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A Multicarrier CDMA Architecture - Presentation

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A Multicarrier CDMA Architecture - Presentation Powered By Docstoc
					A MULTICARRIER CDMA ARCHITECTURE BASED ON
ORTHOGONAL COMPLEMENTARY CODES FOR NEW GENERATION
OF WIDEBAND WIRELESS COMMUNICATIONS


•   BY: COLLINS ACHEAMPONG
•       GRADUATE STUDENT



•   TO: Dr. Lijun Quin
•       DEPT OF ELECTRICAL ENGINEERING
•       PRAIRIE VIEW A&M UNIVERSITY
•       PRAIRIE VIEW, TEXAS

•    A RESEARH PROJECT
•    WIRELESS NETWORKS
•    ELEG:6203
•    FALL 2003 SEMESTER
                           OVERVIEW



•   ABSTRACT
•   INTRODUCTION
•   PROBLEM STATEMENT
•   METHOD OF SOLVING PROBLEM
•   BASIC CODE DIVISION MULTIPLE ACCESS (CDMA) SYSTEM
•   SPREAD SPECTRUM FUNDAMENTALS
•   COMPLETE COMPLEMENTARY CODES
•   MULTIPATH SELF-INTERFERENCE
•   ANALYSIS OF MODEL PARAMETERS AND RESULTS
•   LIMITATIONS, ADVANTAGES, AND REMARKS
•   CONCLUSION
                                     ABSTRACT



•   The proposed Multicarrier Code Division multiple access (MC-CDMA) has a great
    potential for applications in future wideband mobile communications beyond Third
    Generation (3G), which is expected to offer a very high data rate in hostile environment.

•   The paper is a continuation of on going research effort to construct a Multicarrier
    CDMA architecture based on orthogonal complementary codes, characterized by its
    unique spreading modulation scheme, uplink and downlink signature design, and
    receiver implementation for multipath signal detection.

The ongoing research was first conducted by:
 Hsiao-Hwa, National Sun Yat-Sun University
 Jun-Feng, National Chung Hsing University
 Naoki Suehiro, Yeh, National University of Tsukuba
                               INTRODUCTION


•   Code Digital Multiple Access (CDMA) is a predominant multiple access
    technique proposed for the 3G wireless communication systems worldwide.
•   The maturing of 3G mobile communication technologies from concepts to
    commercially deliverable systems motivates us to think about the possible
    architectures for future generations of mobile wireless.
•   The question is how to guarantee such high data rate in highly unpredictable
    and hostile channels?
•   What types of air link architecture are qualified to deliver such high-data-rate
    services?
•   The new MC-CDMA architecture that has a great potential for future mobile
    communications tackles this issues comprehensively.
•   The new CDMA architecture ought to be technically feasible with current
    available digital technology.
•   The proposed CDMA system should preferably have an inherent ability to
    mitigate path problems in mobile channels.
• It can achieve spreading efficiency (SE) very close to one
• It offers Multiple Access Interference (MAI) free operation in both up
  and down link transmission in a MAI-AWGN (Additive White
  Gaussian Noise).
• Able to offer high bandwidth efficiency due to its use of unique
  spreading modulation scheme.
• It is particularly suited to multi-rate signal transmission due to the use
  of an offset stacked modulation scheme.
                     Mothod of Solving Problem


•   Use of direct sequence orthogonal complete complementary codes.


•   The bit error rate (BER) of the proposed CDMA system under MAI and
    Additive White Gaussian Noise (MAI-AWGN) is evaluated using computer
    simulations.

•    The obtained BER performance of the new CDMA system will be compared
    to the conventional CDMA system using Gold codes and m-sequences under
    identical operation environments
     BASIC CODE DIVISION MULTIPIPLE ACCESS (CDMA)
                        SYSTEM


