OFDM-CDMA
Outline
OFDM-CDMA - Introduction
OFDM-CDMA system
Conceptual Implementation
Coding
PAPR in OFDM-CDMA Systems
Multi-antenna OFDM-CDMA
Comparison with Other Technologies
OFDM-CDMA vs. MC-CDMA
OFDM-CDMA vs. MC-DS-CDMA
Conclusion
References
Introduction
•Consider an 8-bit data “word.”. The user data bits are initially fed
into S/P converter.
•After this, each parallel bit is subjected to an 8-bit Walsh spreading
using the code for that particular user in the FD.
•Unlike MC-CDMA where the
spread signal is not summed,
but subjected to an 8-point
OFDM modulation
•In the receiver, an FFT is
carried out, followed by RAKE
combiners for summing.
[1]
OFDM-CDMA system
OFDM-CDMA transmitter with channel coding
[2]
OFDM-CDMA system (contd.)
From each sub-system M data bits are serial to parallel converted
producing ML / K parallel output streams of duration MT
b
seconds.
These streams are sent to the frequency interleaver which
scrambles all of the outputs and passes them onto the OFDM
modulator.
The frequency interleaver attempts to separate chips from the same
data bit further apart than the coherence bandwidth to achieve
frequency diversity. The total number of carriers is ML.
Finally a guard interval ∆, is inserted which is longer than the delay
spread in the channel to prevent ISI and ICI. This gives a total
symbol duration of T = MT
a
+ ∆.
[2]
