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Mainly relied on:
• To present the idea behind combining DS-CDMA systems with OFDM
Objective
Richard Van Nee, Ramjee Prasad, “OFDM For Wireless Multimedia Communications”
Presentation Layout
• CDMA Reminder/Overview
• Multicarrier Modulation Schemes
• OFDM/CDMA
• Some results: DS-CDMA vs. MC-CDMA
Classification of CDMACDMA
Pure CDMA Hybrid CDMA
DS
FH
TH
Wideband
Narrowband
Slow
Fast
Hybrid-pureCDMA
Hybrid-contentionless
CDMA
Hybrid-contentionCDMA
Hybrid-OFDM/CDMA
DS/FHFH/THDS/TH
DS/FH/TH
TDMA/CDMATDMA-FDMA/CDMA
CDMA/ISMACDMA/ALOHA
MC-CDMAMT-CDMA
Pure CDMA - Direct Sequence
• Multiple access: Coherent detection, cross-correlation among codes small
• Multipath interference: If ideal code sequence, zero out of [-Tc, Tc]
• Narrowband interference: Coherent detection, spread the interferer
• LPI: Whole spectrum, low power per Hz
Pure CDMA - Direct Sequence
PROs
• Coded signals implemented by multiplication
• Simple carrier generator• No synchronization
among users necessary
CONs
• Difficult to acquire and maintain synchronization (fraction of the chip time)
• Bandwidth limited to 10 to 20 MHz
• Near-far problem - power control needed
Pure CDMA - Frequency Hopping
• Multiple access: One user at one frequency band (FEC when not)
• Multipath interference: Responses at different hop. freqs are averaged (noncoherent combining)
• Narrowband interference: Gp hopping freqs -> 1/Gp percent of time (average)
• LPI: Low power, catch me!
Pure CDMA - Frequency Hopping
PROs
• Synchronization easier than DS (fraction of the hop time)
• Larger bandwidth (need not be contiguous)
• Better near-far performance• Higher possible reduction of
narrowband interference
CONs
• Sophisticated frequency synthesizer needed
• Abrupt changes lead to wider occupied spectrum
• Coherent demodulation difficult
Pure CDMA - Time Hopping
• Multiple access: One user at a time (FEC when not)
• Multipath interference: Signaling rate up -> dispersion -> no advantage
• Narrowband interference: 1/Gp percent of time, reduction by Gp
• LPI: Short time, catch me when, multiple users
Pure CDMA - Time Hopping
PROs
• Simple implementation• Useful when transmitter
avg. power limitted, but not peak
• Near-far is not a problem
CONs
• Long time until synchronized
• Good FEC code and data interleaving needed
Hybrid CDMA
• The goal is to combine two or more of spread-spectrum modulation techniques in order to improve the overall system performance by combining their advantages:
1. Combination of Pure CDMAs lead to 4 hybrids
2. Combination with TDMA
3. Combination with multicarrier modulation
Conventional vs. Orthogonal
Ch.1 Ch.2 Ch.3 Ch.4 Ch.6Ch.5frequency
Conventional multicarrier
1/T
Ch.1 Ch.3 Ch.5frequency
Orthogonal multicarrier
1/T
Ch.6Ch.4Ch.2
Transmitter
IFFT
b (=RT)bit buffer
andEncoder
DAC1/T' = N/T
data stream R bps P/S
N QAMsymbols
2N timedomain
samples
LPF
Time-frequency occupancy
T’-symbol period; J symbols in parallel; T-OFDM symbol period (in practice T = J*T’ + Tg)
symbol 1
frequency
time
symbol J
T=JT'T'
OFDM
PROs• Efficient way to deal with
multipath• Possibility to enhance the
capacity• Robust against narrowband
interference• Single-frequency networks
possible
CONs• More sensitive to frequency
offset and phase noise• Large PAPR
Why Multicarrier CDMA ?
