Interference Suppression

1
Interference Suppression Techniques for 2nd /
3rd Generation Mobile Radio
Cameron B Shaw BEng(Hons), MEng,
AMIEE C.B.Shaw_at_staffs.ac.uk http//www.soc.staffs.
ac.uk/cmrcs
2
Interference Problem Statement
Signal interference and distortion effects such
as channel noise, fading, multi-path signals and
inter-symbol interference (ISI) create a complex
distortive environment which degrades system
performance and causes a major impediment to
reliable communication.
Multi-path fading channel Wave propagation
depends on the environment buildings, roads,
trees and terrain. Each path will experience an
attenuation (fade), a time delay (phase shift)
and a possible frequency shift (Doppler) due to
the environment and relative motion of the
handset The superposition of all the
signal paths produces a distorted received signal
Fast acting power control is essential in a 3rd
Generation cellular system.
Each mobile has to have the same received power
at the base station to avoid signal blocking by
overpowering close mobiles.
Power control signals are sent periodically to
the mobiles to ensure this.
3
A cellular radio infrastructure which, although
flexible and ideally suited to mass mobile radio
communications, has inherent problems which
produces a degradation of the system performance
Performance degradation due to Inter-cell
(adjacent cell) interference

• Intra-cell interference (when mobiles
• within the same cell produce multiple access
• interference (MAI)).

• Inter-cell interference (when mobiles from
• adjacent cells interfere with one another).

• Jamming which occurs when mobiles from adjacent
  cells that
• are within radio range of each other use codes
  that are similar or
• the same to one another (i.e. non-orthogonal).

Performance degradation due to Intra-cell (same
cell) interference
4
Mobile radio channel (air interface)

The Radio Channel environment can be
mathematically recreated on computer
2sinb1
2cosb1
cosw1t
2sinbN0
2cosbN0
coswN0t
Computer simulations can be performed to test the
different channel models and interference
suppression techniques developed.
2cosa
2sina
?
?
xc(t)
xs(t)
signal source (?c)
p/2

• 6 Tap channel model simulates frequency selective
  behavior.
• Advantage Realistic, Accurate.
• Disadvantage Computationally intensive

?
offset oscillators
y(t)
Output
S

• Jakes channel model simulates
• non-frequency selective behavior.
• Advantage Computationally
• un-intensive
• Disadvantage Approximate results produced

A
(0)
A
(800)
A
(1760)
A
(2720)
A
(4640)
A
(4960)
0
0
0
0
0
0
Input
Discard
T
T
-T
T
T
-T
T
T
-T
T
T
-T
T
T
-T
T
1
2
1
2
3
2
3
4
3
4
5
4
5
6
5
6
Sample
5
Channel Simulation Results

• Quadrature Phase Shift Keying (QPSK) Modulation
  (2 bits/sec/Hz)
• Rayleigh amplitude fading 6 tap channel model
  considered
• 2 GHz operating frequency (allocated for 3rd
  Generation access)

• Environments Considered
• Indoor
• Outdoor / Pedestrian
• Vehicular

6
Proposed System Structure
The purpose of this project is to investigate and
develop a new strategy for the interference
suppression techniques related specifically to
the 2nd / 3rd generation of cellular mobile
radio and the inherent associated problems.
Below is a block diagram of a possible approach
for reducing interference.
Air Interface (channel)
Model
Path Loss
Data
Air Interface
Source
IP
W-
CDMA
Coded
Space Diversity
Routing
Spreading
Modulator
Fading
Dispersion
Channel
Intra-cell
SIR Estimation
Interference
Power Control
Inter-cell
Data
Interference
Destination
IP
W-
CDMA
SOVE
Routing
Despreading
Air Interface
S
AWGN
Space Diversity
Channel
Estimator
Causal Simulation (C)
Event Based Simulation (
OPNET)
7
Trellis Coded Modulation
Output Bit 1
S
Rate 2/3 Convolutional Encoder
D
D
D
Input Bit 1
S
Output Bit 2
Input Bit 2
Output Bit 3
Using the encoder (above) two bits are encoded
into 3 bits which are then assigned to the 8
different phases
Set partitioning of 8 PSK (phase shift keying)
to assign binary data to allowed waveforms
Only certain phase change sequences are allowed
to be transmitted. Using the trellis (above) in
the decoder, the most likely transmitted sequence
can be determined
8
Space diversity combining algorithms
output patterns

• The effect of fading can be reduced by combining
  the signals from different antennas separated
  spatially
• Received signals are statistically independent
  (do not fade at the same time)
• The antennas need to be spaced sufficiently apart
  so that their fading envelopes are uncorrelated
  (normally ? ?/2)

. . .
. . .
processing layer of
v
(
n1
)
v
(
n1
)
hidden and output neurons
1
n1
feedforward and
w
(
n
)
feedback connections
n1n0,n1
w
(
n
)
11
concatenated
. . .
input-output layer
. . .
v
(
n
)
v
(
n
)
u
(
n
)
u
(
n
)
u
(
n
)
u
(
n
)
1
n1
1
2
n0
n0-1
z-1
z-1
z-1
input patterns
xN1(n)
xNc-N1(n)
.
x1(n)
z-1
z-1

• Non Linear space diversity combining
• using artificial neural network
• structures, the minimum BER
• criterion optimises the performance
• MLP, RNN

c1(n)
cN1(n)
cNc-N1(n)
.
xN2(n)
xNc-N2(n)
x2(n)
z-1
z-1
.
c2(n)
cN2(n)
cNc-N2(n)
y(n)
?
.
output after decision device
x2N(n)
xNc(n)
xN(n)
z-1
z-1
.

• Linear space diversity combiner,
• based on the minimisation of the
• MSE
• LMS, RLS, SRK

cNc(n)
cN(n)
c2N(n)
.
linear adaptive algorithm
training sequence
d(n)
9
Context and future work
The overall objective of the project is to
develop a new strategy for interference
suppression by investigating the performance and
practicality of the aforementioned interference
suppression techniques when combined together and
applied within the context of a mobile cellular
radio system of either 2nd or 3rd generation with
the aim of providing a realistic and commercially
exploitable solution to the problem of combating
interference within the high data rate packet
orientated mobile cellular radio system as
envisaged for the third generation by the
standardisation bodies (UTRA / IETF / 3GPP). The
development of this project will be in line with,
but not restricted by the 3GPP standards (ie.
3GPP R99 also known as 3GPP R3). This work
will be tested and evaluated within a packet
switched IP based environment using 2nd and 3rd
generation mobile cellular radio platforms
incorporating the UMTS (W-CDMA / cdma2000) and
GSM (TDMA) standards.