•   CDMA IS A SPREAD SPECTRUM COMMUNICATIONS TECHNIQUE
    THAT SUPPORTS SIMULTANEOUS DIGITAL TRANSMISSION
•   CDMA IS SIMILAR TO THAT OF FDMA (FREQUENCY DIV.
    MULTIPLE ACCESS) AND TDMA (TIME DIV. MULTIPLE ACCESS).
•   CDMA HAS THE UNIQUE PROPERTY OF SUPPORTING A
    MULTIPLICITY OF USERS IN THE SAME RADIO CHANNEL
•   CDMA HAS GRACEFUL DEGRADATION IN PERFORMANCE DUE TO
    MULTI-USER INTERFACE
•   FREQUENCY RE-USE FACTOR IN CDMA CELLULAR ENVIRONMENT
    CAN BE AS HIGH AS UNITY.
•   CDMA BEING A WIDEBAND SYSTEM CAN CO-EXIST WITH OTHER
    NARROWBAND MICROWAVE SYSTEMS
•   THE MOST ADVANTAGE OF CDMA IS ITS ABILITY TO COMBAT
    FROM MULIPATH FADING.
• CDMA system allows unique pseudo-noise codes to individual users
• Any user is allowed to access the air at ant time
• Spreading efficiency (SE) for all conventional CDMA equals 1/N
  which is less than one.
• Low rate data symbols are multiplexed by CDMA code resulting in a
  wideband waveform – “a process known as spreading”
• Multi-code methods assigns one or more codes with fixed spreading
  factor to any one user.
• This results in a waveform with high peak power.
• A users requiring high throughput is allocated a CDMA code with a
  low spreading factor
• CDMA codes may be time slotted to regulate access.
                     Figure 1. CDMA in the code and time domain
CDMA
Code space
              S0 S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11S12 S13 S14 S15 S16
     User 0


    User 1
    User 2
    User 3


     Time

                Figure 1: CDMA in the code and time domain
                 Spread Spectrum Fundamentals

•   Originally develop for military applications to provide anti-jam.
•   Qualcom Inc., developed it in the late 1980’s and early 1990’s which led to
    IS-95 cellular standard.
•   Spread spectrum characteristic:
•   Bandwidth used is much greater than the message bandwidth.
•   Bandwidth spreading is achieve by using a spreading code or (pseudo-
    noise) p-n sequence, while the spreading code is independent to the
    message signal.
•   Two types of spread spectrum are:
•   Direct sequence (DS) and Frequency hopped (FH) spread spectrum.
•   DS uses p-n sequence to introduce rapid phase transition into the carrier
    containing the data.
•   FH uses p-n sequence to pseudo randomly hop the carrier frequency
    throughput a large band.
•   The original information signal which occupies a bandwidth of B Hz is transmitted
    after spectral spreading to the bandwidth N times higher.
•   N is known as the processing gain typically in the range of 10-30 dB.
•   Power of transmitted spread spectrum is spread over N times the original
    bandwidth while its density is correspondingly reduced by the same amount.
•   Hence, the processing gain is given by N = Bs/B, where Bs is the bandwidth of
    spectral signal while B is the original information signal.
•   The unique technique of spreading is the key to improving its detection in a mobile
    radio environment.
•   This allows narrowband signals exhibiting higher spectral density to share the same
    frequency band.
•   Hence, the spectral efficiency is defined as:
           Rd
             bits / s / Hs
           Brf

where Rd is the transmitted data rate, and Brf the system bandwidth.
         COMPLETE COMPLEMENTARY CODES


•   Use of direct sequence orthogonal complete complementary codes
•   The orthogonal complete complementary code is based on a “flock” of
    element codes jointly instead of a single code as in traditional CDMA.
•    Each user in the MC-CDMA system will be assigned a flock of element
    codes as its signature code.
•   Each bit is spread by one single continuous code comprising of N
    contiguous chips to attain certain processing gain or spreading factor.
•   User that require higher throughput is allocated more codes with lower
    spreading factor
•   Codes ought to be transmitted possibly via different frequency channels,
    and arrive at a receiver at the same time.
•   The information bits correlator are spreading modulated by element
    codes that are “offset stacked”, each shifted by one chip relative to one
    another.
•   Every signature code is spread into several segments (or element code)
•   New bits will start right after one chip is delayed relative to the previous
    bits.
•    Bit is spread by element code length L.
•   The bit error rate (BER) under the MAI and AWGN is evaluated using
    computer simulations
 COMPLETE COMPLEMENTARY CODES

Element code length L, = 4 Element code length L = 16
Processing gain are equal to 4x2 = 8 and 16x4 = 64 respectively
   Flock 1    A0: + + + -   Flock 1   A0: + + + + + - + - + + - - + - - +
                                      A1: + - + - + + + + + - - + + + - -
                                      A2: + + - - + - - + + + + + + - + -
                                      A3: + - - + + + - - + - + - + + + +

              A1: + - + +   Flock 2   B0: + + + + - + - + + + - - - + + -
                                      B1: + - + - - - - - + - - + - - + +
                                      B2: + -+ - - - + + - + + + + - + - +
                                      B3: + - - + - - + + + - + - - - -

