FIELD OF THE INVENTION
[0001]
This invention relates to Adaptive Diversity Combining for Wide Band Code Division Multiple Access (W-CDMA) based on Iterative Channel Estimation.
BACKGROUND OF THE INVENTION
[0002]
Maximal Ratio Combining (MRC) is a technique used to combine the different multipath components from the fingers of a Rake Receiver in CDMA systems. In ideal conditions, MRC is known to provide better Signal to Interference Ratio (SIR) than Equal Gain Combining (EGC) which is another well understood combining technique.
[0003]
However, MRC requires estimation of the channel impulse response. When the channel estimates are not reliable, as could happen in severe channel conditions, the performance of MRC degrades. With non-ideal channel estimates the performance of MRC can degrade below that of EGC. This is because EGC does not use any channel estimates for combining the paths and therefore its performance is independent of the channel conditions.
[0004]
Channel Estimation is done on pilot symbols. In the downlink of W-CDMA, the only source of interference is the multipath interference. The pilot symbols are degraded by multipath interference. The level of degradation will depend on the level of multipath interference. Hence the reliability of the channel estimates too will depend on the level of multipath interference. Therefore for more reliable channel estimates to be obtained, the effect of multipath interference on the pilot symbols has to be removed.
[0005]
It follows that a better receiver would be the one, which chooses its combining technique based on the channel conditions. The problem of the performance degradation of MRC due to non-ideal channel estimates has not been addressed so far.
THE OBJECT AND SUMMARY OF THE INVENTION
[0006]
The object of this invention is to obviate this drawback.
[0007]
To achieve the said objective, this invention provides a method for adaptive diversity combining for Wide Band Code Division Multiple Access (W-CDMA) comprising:
[0008]
splitting multi-path interference into I_{known }and I_{unknown }components,
[0009]
precomputing said I_{known }interference components,
[0010]
iteratively estimating channel interference after removing I_{known }components of the multi-path interference from the pilot symbols,
[0011]
using the convergence/divergence of the iteration as an indicator of the channel condition, and
[0012]
selecting the combining technique (ARC or EGC) automatically depending on the channel condition.
[0013]
EGC is selected if said iteration diverges and MRC is selected if it converges.
[0014]
The precomputation of said I
_{known }interference components is calculated by the following formulae
${c}_{l}=\frac{{d}_{l}-\sum {c}_{m}\ue89e{I}_{\mathrm{known}}-{I}_{\mathrm{unknown}}}{{S}_{l,l}}$
[0015]
where,
[0016]
c_{l}=refined channel estimate of lth finger
[0017]
d_{l}=de-spread data of lth finger
[0018]
c_{m}=channel estimate of mth path
[0019]
S_{ll}=signal component that will be received if no path other than path l is present
[0020]
The refined channel estimates c_{l} ^{n }are obtained using the following iteration
c _{l} ^{(n)} =d _{l}(j)−Σc _{m} ^{(n−1)} I _{known }
[0021]
where
[0022]
c_{l} ^{(n)}=nth iterative value of channel estimate
[0023]
d_{l}(j)=jth de-spread data of lth finger
[0024]
c_{m} ^{(n−1)}=(n−1)th refined channel estimate of mth path
[0025]
The said convergence of the iteration is monitored using the following formula
δ=|c _{l} ^{(n)} −c _{l} ^{(n−1)}|
[0026]
where
[0027]
δ=convergence measurement parameter
[0028]
c_{l} ^{(n)}=nth iterative value of channel estimate
[0029]
c_{m} ^{(n−1)}=(n−1)th refined channel estimate of mth
[0030]
An increasing value of convergence measurement parameter indicates a diverging iteration while a reducing value of convergence measurement parameter indicates a converging iteration and the iteration is stopped when the following condition is satisfied wherein δ_{th }is fixed depending on the degree of refinement required for the channel estimates
δ<δ_{th }
[0031]
where,
[0032]
δ=convergence of iteration
[0033]
δ_{th}=predefined threshold value of refinement required for channel estimate
[0034]
The instant invention also provides a system for adaptive diversity combining for Wide Band Code Division Multiple Access (W-CDMA) comprising:
[0035]
means for splitting multi-path interference into I_{known }and I_{unknown }components,
[0036]
means for precomputing said I_{known }interference components,
[0037]
means for iteratively estimating interference after removing I_{known }components of the multi-path interference from the pilot symbols,
[0038]
means for monitoring the convergence/divergence of the iteration as an indicator of the channel condition, and
[0039]
means for selecting the combining technique (MRC or EGC) automatically depending on the channel condition.
