Cellular Systems
BASE
STATION
• Frequencies (or time slots or codes) are reused at
spatially-separated locations  exploit power falloff
with distance.
• Best efficiency obtained with minimum reuse
distance  system capacity is interference-limited.
• Mainly designed for circuit-switched communications
• Base stations perform centralized control functions.
(call setup, handoff, routing, etc.)
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DESIGN CONSIDERATIONS
• Access Technique: CD, TD, FD, or hybrid
– Efficiency within a cell
– Interference to other cells
– Cellular system “capacity”
– Other considerations:
- Frequency planning
- Synchronization requirements
- Soft handoff
- Need for power control
- Frequency reuse requirements
• Reuse Distance (D)
– distance between cells using the same
frequency, time slot, or code
– smaller reuse distance packs more users
into a given area, but also increases
co-channel interference
• Cell Radius
– decreasing the cell size increases system
capacity, but complicates the network
functions of handoff and routing
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TD Access
• FDD separates uplink and
downlink.
• Timeslots allocated between
different cells.
– FDD separates uplink and downlink.
• One of the US standards for
digital cellular
– IS-54 in 900 MHz (cellular) band.
– IS-136 in 2 GHz (PCS) band.
• IS-54 compatible with US analog
system.
– same frequencies and reuse plan.
GSM Access
• FDD separates uplink and downlink.
• Access is combination of FD,TD, and
slow FH
– Total BW divided into 200Khz channels.
– Channels reused in cells based on signal
and interference measurements.
– All signals modulated with a FH code.
• FH codes within a cell are orthogonal.
• FH codes in different cells are semiorthgonal
– FH mitigates frequency-selective fading via
coding.
– FH averages interference via the
pseudorandom hop pattern
Access in IS-95 (CDMA)
• Each user assigned a unique
DS spreading code
• Code is reused in every cell
– No frequency planning needed
– Allows for soft handoff is code
not in use in neighboring cell
• Power control required due to
near-far problem
– Increases interference power of
boundary mobiles.
Capacity Comparison
• Shannon Capacity
– Shannon capacity does no incorporate
reuse distance.
– Some results for TDMA systems with
joint base station processing (Wyner,
Wyner and Shamai).
• User Capacity
– Calculates how many users can be
supported for a given performance
specification.
– Results highly dependent on traffic,
voice activity, and propagation models.
– Can be improved through interference
reduction techniques.
• Area Spectral Efficiency
Area Spectral Efficiency
• Defines as the total throughput per unit
area.
• Captures the design tradeoffs for reuse
distance as well as other parameters
• General performance metric that can
be applied to any system
• Rates can be computed based on
analytical model or simulation
• ASE for equal rate users
Ae =
K • Rb (S/I)
(.25 D p)
2
bps/Hz/km2
-K is the number of users per cell
-S/I is a (time-varying) function of the access
method, reuse distance, and propagation.
- Rb is the data rate per user
- D is the reuse distance
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Average Area Spectral Efficiency
[Bps/Hz/Km2]
ASE vs. Cell Radius
fc=2 GHz
10
1
D=4R
D=6R
10
0
D=8R
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Cell Radius R [Km]
0.8
0.9
1
METHODS TO IMPROVE
SPECTRUM UTILIZATION
• Interference Averaging (CDMA, FH)
• Interference Reduction
(power adaption, sectorization)
• Interference Cancellation
(smart antennas, multiuser detection)
• Interference Avoidance
(dynamic resource/channel allocation)
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SECTORIZATION
5
7
2
5
6
5
3
1
5
4
5
5
• 120° sectoring reduces interference from
co-channel cells.
• Out of the 6 co-channel cells in the first tier,
only 2 of them interfere with the center cell.
• If omni-directional antennas were used at
each base station, all 6 co-channel cells
would interfere with the center cell.
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SMART ANTENNAS
• Multiple antenna elements at the receiver
and/or the transmitter form an antenna array.
• Space-time processing of the received signal
at the array reduces interference, and also
compensates for flat-fading and delay spread.
• Methods
– switched beam
– adaptive array
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MULTIUSER DETECTION
• Goal: decode the interfering signals to remove them
from the desired signal
• Interference cancellation
– decode strongest signal first, and subtract it from
the remaining signals
– repeat the cancellation process on the remaining
signals
– works best when signals are received at very
different power levels
• Optimal multiuser detector (Verdu Algorithm)
– cancels interference between users in parallel
– complexity increases exponentially with the
number of users
• Other techniques tradeoff performance and complexity
– decorrelating detector
– decision-feedback detector
– multistage detector

multiuser detection often requires knowledge of the
channel parameters, which is difficult to obtain in a
rapidly-changing environment
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