Cryptography Reference
In-Depth Information
MMSE turbo equalizer, under the hypothesis of ideal interleaving. The reader
will find further information on this subject in [11.8] or [11.50], for example.
Example of implementation and applications
The implementation of an MMSE turbo equalizer on a signal processor was
reported in [11.9]. The target was the TMS320VC5509 processor by Texas
Instruments. This is a 16-bit fixed-point DSP with low power consumption,
which makes it an ideal candidate for mobile receivers. The considered trans-
mission scheme included a 4-state rate
1
/
2
convolutional encoder and a 1024
bit interleaver followed by a QPSK modulator. The whole turbo equalizer was
implemented in C language on the DSP, with the exception of some processing
optimized in assembly (filtering and FFT) provided by a specialized library. The
equalizer included 32 coecients. The decoding was performed using the Max-
Log-MAP algorithm. The simulation results showed that, subject to carefully
choosing the representation in fixed decimal points of the data handled (within
the limit of 16 bits maximum granted by the DSP), data quantization did not
cause any loss in performance in comparison with the corresponding floating-
point receiver. The final data rate obtained was of the order of 42 kbits/s after 5
iterations, which shows the feasibility of such receivers using current technology.
The challenge now involves defining appropriate circuit architectures, capable of
operating at several Mbits/s, in order to respond to emerging demands for high
data rate services.
MMSE turbo equalization is a relatively recent technology. Therefore, at
the moment of writing this topic, there have been few studies on the poten-
tial applications of this technique at reception. Generally, resorting to MMSE
turbo equalization is an effective solution in the context of high data rate trans-
missions on highly frequency-selective channels. In particular, this system has
shown excellent performance on the ionospheric channel typically used in the
context of HF military communications. Indeed, the long echoes produced by
this channel prevent the use of MAP equalizers. On the other hand, conven-
tional linear equalization schemes do not make it possible to reach a transmission
quality acceptable when high-order modulations (
e.g.
8-PSK or 16-QAM) are
considered. MMSE turbo equalization is thus an attractive solution to the prob-
lem of increasing the data rate of military transmissions. In the context of HF
communications, the interest of MMSE turbo equalization for high spectral e-
ciency modulations has been validated by the work of Langlais [11.29] and Otnes
[11.41], which shows that this technique can offer gains up to 5 dB compared
to conventional receivers. To our knowledge, MMSE turbo equalization has not
yet been implemented in standardized modems. However, it is important to
note that this reception technique enables the transmission performance to be
notably improved while keeping standardized transmitters.
