CDMA Solutions: Smart Antenna Technology

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CDMA Solutions Smart Antenna Technology

• Wei-Chiang Wu

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Multiple Access Techniques for Wireless
Communication

• Frequency Division Multiple Access (FDMA)
• Time Division Multiple Access (TDMA)
• Code Division Multiple Access (CDMA)
• Space Division Multiple Access (SDMA)
• Orthogonal Frequency Division Multiple Access
  (OFDMA)
• Purpose Allow different users to communicate
  over the same channel

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FDMA - Frequency
divide up the spectrum into non-overlapping
sub-bands called channels but can transmit all
the time Orthogonal in frequency domain
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TDMA - Time
divide up the time domain into non-overlapping
time slots Orthogonal in time domain GSM uses
200 kHz channels and 8 slots
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TDMA - Time
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Multiple transmitters are divided in time for
multiple access to the same carrier
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CDMA - Code
CDMA uses a code division scheme, each user
utilizes the entire bandwidth all the time with a
user-specific signature sequence
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SDMA - Spatial
Frequency reuse Same frequency is used in
different geographic areas or "cells"
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Cell Sectoring
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Cell Sectoring

• 120 cell sectoring reduces the number of
  co-channel base stations from 6 to 2.

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Characteristics of CDMA (1)

• A multiple access technique that each user is
  assigned a unique signature waveform (PN code)
  upon which data bits are modulated
• In FDMA, all users transmit simultaneously, but
  use disjoint frequency bands
• In TDMA, all users occupy the same BW, but
  transmit sequentially in time
• In CDMA, users are allowed to transmit
  simultaneously in time and occupy the same BW

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Characteristics of CDMA (2)

• Multiple Access Interference (MAI) limited
• The signature waveforms should perform
• Delta-like autocorrelation for identification and
  synchronization
• Low crosscorrelation (approximate orthogonal) to
  suppress MAI.
• The signature waveforms among each user should be
  linear independent in order not to cancel each
  others transmission

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Characteristics of CDMA (3)

• Anti-multipath fading capability
• Relatively easy to add additional users to the
  system
• CDMA Capacity How many users can coexist in a
  CDMA cell?
• mainly determined by processing gain and the
  required SIR
• The capacity is soft
• Same frequency can be used in all cells, BW
  efficient for multiple users
• CDMA air-interface is well-standardized

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Multipath Fading
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Tapped Delay Line Channel Model
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CDMA Path Diversity_ RAKE Receiver
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RAKE Receiver for CDMA

• CDMA takes advantage of the multipath signals to
  improve signal quality
• Using multiple correlating receivers that matched
  to each path
• Maximum ratio combining
• coherently recombine the output signals from each
  finger

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Characteristics of CDMA (4)

• Anti-jamming capability
• Security, ECCM
• FH/CDMA systems achieve their processing gain
  through Interference avoidance
• DS/CDMA systems achieve their processing gain
  through Interference attenuation
• Soft-handover
• Near-Far Problem rx powers from different users
  are unequal such that stronger user may degrade
  weak users transmission

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Characteristics of CDMA (5)

• Power control counter the near-far problem by
  dynamically adjusting transmitted power, based on
  feedback information from the receiver
• Multiuser detection robust signal processing
  technique to design near-far resistant detectors,
  timing estimators,

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Power Control

• A way to solve the near-far problems such that
  all the mobiles transmitting powers are
  equally-received by the Base station
• For the near-end mobile, it can be asked to
  transmit lower power
• For the far-end mobile, it can be asked to
  increase transmit power
• All mobiles are power controlled to the minimum
  power so as to maintain the link

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Advantages of Cellular CDMA

• Multipath diversity can be gained with a RAKE
  receiver
• High frequency reuse efficiency
• Soft handoff capability
• Rejection of narrowband interference
• Ability to adapt to nonstationary traffic
• No need for guard intervals as in TDMA

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Soft Handoff and Hard handoff

• Hard handoff
• a physical change in the assigned channel
• Break before make
• Soft Handoff
• Soft Handoff is performed by exploiting two or
  more base stations as a giant diversity system
  such that multiple BSs simultaneously talk to the
  mobile during a handoff
• a different BS handles the communication

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Soft Handoff
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Soft Handoff (1)

• Power control during soft handoff
• If ANY BS orders the MS to reduce its power, it
  must do so, but it may increase power only when
  directed by ALL BSs
• Macro-diversity
• On the reverse link, each BS receives the signal
  from MS and the demodulated signal are combined
  (fade resistant)
• On the forward link, MS receives the signal from
  BSs by RAKE and then combined

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Soft Handoff (2)

• Advantage
• improve link performance at the edge of the cell
• Disadvantage
• Reduce forward link capacity since each takes up
  a traffic channel

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Overview of Smart Antenna System (SAS) for Mobile
Communications

• SAS is equipped in basestation (BS) with a
  pattern that is not fixed but adapts to the
  current radio conditions.
• Traditional omnidirectional or sectored antennas
  not only Waste power but also interfere to
  other users
• Smart antenna is a Spatial Filter that directs
  a beam toward desired user only
• It consists of a number of radiating elements, a
  combining/dividing network, a control unit (DSP).