   Flock 2    B0: + + - +   Flock 3   C0 : + + + + + - + - - - + + - + + -
                                      C1: + - + - + + + + - + + - - - + +
                                      C2: + + - - + - - + - - - - - + - +
                                      C3: + - - + + + - - - + - + - - - -

              B1: + - - -   Flock 4   D0: + + + + - + - - + + + - - +
                                      D1: + - + - - - - - - + + - + + - -
                                      D2: + + - - - + + - - - - - + - + -
                                      D3: + - - + - - + + - + - + + + +


  Table 1. Two examples of complete complementary codes
  with element code lengths L = 4 and L = 16
    Performance Analysis Under Multiple Access Interference

•   Compared with traditional spreading modulation used in conventional
    CDMA systems, the new system has the following salient features:
•   The new system is no longer aligned in time one bit after another.
•    Instead a new bit will start right after one chip delay relative to the
    previous element L.
•   The unique offset stacked spreading method can easily slow down data
    transmission by simply shifting more than one chip (at most L chips)
    between two neighboring offset stacked bits.
•   Thus, the MC-CDMA architecture is capable of delivering much higher
    bandwidth efficiency than a conventional CDMA architecture under the
    same processing gain.
•    The “inherent” ability of the MC-CDMA system to facilitate multirate
    transmissions is based on its innovative offset stacked spreading
    technique, which cannot be applied to traditional spreading codes.
•   The MAI-independent property is significant in terms of its potential to
    enhance its system capacity in a multipath channel in MC-CDMA.
                   Multipath Self-Interference

• CDMA rake receiver achieves perfect diversity gain from multipath
  reception if it can perfectly decouple the different paths.
• In practice however, each path acts as interference for other paths.
• Interference becomes especially significant if the CDMA signal is
  transmitted over many code channels or the spreading factor is small.
• Conventional CDMA receiver usually uses a Rake receiver to collect
  dispersed energy among different reflection paths to achieve multipath
  diversity at the receiver.
• Therefore, the Rake receiver is a must for all conventional CDMA
  systems, including current operational 2G and 3G systems.
• In MC-CDMA architecture, the Rake receiver becomes inappropriate
  due to the nature of the unique spreading modulation technique
  employed in the system.
                               Power Control


•   CDMA technology requires the use of power control for subscriber terminals.
•   The effectiveness of power control determines the capacity of the network.
•   In CDMA if one terminal transmit excessive power, it increases the
    interference on all other remaining terminals and reduces their link quality.
•   In ODFM power control is not as critical as CDMA.
•    There is no fundamental requirement for the use of power control since the
    user terminals in the same sector do not share the same frequency at the same
    time.
•   Reason for using power control in an OFDM system is to minimize co-channel
    interference.
     A Multicarrier CDMA Architecture




Figure 10: MC-CDMA simulation used for forward traffic
         ANALYSIS OF MODEL PARAMETERS AND
                      RESULTS




                                        Figure 13: The simulation in operation
This is the start of the simulation for MC-CDMA (with BER values shown) in Figure 13.
Comparison of MC-CDMA and CDMA Spread Model
               CDMA and MC-CDMA Comparison Summary
                                     MC-CDMA                                    CDMA

Spectral efficiency                  More efficient                             Less efficient

Multipath                            Handles larger number of paths             Some diversity benefits; performance
                                                                                sensitivity to number of path coherently
                                                                                combined


Multiple Modulation support          Downlink and Uplink                        Downlink only


Resistance      to      Narrowband   Suited     for     multirate     signal    Spread spectrum provides that
Interference                         transmission due to the use of an offset
                                     stacked spreading modulation scheme

Network Planning                     It simplifies the rate-matching            Difficult for cell overlays; PN offset
                                     algorithm relevant to multimedia           planning cell-breathing complications
                                     services and asymmetric traffic in up
                                     and downlink transmission for IP-
                                     based applications
Power Control                        Required function                          Required function

Peak-to-Average Ratio                Varies                                     Varies; can be as high as 11dB

Standards Adoption                   WLAN, 3G, 4G                               WLAN, 3G

Cost                                 Less expensive                             Varies, may depend modem.
 The Bit Error Rate (BER) Test Results for MC-CDMA


TX Error       1   2         3         4         5         6         7
Rate
Calculation


