Leif Wilhelmsson

On the effect of imperfect interleaving for the Gilbert-Elliott channel

By using the Gilbert-Elliott (1960, 1963) model to study the performance of block-coded transmission over the land mobile channel, a new analytical expression illustrating the effect of various parameters, e,g., mobile speed, delay constraint, and parameters for the error correcting code, is found. Comparisons between the results obtained by this analytical expression and results obtained by computer simulations show that the analytical results are accurate for a broad range of channel parameters. The Gilbert-Elliott model is then used to compare the performance of different binary BCH codes when the delay constraint does not allow the assumption of infinite interleaving. In contrast to the memoryless case, where the performance typically is improved with increased block length, short codes are found to be as good, or even superior, due to the fact that the interleaver works better for shorter codes.

A receiver for LTE Rel-11 and beyond supporting non-contiguous carrier aggregation

Carrier aggregation (CA) is introduced in 3GPP LTE Rel-10 [1] to meet the demand for further increased bitrates. While LTE Rel-10 supports simultaneous reception of two carriers either in contiguous intra-band or in inter-band CA configuration, the upcoming LTE Rel-11 will add support for non-contiguous (NC) carriers within bands. Supporting NC CA in handsets is a demanding challenge for several reasons. Foremost, the total bandwidth spanned by the carriers may be several times the bandwidth of the individual carriers, possibly spanning an entire band with interfering signals between desired carriers. Furthermore, the distance between TX and RX carriers will vary and worse, may be much smaller than the fixed duplex distance for LTE Rel-8 and W-CDMA single carrier operation [2-5].

Low-Complex ICI Cancellation for Improving Doppler Performance in OFDM Systems

A combination of receiver windowing and inter-carrier interference (ICI) canceling is proposed for improving the Doppler performance of orthogonal frequency division multiplex (OFDM) systems. The windowing reduces the ICI, and also reduces the required number of canceled bins. The effect of windowing is analytically derived, and bounds are given on the possible gain that can be obtained. Since successful ICI cancellation relies on that the channel can be accurately estimated, the paper also give examples for how different one-dimensional and two-dimensional channel estimators can be expected to work for various Doppler spread and delay spread of the channel. To verify that the proposed approach for ICI canceling works, as well as studying the impact of channel estimation errors, simulations are performed using the key parameters of DVB-H. As a result it is found that with proper channel estimation, windowing but no ICI cancellation can increase the allowed Doppler by 20%, ICI cancellation but no windowing can also increase the allowed Doppler by 20%, and if both windowing and ICI cancellation are used, the allowed Doppler can increase by more than 40%.

Complex IF Harmonic Rejection Mixer for Non-Contiguous Dual Carrier Reception in 65 nm CMOS

This paper presents a complex IF mixer for a double conversion receiver architecture to be used for non-contiguous dual carrier reception as specified in upcoming releases of 3GPP standards. The complex IF mixer contains four harmonic rejection (HR) mixers, each of which is implemented with 64 passive unit cell mixers, clocked by a ring-oscillator based phase-locked loop and driven by sequencers that represent thermometer-coded oversampled sinusoidal LO waveforms. Each HR mixer is followed by a buffer and a signal distribution network to enable separation of the two carriers as well as IQ-imbalance correction. The complex IF mixer supports reception of two carriers with up to 65 MHz separation using 12 samples per IF LO period and a clock frequency of 390 MHz. The IF mixer is implemented in 65 nm CMOS, has an area of 0.74 mm2, draws 26 mA, and has a harmonic conversion lower than -68 dBc per harmonic.

A sign-bit auto-correlation architecture for fractional frequency offset estimation in OFDM

This paper presents an architecture of an autocorrelator for Orthogonal Frequency Division Multiplexing systems. The received signal is quantized to only the sign-bit, which dramatically simplifies the frequency offset estimation. Hardware cost is reduced under the assumption that synchronization during acquisition does not have to be very accurate, but sufficient for coarse estimation. The architecture is synthesized towards a 65nm low-leakage high threshold standard cell CMOS library. The proposed architecture results in area reduction of 93% if compared to typical 8-bit implementation. The area occupied by the architecture is 0.063 mm2. The architecture is evaluated for WLAN, LTE and DVB-H. Power simulations for DVB-H transmission shows a power consumption of 4.8µW per symbol.