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Difference of BS radiation pattern between a
traditional antenna and SAS
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Principle of a SAS
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Sharing the radio spectrum SDMA (Space Division
Multiple Access)

• Users in the same cell can use the same physical
  communication channel (carrier frequency, time
  slot, spreading code) as long as their angles are
  different
• The next step in an evolutionary path toward
  increasing the capacity of cellular systems
• Dr. Andrew Viterbi

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Advantages of Smart Antenna Technology

• Enhance coverage through Range extension
• Improve building penetration and Hole filling
• Reduce delay spread (time dispersion) because
  fewer scatters are illuminated
• Reduce co-channel interference (CCI) and multiple
  access interference (MAI)
• Link quality can be improved through multipath
  management
• Improve system capacity
• Helps to isolate the uplink signals from
  different users, reduce the power control burden

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Level of intelligence

• Switched Beam system
• A switch is used to select the best beam to
  receive a particular signal
• Rx power level fluctuates as a subscriber
  travels in an arc
• Unable to take advantage of path diversity of
  multipath signal
• Dynamically phased array
• Adaptive antenna system
• Interference nulling
• Multipath diversity
• Main beam steering

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Different levels of SAS
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Challenges of optimizing CDMA capacity

• Traffic load balancing
• Handoff overhead management
• Interference control

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Traffic loading Challenge

• The time-varying traffic load is usually
  distributed unevenly among the cell sites or even
  within the sectors of an individual cell.
  Consequently, the capacity limit may be attained
  in the heavily loaded sectors (cells) even though
  quite a few channels (codes) remain available in
  the lightly loaded sectors (cells). The problem
  arises that as the capacity limit is reached in a
  specific heavily loaded sector, the unused
  capacity in some more lightly loaded sectors is
  inaccessible to subscribers.

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Load Balancing SAS rotates sectors and changes
beamwidth
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Traffic load balancing

• Consider a conventional 3-sector cell, one has to
  climb the antenna tower, physically rotate the
  orientation of the antennas such that the traffic
  is redistributed, which seem to be time-consuming
  and inefficient
• Smart antennas not only redirect the sector
  orientation, the beamwidth or coverage area of
  each sector is also changed. Furthermore, to
  reflect the fact that the load distribution
  pattern is time-varying, a software controlled
  sector orientation and beamwidth should be
  applied.

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Dynamic sector synthesis
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Handoff Overhead Challenge

• In the existed or third generation CDMA network,
  e.g. IS-95, cdma2000, and W-CDMA, Soft/softer
  handoff is a well-published mean to enhance the
  link quality in the cell edge. It is a giant
  diversity system that several base stations (BSs)
  simultaneously communicate to a single mobile
  station. Reliable link performance and fade
  resistance can be attained in the handoff
  process. However, since a MS contacts to several
  BSs via different channels (codes), Soft/softer
  handoff does exact a significant cost in
  capacity.

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Handoff Overhead Management

• Goals
• Reduce the size of handoff zones
• Shift the handoff zone from heavy to light
  traffic area
• Cannot be overcome by conventional antennas,
  since the off-the-shelf antennas typically
  display very gradual roll off characteristics
• Best solution apply array of antennas to produce
  narrower beams
• Smart antennas can create beams with sharper roll
  off especially in the high traffic area, the
  unnecessary handoff zones can be reduced and
  handoff overhead is lessened. Thereby, system
  capacity is increased.

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Handoff Overhead plot
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Interference Challenge

• Pilots interference The MS usually receives
  various pilot signals from the surrounding cell
  sites and always chooses the strongest one to
  register. In some environment, the strength of
  the received pilot signals are approximately
  equal. This leads to unreliable handoff,
  synchronization problem, and moreover, reduced
  system capacity. It is not uncommon since many
  high elevation sites with RF coverage footprints
  much larger than normal sites due to
  line-of-sight. To reduce the transmitting power,
  downtilt or reduce the elevation height to the
  offending antenna seem to be the solution to
  prevent interference from too many pilots.
  However, the overall coverage is reduced.
• Multiuser interference and near-far problem

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Pilot pollution results from too many dominant
servers
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Interference Control

• It is required to propose a per-beam gain control
  scheme
• the beam that is dedicated to provide service to
  the in-building or tunnel should have a larger
  gain
• the beam that is directed to an open area should
  have lower gain in order not to induce
  interference to neighboring sites

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Per-beam gain control
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The CDMA Spatial Processing RAKE receiver

• Each RAKE finger uses the adaptive antenna to
  reject multipath component to which the finger is
  locked
• Diversity combining (MRC) is then used to combine
  the output from each RAKE finger to maximize the
  SINR for that finger

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Downlink Beamforming for CDMA (1)

• Downlink Beamforming is used to significantly
  reduce the overall transmitted power of the BS,
  since it focuses only sufficient power needed to
  meet FER requirements for a subscriber receiver
• CDMA BS transmitter uses a broad beam to provide
  universal pilot, sync, paging channels throughout
  coverage (otherwise, MSs in those areas may not
  detect that the cell is available to provide
  service)

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Downlink Beamforming for CDMA (2)

• Individual traffic channel is focused where they
  are required using Downlink Beamforming technique
• Coherent demodulation is degraded
• Traffic signals (narrow beam) are phase-modulated
  relative to pilot (broad beam)
• They may encounter phase differences, since broad
  beam encounter more scatters
• Add auxiliary pilot in cdma2000

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Conclusion (1/2)

• CDMA network capacity strain is a paradox,
  because most networks use far less capacity than
  they have available
• Key constraint on capacity is peak loading in hot
  spots. Load balancing can unlock capacity by
  reducing peak loading
• SAS can generate a set of flexible beams. In
  what follows, the pattern, beamwidth, and gain of
  each beam can be adjusted even adapted
  individually to optimize the capacity of a CDMA
  network.

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Conclusion (2/2)

• Specifically, unlike the conventional scheme, no
  tower climbing and no physical changing of
  antenna orientation are needed. All the required
  sector orientation and beamwidths are
  software-controllable, remotely configurable.