interference cancellation have been in widespread use for
CANCELLATION SCHEME IN CELLULAR SYSTEMS.
years. Although the phrase has been used fairly loosely,
Considering the D2D subsystem is an ad-hoc like system, in
interference cancellation should be interpreted to mean the
our work we assume a Carrier Sense Multiple Access with
class of techniques that demodulate and/or decode desired
Collision Avoidance (CSMA/CA) type MAC protocol is
information, and then use this information along with channel
applied for D2D transmission, that means a dedicated
estimates to cancel received interference from the received
common control channel is used for the D2D handshaking
signal.In the DFE, the desired symbol x[n] at some time n is
procedure where all control related signallings are transmitted
decoded. Since this symbol will interfere with many future
to facilitate radio resource management (RRM) and D2D data
symbols – i.e. from times n+1; n+2; : : : – given knowledge of
transmission. The peculiarity utilized by the hybrid system is
the channel, this inter symbol interference can be cancelled.
that D2D users operate in an underlay mode which means that
The DFE is known to work well in practice, and achieve far
the cellular eNB can control D2D users loosely by sending
better performance than linear equalizers which suffer from
limited assisting information to facilitate D2D RRM.
noise enhancement.
Furthermore, the sensed signals from CCCH are assumed to
be reliable. Denote C_UE and D_UE a cellular UE and a D2D
UE respectively, the proposed scheme to share cellular UL
resources in a hybrid system is presented in detail as follows.
Step 1: At the beginning of D2D transmission, eNB broadcasts
the reserved resource including time and frequency of D2D
CCCH in the broadcasting channel such that all C_UEs and
D_UEs can know the information of D2D CCCH.
Figure 3: Block Diagram of the Decision Feedback Equalizer.
Step 2: After that, cellular UEs periodically listen to
The same reasoning applies to analogous types of interference,
thesignalings on CCCH from D2D UEs to ensure whether
such as multiuser interference or spatial interference. The
D2D
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UEs
are
near
to
them.
Here
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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013
signal-to-interference-plus-noise ratio (SINR) threshold and
listening period are needed to identify the adjacent D2D UEs
accurately. According to the peculiarity of the cellular system,
two criteria are proposed for the cellular users:
Criterion I: Cellular UEs only monitor the power ofCCCH
regularly without decoding the information they
received.
Criterion II: Cellular UEs decode the information obtained
from CCCH to find the IDs of neighboring D2D UEs and then
report their IDs to eNB in the next step. To utilize this, the
information such as Cell ID and C-RNTI should be included
Figure 4: Parallel Interference Cancellation.
in the handshaking procedure.
This process can be repeated over several stages, hence PIC is
Step 3: Once a C_UE senses an SINR from CCCH beyond the
sometimes called multistage interference cancellation . Since
predefined threshold, it will report the measurement result to
the first stage generally results in very noisy data estimates, soft
eNB in the earliest available UL time slot in a dedicated
interference cancellation is necessary .
channel. According to the two different criteria in step#2, two
reporting formats will be utilized.
V. MULTIUSER INTERFERENCE CANCELLATION
Interference cancellation for multiuser systems has generally
been broken into two categories, parallel and successive,
although recent developments in iterative interference
cancellation have blurred the distinction.Both SIC and PIC
have the important advantage over other types of multiuser
receivers that error correction coding is integrated into the
Figure 5: Successive Interference Cancellation.
multiuser detection process. As previously noted, both SIC and
This can be subtracted from the composite received signal,
PIC are primarily applicable to the uplink in a many user
which then allows subsequent users to experience a cleaner
CDMA system. However, for a non-CDMA system, these same
signal. All users have improved performance: earlier users
powerful techniques, suitably modified,can help suppress
because they can have disproportionately high received power,
other-cell interference in the downlink. Parallel interference
and later users because a large fraction of the total interference
cancellation (PIC), as shown in Fig. 4, detects all the users
has been removed by the time they are detected.There are a
simultaneously. This initial very coarse estimate can then be
variety of trade offs between SIC and PIC. PIC has decreased
used to cancel some interference, and then the parallel detection
latency, but higher overall complexity because K users must be
can be repeated. Successive interference cancellation (SIC),
detected in parallel, plus there are P cancellation stages. So the
shown in Fig. 5, detects just one user per stage.
latency is proportional to P which is generally much smaller
than K for cellular systems, but complexity is proportional to
PK. SIC on the other han has complexity and latency
proportional to K, and this latency may be prohibitive if there
are many users with real-time data. Some authors have
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attempted to provide a smoother trade off between these
results in power control theory that the optimum SIC power
techniques by introducing multistage SIC: a group of users are
distribution, even with channel estimation error accounted for,
detected in parallel, and then has their aggregate interference
can easily be accomplished using binary iterative feedback
subtracted from the composite received signal, and then
algorithms. Multipath channels are challenging for all wireless
another group is detected in parallel.
