If we use Multiple antennas at the transmitter as well as receiver. We also apply turbo codes and or bit-interleaved coded modulation with soft input soft output Viterbi decoders as the baseband techniques for both CDMA and OFDM, then which one is more reliable and give better rates in actual field scenarios. 3G and EVO has a more successful experience. Any comment? OFDM relies on FFT which is efficient but still require complex additions and multiplications. In case of equalizer, I guess the number of multiplications and additions are usually less as compared to FFT.
Well it depends on the following and may be many other factors like
Metric used for comparison
Channel conditions
System complexity, etc..
Mostly the comparison is done in terms of TSE(Transport Spectral Efficiency) defined as the distance weighted sum rate of all users in the cell per unit frequency.
If i compare 4 schemes as under
CP-OFDM, ZP-OFDM, CDMA-LMMSE, CDMA-RAKE then we get the following comparisons for TSE
Transport Spectral Efficiency:
(Flat fading) The TSE of all four schemes are approximately the same;
(Multipath fading) CP-OFDM>CDMA-LMMSE=ZP-OFDM>>CDMA-RAKE;
Complexity:
CDMA-LMMSE=ZP-OFDM>>CP-OFDM=CDMA-RAKE;
Base station density needed to achieve optimal TSE:
CDMA-RAKE
It depends on the channel.
The OFDM divides the entire bandwidth into subchannels (subcarrier) to do solve the issue of frequency-selective channel.
Regards.
Reliability factors for CDMA and OFDM based systems varies from time to time. and from scenario to scenario.
Well it depends on the following and may be many other factors like
Metric used for comparison
Channel conditions
System complexity, etc..
Mostly the comparison is done in terms of TSE(Transport Spectral Efficiency) defined as the distance weighted sum rate of all users in the cell per unit frequency.
If i compare 4 schemes as under
CP-OFDM, ZP-OFDM, CDMA-LMMSE, CDMA-RAKE then we get the following comparisons for TSE
Transport Spectral Efficiency:
(Flat fading) The TSE of all four schemes are approximately the same;
(Multipath fading) CP-OFDM>CDMA-LMMSE=ZP-OFDM>>CDMA-RAKE;
Complexity:
CDMA-LMMSE=ZP-OFDM>>CP-OFDM=CDMA-RAKE;
Base station density needed to achieve optimal TSE:
CDMA-RAKE
MIMO systems normally refer to using multiple antennas both at the transmitter and the receiver for improving system performance since each antenna is sufficiently separated from each other in space, the signal will be uncorrelated, hence at least one of these signals will be of sufficient quality for successfull system performance. Depending on the
Apparently, if you want to achieve better system performance and create the flexible air interface, with dense adjustment to current conditions, OFDM system is more preferable. Spectrum division into subcarriers allows OFDM system to allocate the exact amount of spectrum needed to perform certain connection. CDMA system is not capable to provide such flexibility.
Thanks to existing processing capabilities, complexity of OFDM is not an issue now. The hardware performance growth much quicker, than signal processing algorithms complications.
MIMO refers to the use of multiple antennas (or any other means) to produce statistically independent signal replicas at the receiver. In the case of, particularly, frequency selective multipath fading channels where the channel impairments and ISI are the most severe, the probability of all the copies of statistically independent signals at the receiver to be severely effected at any time is very low. Therefore, the receiver will have to use these signal copies in order to resolve the channel paths and use them in detecting.
The CDMA system is inherently suitable for benefiting from the diversity due to the use of RAKE receiver for signal resolving in time and detection provided that the system sampling time is small enough to resolve all channel paths. A suitable equalisation technique cane be used to detect each temporal component successfully.
In OFDM, the statisticallt independent multiple copies of signals are provided in frequency domain and these signals are resolved successfully provided that the frequency resolution of the OFDM system is sufficiently high to separate each frequency component successfully. A suitable equalization technique can be applied to detect each frequency component successfully.
In conventional OFDM systems . One can benefit from both Frequency diversity( adapting Frequency Diverse Scheduling ), multi user diversity(adapting Frequency Selective Scheduling) and Spatial Diversity (MIMO) . hence these systems can be very reliable. actually these methods performances are good enough in fading channel to the extend that most communication systems can move from spatial diversity into spatial multplexing to enhanced the available bandwith.
In regard of complexity, FFT is not a big issue these days as all of the 4G systems(LTE, Wimax) are based using OFDM and OFDMA in their downlink and it is easily implemented in practise.
As you are working on MIMO and coding give extra attention to the effect of your code rate on the DOF of your MIMO system as it has great impact and normaly researchers who work solely on coding forget to take it into account.
If you have a selective fading channel (or, equivalently, a channel with delay spread), OFDM is much easier to be used in combination of MIMO in spatial multiplexing mode. You can treat each sub-carrier as a flat-fading channel and use the standard MIMO technology for each sub-carrier. If you use CDMA, you probably need a generalized DFE to process at the receiving end. This is especially difficult because you use a strong channel code, so your receiver cannot perform immediate decision feedback.
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