DS-CDMA is divided into uplink and downlink communications.
As for uplink, the users suffer from different channel impulse responses (CIRs). How the receiver estimates these CIRs and cancel it?
As for downlink, does the receiver only know own signature waveform? If say yes, how does it uses multiuser detection to cancel CIR and multiuser interference (MUI)?
LMS and RLS are popular methods for channel estimation. However, in the DS-CDMA system, there are not only CIR but also MUI. How can we let LMS and RLS adapt to this situation?
Many thanks.
For uplink matrix inversion based ML channel estimation DS-CDMA systems. It requires high complexity. Channel estimators using MSE is also used whose performance is achieved in an iterative manner without any matrix inversion.
EM/SAGE based channel estimation is also used for time varying fading channels.
LMMSE is used for uplink multiuser detection as mentioned before in my answer
The RAKE Receiver is used to resolve the channel impulse response, provided that the resolution of the system (sampling frequency) should be high enough so that the Rake receiver resolves all the significant paths. You will find the answer to this question in my publications with relayed names.
If you use NLMS, it is possible to overcome the mentioned problem. Because it works based on normalization, the complexity will be less.
In DS-CDMA (standard) systems there is no interference cancellation. Not sure if MU interference cancellation was adopted in updated standards though interference cancellation using successive cancellation methods was suggested in research. It is necessary to clarify that the DS-CDMA systems all transmitted signals in the cell are included and the job of the receiver is magnify the wanted signal and to mute all other signals thereby improving the S/N ratio. Those all other signals are the other signals of other users and copies of the signal itself due to channel dispersion plus the thermal noise.. In other words, the signal copies are treated like other noise sources. However, when RAKE receiver is used it makes advantage of the signal copies (coherently align the channel responses but not cancelling them) and magnify the signal more over the noise floor.. Thanks. @AlDmour
RAKE receiver performs correlation function Rxy(tau) x, being received signal and y the code of the user. If the user signal is present in the received signal, Rxy(tau) has a maximuma at every repetition (multipath) of the signal, provided that the delay of the repetition is greater than the width of autocorrelation function Ryy(tau) of the code.
Thank you all for helping me in this question. Probably, I have solved this problem.
As above comments, actually, both uplink and downlink of DS-CDMA system over frequency dispersive channel can use LMS or RLS to train the equaliser and cancel ISI.
In this case, as for training sequence, the unique spreading code is not used in receiver, which is able to recover the signal it want only via the equaliser trained by LMS or RLS.
Noted that the length of equaliser may be required to equal the length of spreading code.
The downlink system model can be established according to reference from
http://eprints.soton.ac.uk/252953/1/lbermu.pdf
Thanks
Indeed, yes. I have done quite some work on DS CDMA in multipath channel. What do you want to learn
There are some usefull methods for carrier offset recovery published in the literature such as those you can list by making a google search with the keywords "how can I eliminate carrier frequency offset before channel estimation in CDMA"
or you can try combined methods of carrier offset recovery and channel estimastion together. The above search will lead you. Check the list and follow the articles references. They will lead you...
- Badges
- Science topic
- Similar topics
- Engineering
- Telecommunications Engineering