Comments on IEEE MIMO Channel implementation

1. Comments on IEEE MIMO Channel implementation Angelo Poloni and StefanoValle STMicroelectronics (angelo.poloni@st.com, stefano.valle@st.com) A. Poloni, S. Valle, STMicroelectronics

2. Outline • Topic: The generation of fading coefficients for MIMO channels • Undesirable artifacts of the current wrap-around technique: • Discontinuities in fading coefficients; • Periodicity in flat-fading channels. • A straightforward solution is proposed. A. Poloni, S. Valle, STMicroelectronics

3. n fD Generation of fading coefficients in the present IEEE MIMO channel Spectrum modulus shaping Uniform Random phase n fD IFFT Wrap-around buffer interpolation A. Poloni, S. Valle, STMicroelectronics

4. Wrap-around technique artifacts:fading coefficient discontinuities • The wrap-around technique allows us to reuse the same fading coefficients for long simulations (SISO or MIMO). • In order to avoid periodic replications of the channel transfer function, each path is wrapped-around not at the last sample (which matches with the first sample--thanks to the FFT properties), but at a certain sample chosen among the prime numbers (close but less than the IFFT length). • In this case, a discontinuity is introduced in a single path’s fading coefficients. Refer to the next slide, where for a path, the fading process (before interpolation) and its numerical derivative (x(k)-x(k-1)) are plotted. • Interpolation of the samples smoothens such discontinuities. However, the slope of the fading process at the discontinuity remains significantly steeper than elsewhere. As a result, the receiver will face a faster (and maybe not realistic) Doppler process for a certain time. • This problem is more evident with simple MIMO channels (i.e. few antennas and few delays), but is smoothened in richer channels. A. Poloni, S. Valle, STMicroelectronics

5. Wrap-around technique artifacts: fading coefficient discontinuities discontinuity Wrap-around period A. Poloni, S. Valle, STMicroelectronics

6. Wrap-around technique artifacts:periodicity in case of flat fading channel • The wrap-around technique, as currently implemented, adopts a single prime number for the fading coefficients belonging to the same tap (delay), in order to not destroy the spatial correlation of the fading samples. • In flat fading MIMO channels, which have only one tap, the channel simulation becomes periodic. A. Poloni, S. Valle, STMicroelectronics

7. Wrap-around technique artifacts:some considerations • IFFT is a very useful technique to generate fading coefficients which have “special” Doppler Power Spectra (e.g. reproducing actual measurements). • If only “standard” Doppler Power Spectra are going to be adopted in IEEE channel model, our suggestion is to generate the fading coefficients by filtering AWGN with the proper filter (see next slide). In this way, both discontinuities and periodicity can be avoided. • We are available to support the introduction of this generation method into the IEEE model. A. Poloni, S. Valle, STMicroelectronics

8. AWGN H(f) interpolation an(kT) AWGN H(f) interpolation Generation of fading coefficients • The filter H(f) has a fixed normalized cut off frequency fDN. • The desired Doppler Spread fD is obtained via interpolation with a variable interpolation factor b given by where fW is the simulation sampling frequency A. Poloni, S. Valle, STMicroelectronics