Performance Analysis of Sign-Based Pre-FFT Synchronization in OFDM Systems

This paper treats the feasibility to use only the sign bit of the in-phase and quadrature components when estimating time and frequency in OFDM systems. Using only the sign bit is shown to result in a frequency dependent bias, which can be easily compensated. The approach is evaluated for LTE and DVB-H, when the estimation is performed using the cyclic prefix, and for WLAN 802.11g, when the estimation is done using the short training field (STF). The performance is compared to a floating point implementation, and it is also compared to what is believed to be reasonable requirements for initial time and frequency estimation.

A reconfigurable OFDM inner receiver implemented in the CAL dataflow language

This paper presents a reconfigurable inner receiver for the LTE, DVB-H, and IEEE802.11n (WLAN) radio systems, all of which are based on orthogonal frequency division multiplexing (OFDM). The receiver is implemented in the CAL language. An FPGA-based hardware implementation is synthesized from RTL generated from the CAL description. The purpose of our work is to investigate the feasibility of dataflow methodology for high-level description of digital radio transceivers.

Evaluating the Performance of Raptor Codes for DVB-H by Using the Gilbert-Elliott Channel

The Gilbert-Elliott channel is used to evaluate the performance of Raptor codes for file download in the standard digital video broadcasting for handhelds (DVB-H). This allows for an analytical evaluation without assuming the log-normal fading affecting the different time-slices to be uncorrelated. The approach is also a complement to performing evaluation using prototypes in laboratory, which during the standardization work was found to be rather cumbersome when the channel was slowly varying. The choice of performance criterion is discussed, and some of the results that at first might seem counter intuitive are found to be a consequence of the used criterion. Finally, the coding gain obtained by using Raptor codes in addition to MPE- FEC is evaluated for various speeds of the receiver, showing a substantial gain, although noticeable reduced when the receiver is moving slowly.

The kappa-#181;/Ig Composite Statistical Distribution in RF and FSO Wireless Channels

The aim of this work is the proposition of the κ-μ/Inverse Gaussian distribution which corresponds to a physical fading model. This is a composite distribution which is based on the κ-μ multipath model and the recently proposed Inverse Gaussian shadowing model. The former is a generalised model which includes as special cases the widely known Nakagamim, Rayleigh, Rice and one sided Gaussian distributions and accounts particularly for Non-Line-of-Sight communications. The latter is a convenient model which was recently shown to characterize shadowing effect more accurately than the widely used gamma distribution. As a result, the proposed composite model provides an overall efficient characterisation of multipath and shadowing effects which typically occur simultaneously. The offered modelling accuracy is achieved thanks to the remarkable flexibility of its parameters which is verified by the fact that the model is capable of providing good fittings to measurement data from realistic communication scenarios. Novel analytic expressions are derived for the probability density function (pdf) and the moments of the κ-μ/Inverse Gaussian composite fading model which includes as special cases the Nakagami-m/Inverse Gaussian, Rayleigh/Inverse Gaussian and Rice/Inverse Gaussian composite fading distributions. The offered expressions can be readily utilized in the derivation of vital performance measures in radio and free-space-optical communications over multipath/shadowing and medium-to-strong atmospheric turbulence, respectively. In this context, a closed-form expression is derived for the Amount of Fading over κ-μ/Ig fading channels.

Analysis of a novel low complex SNR estimation technique for OFDM systems

Signal-to-noise ratio (SNR) estimation is commonly used in wireless receivers to enhance the performance in different ways. In this paper a novel low complexity SNR estimator for OFDM is proposed. The estimator might be implemented using floating point representation or by using only the sign-bit, and can if desired be effectively implemented by reconfiguring the standard correlator used for time- and frequency estimation. Closed form expressions for the SNR estimate are derived for both the floating point implementation and the sign-bit implementation, and compared to simulation results both for an additive white Gaussian noise (AWGN) channel and for a frequency selective channel showing the feasibility of the proposed algorithm.