systems, but particularly for multiuser receivers. The reason is
Channel estimation error and error propagation-The entire
that each multipath component can appear to be a user of this
concept of interference cancellation is based on the premise
system,so the quantity of perceived users grows not just with
that the received signal can be reliably estimated. Whereas
the number of users K, but also roughly with the number of
communication systems are by definition designed to allow the
multipath components L. Although interference cancellation
transmitted signal to be recovered, reconstructing the received
receivers, which are based on conventional receivers, can easily
signal requires an accurate description of both what was
employ a RAKE receiver to handle multipath, it may be
transmitted, and what the channel did to that transmission.
difficult to accurately regenerate the interference for
Inaccurate channel estimation is a problem for both PIC and
cancellation if there are many multipath components. If each
SIC, especially for SIC since historically, the optimal received
multipath component has an independent amplitude and phase,
power distribution is based on the assumption that interference
then generally the estimation error for each will be
has been completely cancelled, which is never fully achieved in
independent. This can cause the capacity to decrease rapidly as
practice. This residual interference then causes the later users to
the multipath profile worsens,since channel estimation errors
have unacceptably bad performance, causing a major fairness
effect all the users signals in every dimension when the
problem as well as an overall degradation in bit-error rate and
multipath interference is regenerated in the time-domain by the
system capacity, it can be seen that if the channel estimation
RAKE “encoder”.Exploiting the characteristics of GMSK
error is on average larger than about 20%, then the system was
modulation allows signal reception to be split into two virtual
better off without SIC, assuming the traditional power control
paths, which can then be processed using classical linear
for SIC is used that assumed perfect interference cancellation.
processing techniques such as ZF or MMSE.However, joint
Whereas with a modified power control formulation that
detection is also possible in asynchronous networks provided
accounts for the statistics of the channel estimation error, even
the mobile terminal platform can handle the complexity.
with dramatic estimation error as high as 50%, SIC nearly
Collectively, these joint detection and hybrid/linear receivers
doubles the system capacity relative to no interference
are referred to as Single Antenna Interference Cancellation
cancellation.Whereas
cancellation
(SAIC) receivers. It should be noted that these kinds of
functions best in the familiar case where all the received powers
interference “cancelling” receivers may employ either
are equal, we have just seen that successive interference
maximum likelihood detection or pre-detection processing
cancellation works best when a specific and unequal
rather than the post-detection interference cancellation
distribution of user powers is maintained, and furthermore
emphasized in this paper.
parallel
interference
when the distribution specifically considers imperfect
interference
cancellation.
An unequal received
power
.
distribution has also been shown to be highly preferable for
iterative interference cancellation . This apparent complication
of CDMA power control has been frequently cited as a major
shortcoming of SIC. But recently it has been shown using new
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International Journal of Engineering Trends and Technology (IJETT) - Volume4Issue4- April 2013
University of Illinois at Urb ana- Champaign, Aug. 1984.
CONCLUSION
Through this paper we can know how interference can be
[5] Multiuser Detection. Cambridge, UK: Cambridge, 1998.
cancelled in mobile cellular systems.MIMO systems are gaining
[6]IEEE Journal of,Volume:36,Issue:4,oct.2005.
much more attention and efforts in wireless communication research
[7] L. B. Michael, S. Kikuchi, T. Adachi, and M. Nakagawa,
due to their potential to increase considerable capacity in mobile.
“Combined
The synchronous downlink will likely employ opportunistic
communication,”
scheduling across time, frequency, and/or codes, and so the
Symposium, Oct. 2000, pp. 534–539.
majority of the interference at the mobile stations will come
[8] X.Wang and H. Poor, Wireless Communication Systems.
cellular/direct
in
Proc.
method
IEEE
of
inter-vehicle
Intelligent
from a small number of neighboring base stations, rather than
the present scenario where much of the interference in intracell.
For these reasons, multiuser receivers will play an important,
but different, role in the downlink and uplink of future cellular
systems. In the uplink, there will be many asynchronous users,
although fewer than in a 2G or 3G system as the cells will
continue to grow smaller while data will be burstier and higher
rate, and hence the spreading factor smaller. This makes
interference cancellation techniques all the more desirable and
practical. Recent research on interference cancellation,
highlighted in this paper, has made significant strides, but more
research and development is required to prototype these
systems and adapt them to real-world environments.For the
downlink, the need will be to attenuate the interference from a
small number of neighboring base stations while maintaining
the orthogonality of the users within the cell in time, frequency,
code,and/or space, as the case may be. This implies that the
success of the recent research and implementation of Single
Antenna Interference Cancellation (SAIC) techniques for
single-carrier GSM systems should be extended through future
research to higher-bandwidth and more complex multi carrier
and CDMA systems.
REFERENCES
[1]“S. R0023, High Speed Data Enhancement for CDMA2000
1x-Data Only,” 3GPP2, June 2003.
[2] “High Speed Downlink Packet Access (HDSPA), release
5,” 3GPP, 2003.
[3] “Wideband CDMA Standard, Release 99,” 3GPP, 1999.
[4] “Optimum multiuser signal detection,” Ph.D. dissertation,
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