BER            0   0.00072   0.00833   0.00227   0.00077   0.00091   0.00119
Error Count    0   0         1         2         3         4         6

Sample Count 0     80        120       880       3880      4360      5040

Total Sample                                                         T=0.085
Time                                                                 secs
      The Bit Error Rate (BER) Test Results for CMDA

Number of     1   2      3        4        5         6
TX Error
                                                              7
Rate
Calculation
BER           0   0      0.0174   0.0290   0.03516   0.0195   0.01896

Error Count 0     0      30       85       127       131      137

Sample        0   1032   1720     2924     3612      6078     7224
Count
Total                                                         T=0.092
Sample time                                                   secs
                                Limitations

   There exit some technical limitations for the proposed cc-codes based MC-
    CDMA which ought to be addressed.
   If a long cc-code is employed in the MC-CDMA system, the number of
    different levels generated from the baseband could be a problem.
   For example, cc-codes with L = 4 will generate five possible levels from the
    “offset stacked” spreading; 0, ±2 ,and ±4.
   For cc-code with L=16 becomes 0, ±2, ±4 ±16, comprising 17 different levels.
   In general, it will yield L+1 different levels.
   Another concern is with the cc-code based CDMA is that, a relatively small
    number can be supported by a family of cc-codes.
   For cc-codes family with L = 64, only eight flock of codes, each of which can
    be assigned to one channel. If more users should be supported, long cc-codes
    have to be used.
   The cost for introducing a multilevel digital modem may be expensive varies
    depending on a user’s requirements.
                           Advantages and Remarks


   The MC-CDMA can achieve spreading efficiency very close to one.
   It offers multiple access interference (MAI free operation in both up and down link
    transmission in MAI-AWGN channel.
   It can significantly reduce reduce the co-channel interference responsible for capacity
    decline in CDMA systems.
   Able to offer high bandwidth efficiency due to its use of unique spreading modulation
    scheme.
   Particularly suited for multi-rate signal transmission due to the use of an offset stacked
    modulation scheme.
   One possible solution to the MC-CDMA problem is to introduce a of multilevel digital
    modem capable of transmitting L+1 different levels in a symbol duration. An L+1
    quadrature modulation (QAM) digital modem can be a suitable choice for its robustness
    in detection efficiency.
   Another possible solution is to introduce frequency division on top of the code division
    in each frequency band to create more transmission channels.
                                  Conclusion


•   The paper addresses several advantages of the MC-CDMA over conventional
    CDMA such as higher bandwidth efficiency, MAI-free operation in both
    synchronous and asynchronous MAI_AWGN channels.
•   Also reduces co-channel interference and capacity increase in mobile cellular
    system.
•   The paper addresses technical limitations of the new MC-CDMA architecture,
    such as relatively small family codes and the need for complex multilevel
    digital modems.
•   Both CDMA and the MC-CDMA both have their respective advantages.
•   Nevertheless, the proposed MC-CDMA architecture based on complete
    complimentary codes offers a new option to implement future wideband
    mobile communications beyond 3G.
•   Future research work on this paper will be needed to be continued as I could
    not find all the answers to limitation problems.
                              References


•   1. Hsiao-Hwa Chen, Jun-Feng Yeh, Noaki Suehiro, “A Multicarrier CDMA
    Architecture Based on Orthogonal Complementary Codes for New Generation
    Wideband Wireless Communications,” IEEE Communication Magazine,
    October 2001, Page 126-134.
•   2. P. Nicopolidis, M.S. Obaidat, and A. S. Pomportsis, “Wireless Networks,”
    Institute of Electrical Engineers, Inc, New York, NY 2001; Pages 365-389.
•   3. Young-Hwan You, Won-Gi Jeon, Jung-Work Wee, and Hyeok-Koo Jung,
    “Effect of Diversity Technique on Performance of OFDM-CDMA Base
    Broadband Wireless Access Networks,” IEICE Transaction Communication,
    Vol. E86-B. No.4, April 2003, Pages 1402-1404.
•   4. Lajos Hanzo, Peter J. Cheerriman, Jurgen Streit, “Wireless Video
    Communications: second to third generation systems and beyond,” IEEE, Inc.
    Park Avenue, 17th Floor, New York, NY 10016-5997, pages 365-368, 378-
    389.
•   5. Israel Vincentzio, “Broadband Wireless Access Solutions Based on OFDM
    Access in IEEE 802.16,” IEEE Communication Magazine, April 2002, pages
    96-99.

				
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