[0040]
The said means for selecting chooses EGC if said iteration diverges and MRC if said iteration converges.
[0041]
The means for precomputation of said I
_{known }interference components uses following formulae
${c}_{l}=\frac{{d}_{l}-\sum {c}_{m}\ue89e{I}_{\mathrm{known}}-{I}_{\mathrm{unknown}}}{{S}_{l,l}}$
[0042]
where,
[0043]
c_{l}=refined channel estimate of lth finger
[0044]
d_{l}=de-spread data of lth finger
[0045]
c_{m}=channel estimate of mth path
[0046]
S_{ll}=signal component that will be received if no path other than path l is present
[0047]
The said means for iterative estimation uses:
c _{l} ^{(n)} =d _{l}(j)−Σc _{m} ^{(n−1)} I _{known }
[0048]
where
[0049]
c_{l} ^{(n)}=nth iterative value of channel estimate
[0050]
d_{l}(j)=jth de-spread data of lth finger
[0051]
c_{m} ^{(n−1)}=(n−1)th refined channel estimate of mth path
[0052]
The said means for monitoring uses:
δ=|c _{l} ^{(n)} −c _{l} ^{(n−1)}|
[0053]
where
[0054]
=convergence of iteration
[0055]
c_{l} ^{(n)}=nth iterative value of channel estimate
[0056]
c_{m} ^{(n−1)}=(n−1)th refined channel estimate of mth
[0057]
The said means for monitoring uses an increasing value of convergence measurement parameter to indicate a diverging iteration and a reducing value of convergence measurement parameter to indicate a converging iteration and the iteration is stopped when the following condition is satisfied wherein δ_{th }is fixed depending on the degree of refinement required for the channel estimates
δ<δ_{th }
[0058]
where,
[0059]
δ=convergence of iteration
[0060]
δ_{th}=predefined threshold value of refinement required for channel estimate
[0061]
The invention further provides a computer program product comprising computer readable program code stored on computer readable storage medium embodied therein for adaptive diversity combining for Wide Band Code Division Multiple Access (W-CDMA) comprising:
[0062]
computer readable program code means configured for splitting multi-path interference into I_{known }and I_{known }components,
[0063]
computer readable program code means configured for precomputing said I_{known }interference components,
[0064]
computer readable program code means configured for iteratively estimating interference after removing I_{known }components of the multi-path interference from the pilot symbols,
[0065]
computer readable program code means configured for monitoring the convergence/divergence of the iteration as an indicator of the channel condition, and
[0066]
computer readable program code means configured for selecting the combining technique (MRC or EGC) automatically depending on the channel condition.
[0067]
The said computer readable program code means configured for selecting chooses EGC if said iteration diverges and MRC if said iteration converges.
[0068]
The computer readable program code means configured for precomputation of said I
_{known }interference components uses following formulae
${c}_{l}=\frac{{d}_{l}-\sum {c}_{m}\ue89e{I}_{\mathrm{known}}-{I}_{\mathrm{unknown}}}{{S}_{l,l}}$
[0069]
where,
[0070]
c_{l}=refined channel estimate of lth finger
[0071]
d_{l}=de-spread data of lth finger
[0072]
c_{m}=channel estimate of mth path
[0073]
S_{ll}=signal component that will be received if no path other than path l is present
[0074]
The said computer readable program code means configured for iterative estimation uses:
c _{l} ^{(n)} =d _{l}(j)−Σc _{m} ^{(n−1)} I _{known }
[0075]
where
[0076]
c_{l} ^{(n)}=nth iterative value of channel estimate
[0077]
d_{l}(j)=jth de-spread data of lth finger
[0078]
c_{m} ^{(n−1)}=(n−1)th refined channel estimate of mth path
[0079]
The said computer readable program code means configured for monitoring uses:
δ=|c _{l} ^{(n)} −c _{l} ^{(n−1)}|
[0080]
where
[0081]
=convergence of iteration
[0082]
c_{l} ^{(n)}=nth iterative value of channel estimate
[0083]
c_{m} ^{(n−1)}=(n−1)th refined channel estimate of mth
[0084]
The said computer readable program code means configured for monitoring uses an increasing value of convergence measurement parameter to indicate a diverging iteration and a reducing value of convergence measurement parameter to indicate a converging iteration and the iteration is stopped when the following condition is satisfied wherein δ_{th }is fixed depending on the degree of refinement required for the channel estimates
δ<δ_{th }
[0085]
where,
[0086]
δ=convergence of iteration
[0087]
δ_{th}=predefined threshold value of refinement required for channel estimate.