Broadband CDMA Techniques Fumiyuki1 Adachi Wireless Signal Processing

1
Broadband CDMA TechniquesFumiyuki1 Adachi
Wireless Signal Processing Networking (WSPN)
Lab.Dept. of Electrical and Communications
Engineering,Tohoku University, JapanE-mail
adachi_at_ecei.tohoku.ac.jphttp www.mobile.ecei.toh
oku.ac.jp/
4G Forum, King's College London, 3 June 2005

• OUTLINE
• Wireless Evolution
• Broadband CDMA

2
Wireless Evolution

• From 2G to 3G
• Then into 4G

3
Wireless Evolution

• Every one wants to communicate instantly any type
  of information with anyone, any time from
  anywhere
• Arrival of ubiquitous society means of
  communication is everywhere
• This is only possible by wireless. Wireless is
  indispensable in our forthcoming ubiquitous
  society
• Every 10 years, a new wireless technology has
  changed our society
• 1980s from point-to-point to anytime,
  anywhere communication
• 1G systems (analog)
• 1990s from voice to any type of information
• 2G systems (digital)
• Access to the Internet
• 2000s wideband data
• 3G systems (wideband) and then 3.5G systems (high
  speed packet)
• 2010s broadband data, ubiquitous
• 4G systems (broadband packet)
• Roaming among heterogeneous networks

4
Cellular Systems Evolution Path

• Have evolved from narrowband to wideband and now,
  into broadband
• On the way to broadband wireless network
• Core is the wireless technology

We are here
5
Internet Access

• In line with the increasing popularity of
  Internet in fixed networks, convergence of
  wireless, computing and Internet is on the way to
  offer the mobile users a convenient access to the
  Internet from anywhere, at anytime
• Cellular systems are evolving from simply
  providing traditional voice commun. services to
  providing broadband multimedia services
• Internet cellphones_at_ March. 2005 (source TCA)
• Total mobile (cellularPHS) users
• 91.47m (penetration 71.6)
• Users connected to Internet 75.15m (86.4)
• i-mode44.02m
• Ezweb18.26m
• Vodaphone live12.87m

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Shift To 3G Systems Is On Going

• 2G (56,644,900)
• PDC55,037,400
• cdmaOne1,607,500
• 3G (30,352,700)
• W-CDMA12,417,800
• CDMA2000 1x
• 17,934,900

As of March 2005
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Cellular Systems Evolution

• It is quite difficult to predict which services
  will become popular in the coming 10 years
• However, it is no doubt that Packet services will
  dominate in 4G
• Even 14Mbps data rate capability will sooner or
  later become insufficient to cope with the
  increasing demands for broadband multimedia
  services

8
Evolution to 4G

• User wants to have much higher speed than 3G
• Video conversation together with high quality
  voice will be a promising wireless service

9
Broadband CDMA

• F. Adachi, D. Garg, S. Takaoka, and K. Takeda,
  Broadband CDMA techniques, IEEE Wireless
  Commun. Mag., Vol. 12, No. 2, pp. 8-18, April
  2005.
• F. Adachi, T. Sao, and T. Itagaki, Performance
  of multicode DS-CDMA using frequency domain
  equalization in a frequency-selective fading
  channel, Electronics Letters, vol. 39,
  pp.239-241, Jan. 2003.
• F. Adachi and K. Takeda, Bit error rate analysis
  of DS-CDMA with joint frequency-domain
  equalization and antenna diversity combining,
  IEICE Trans. Commun., vol.E87-B, no.10,
  pp.2991-3002, Oct. 2004.

10
Propagation Channel Model

• Deep understanding of propagation mechanism is
  necessary for system development.
• The transmitted signal is reflected and
  diffracted by buildings, resulting in a multipath
  channel.

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Time-domain Modeling

• Many impulses are received with different time
  delays t when one impulse is transmitted from a
  transmitter at time t.
• Such a multipath channel can be viewed as a time
  varying linear filter of impulse response h(t,t).
  
• Doubly (frequency-time) selective fading channel

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Channel Characterization

• Channel frequency response randomly varies both
  in frequency and in distance.
• Challenge is to transmit data at high speed
  (close to 1 Gbps) with high quality under such a
  severe doubly selective fading environment.