• Robust to frequency-selective fading (OFDM)• Robust to frequency offsets and nonlinear
distortion (DS-CDMA)• Fast FFT/IFFT devices• Good frequency use efficiency• OFDM/CDMA can lower the symbol rate in
each subcarrier, so longer symbol duration makes quasisynchronization easier
Multicarrier CDMA flavors
• Multicarrier CDMA : MC - CDMA
• Multicarrier direct sequence CDMA : MC - DS - CDMA
• Multitone CDMA : MT - CDMA
MC - CDMA
X
Serialto
ParallelConver
ter
1/J
data stream
b k(t)T' - symbol period
b k,1(t)
T=J*T'
b k,j(t)
b k,J(t)
X
X X
C k1
C kM
cos(2pi f j,1 t)
cos(2pi f j,M t)SUM
SUM
IDFFT
User K; J BPSK (T’) symbols are grouped (T=J*T’); each spread by C=(Ck1,…,CkM) in frequency domain; separation between adjacent carriers = 1/T
Time-frequency occupancy
T’-symbol period; J symbols in parallel; T-OFDM symbol period (T = J*T’ + Tg); J*M total # of carriers
symbol 1 chip 1
frequency
time
symbol n chip M
o
symbol n chip 1
o
T=JT'T'
MC - DS - CDMA
X
Serialto
ParallelConver
ter
1/J
data stream
b k(t)T' - symbol period
b k,1(t)
b k,j(t)
b k,J(t)
X
X X
C k(t)
C k(t)
cos(2pi f j,1 t)
cos(2pi f j,M t)SUM
SUM
IDFFT
User K; J BPSK (T’) symbols are grouped (T=M*J*T’) M times longer; M identical branches of each symbol are spread by Ck(t)=(Ck1,…,CkN) in time domain; N-processing gain; separation between adjacent carriers N/T; total # of carriers is J*M
Time-frequency occupancy
T’-symbol period; J*M symbols in parallel; T-OFDM symbol period (T = M*J*T’ + Tg); J*M total # of carriers
symbol 1 sequence 1
frequency
time
symbol n sequence M
o
symbol n sequence 1
o
T=MJT'T'
Midenticalsymbols
MT - CDMA
User K; J BPSK (T’) symbols are grouped (T=J*T’); each spread by signature waveform Ck(t)=(Ck1,…,CkN) in time domain; separation among carriers = 1/T prior to spreading! - after spreading spectrum overlaps more densely
X
Serialto
ParallelConver
ter
1/J
data stream
b k(t)T' - symbol period
b k,1(t)
T=J*T'
b k,j(t)
b k,J(t)
X
C k(t) cos(2pi f j t)
SUM
IDFFT
Time-frequency occupancy
T’-symbol period; J symbols in parallel; T-OFDM symbol period (T = J*T’ + Tg); J total # of carriers
frequency
time
T=JT'T'
symbol nspreadacrossseveralcarriers
‘MT’ - CDMA
BPSK(T’) streams; N users; each spread by its own signature Ck(t)=(Ck1,…,CkL) in time domain; orthogonal; M user bits per OFDM symbol to transmit (M*T’) (L chips per bit); all users across all carriers; total # of carriers M*L
X
Serial toParallel
Converter
1/LM
X
X
X
C1(t)
C N(t)
cos(2pi f1 t)
cos(2pi fLM t)
SUM
SUM
IDFFT
data stream - d1
data stream - dN
Interleav
er
Time-frequency occupancy
T’-symbol period; M*L symbols in parallel; T-OFDM symbol period (T = M*T’ + Tg); M*L total # of carriers
frequency
time
T=MT'T'
symbol nspread
across allcarriers
Remarks
• The M identical information bearing branches in MC-CDMA and MC-DS-CDMA is to increase frequency diversity
• Carrier separation big enough => uncorrelated fading• J must be large enough to insure that each subchannel be
frequency non-selective• MC-CDMA needs reliable carrier and phase recovery -
coherent modulation• MC-DS-CDMA and MT-CDMA better with non-coherent• MT-CDMA has much denser spectrum, more susceptible to
MAI and ICI
Assumptions:
• fast Rayleigh fading channel (WSSUS)• L received paths• Synchronous downlink channel +
quasisynchronous uplink• Perfect synchronization, no frequency offset, no
nonlinear distortion, perfect phase estimation (OFDM)
• Perfect path gain estimation and carrier sync. (DS-CDMA)
Assumptions:
X
Serialto
ParallelConver
ter
1/J
data stream
b k(t)T' - symbol period
b k,1(t)
T=J*T'
b k,j(t)
b k,J(t)
X
X X
C k1
C kM
cos(2pi f j,1 t)
cos(2pi f j,M t)SUM
SUM
IDFFT
Assumptions:
Numerical values used in simulations:
• Delay spread 20ns• Doppler power spectrum with max fd = 10Hz• Transmission rate R = 3Msyb/sec (BPSK)• MC-CDMA - Walsh Hadamard K=32• DS-CDMA - Gold K=31
Conclusions:• It can be shown that as long as we use the same
frequency-selective fading channel, the BER lower bound is the same for both DS-CDMA and MC-CDMA
• MC-CDMA has no major advantage in terms of signal bandwidth, as compared with DS-CDMA (although when Nyquist filters are used within DS-CDMA, RAKE may wrongly combine paths)
• Also, the number of users in the system depends on the combining strategy for MC-CDMA and on RAKE finger number for DS-CDMA
Downlink:
• It may be difficult for DS-CDMA RAKE to employ all the received signal energy scattered in time domain, whereas MC-CDMA receiver can effectively combine all the received signal energy scattered in the frequency domain
• MMSEC based MC-CDMA - Minimum Mean Square Error Combining (error in the estimated data symbols must be orthogonal to the baseband components of the received subcarriers)
• MMSEC MC-CDMA is promising although noise power estimation and subcarrier references are required
Uplink:
• As compared with the DS-CDMA scheme, MMSEC MC-CDMA performs well only for the single user case (code orthogonality among users is totally distorted by the instantaneous frequency response)
• Multiuser detection scheme is required which jointly detects the signals to mitigate the nonorthogonal properties
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