16-path exponential profile, delay factor of 1.0
dB, time delay separation of 150ns, carrier
frequency of 5 GHz, moving speed of 4km/h
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CDMA Techniques

• CDMA can overcome the channel frequency-selectivit
  y and even improve the transmission performance,
  yet retaining multiple access capability
• DS-CDMA Time domain spreading
• MC-CDMA Frequency-domain spreading
• DS-CDMA single-carrier/time-domain spreading
• MC-CDMA multi-carrier/frequency-domain spreading

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Bandwidth Requirement

• Bandwidth for 100M1G transmissions is about
  100MHz?
• With MIMO techniques, it may be less than 100MHz
• Two possible CDMA approaches
• Both single- and multi-carrier-CDMA approaches
  have the flexibility of providing variable rate
  transmissions, yet retaining multiple access
  capability.
• DS-CDMA
• Chip rate about 100Mcps
• Multipath resolution100ns (30meters)
• MC-CDMA
• FFT/IFFT sampling rate100MHz

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DS vs. MC
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Multi-Carrier Approach

• A number of orthogonal subcarriers is used for
  parallel transmission
• Frequency efficient transmission than single
  carrier transmission
• MC-CDMA is a combination of OFDM and CDMA
• a simple one-tap FDE
• Very robust transmission against frequency
  selectivity

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• MC-CDMA

18

• One-tap frequency-domain equalization (FDE) to
  exploit the channel frequency-selectivity
• Minimum mean square error (MMSE) weight provides
  the best BER performance in a multi-user
  environment

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DS-CDMA vs. MC-CDMA

• Performance of DS-CDMA with Rake significantly
  degrades due to IPI
• BER floor increases as the no. of paths , L,
  increases
• On the other hand, MC-CDMA with MMSE-FDE provides
  much better performance
• Performance improves as L increases

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Single-Carrier Approach

• DS-CDMA

Time-domain spreading
c(t)
Data
Data modulation
Chip shaping
Channel coding interleaving
(a) Transmitter
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Limitation of DS

• Limitation of time-domain Rake
• Number of resolvable paths increases as the
  transmission rate increases. This increases
  inter-path interference, thereby degrading the
  BER performance
• Finite number of Rake fingers cannot collect all
  transmitted power

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Application of FDE

• One-tap FDE can replace Rake combining to have
  much improved performance
• Insertion of guard interval (GI) at the
  transmitter
• FFT/IFFT at the receiver

(a) Transmitter
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FDE vs. Rake

• As the spreading factor SF becomes smaller
  (higher data transmission rate)
• BER performance degrades with Rake combining
• On the other hand, BER performance with MMSE-FDE
  improves and produces no BER floor
• Complexity of MMSE-FDE does not depends on the
  no. of paths L while that of Rake grows as L

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DS-CDMA with FDE vs. MC-CDMA with FDE

• Single user case (C1)
• DS-CDMA is better than MC-CDMA since it can
  benefit from a larger frequency- diversity effect

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• Multi-user case (Cgt1)
• Both CDMA provide almost the same BER performance
  for the downlink (base-to-mobile)
• Multi-user interference (MUI) is predominant
  cause of errors than self interference

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Flexible CDMA Network
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Is It Necessary To Spread?

• When Spread
• Spreading allows multiple users to communicate at
  the same time.
• Total throughput is divided to each user, hence,
  each users throughput is lower.
• Each user is provided continuous transmission but
  the time taken for transmission is longer.
• This may be optimum for real time communication
  with a certain data rate.
• When not spread
• High throughput is given to a single user at each
  moment.
• After completing the transmission of one user,
  channel is assigned to another user.
• Users need to wait for channel being assigned.
• This scheme requires scheduling and may be
  optimum for non-real time data communication.

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Flexible CDMA Network

• The use of OVSF spreading codes allows
  construction of spread and non-spread systems for
  real time and non-real time services
• Cellular(SFgt1)
• Real time and non-real time services with
  relatively low data rate per user
• Hot spot areas (SF1)
• Non-real time services with very high data rate
  per user are provided by random TDMA system with
  appropriate scheduling.
• An SF1 system can be extended to a cellular
  system with the aid of fast selection of transmit
  cell and adaptive antenna array.

F. Adachi, M. Sawahashi, and K. Okawa, Electron.
Lett., vol. 33, pp. 27-28, Jan. 1997.
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Cellular System Isolated-cell System

• Different PN scramble codes are assigned to
  different cells to distinguish different cells.
• The same set of OVSF codes is used to configure
  channels in all cells.

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Conclusion

• Next generation network is a broadband packet
  network and requires Giga-bit wireless technology
  of 1Gbps capability
• Frequency-domain equalization technique can
  improve the transmission performance
• Either CDMA or OFDM can be used since both can
  provide similar performance
• Other important techniques
• HARQ for improved packet transmissions
• Adaptive modulation coding (AMC)
• Scheduling multi-user diversity
• MIMO, etc.