The idea of moving the workload of channel equalization from the receiver to the transmitter was first presented by Nakagawa and Esmailzadeh in 1993, when they proposed their pre-Rake for spread spectrum radio transmission. Today this idea is also known as preceding or receiver orientation (RO). The low receiver complexity enabled by RO makes it especially attractive for mobile radio downlinks (DL) with their demand of low cost mobile terminals (MT). Mostly, the considered RO transmission schemes considered up to now are linear with the result that their required transmit powers are virtually the same as in systems not resorting to RO. However, low transmit powers are of paramount importance both with respect to the public concern about electromagnetic irradiation, and to checking the disturbance of other radio links. In the present paper we ventilate the possibility of transmit power reduction in RO mobile radio DLs by introducing a non-linear feature. We set out from known linear RO schemes, in the demodulators of which discrete valued target points are aimed at in the complex plane in the sense of "hard" spot landings, and then, instead of this, introduce non-linearity by envisaging "soft" landings somewhere in suitably defined more or less extended continuous valued domains. In this way the constraints on the transmit signals are loosened, which opens the possibility to minimize the transmit powers. We term our approach minimum energy soft precoding (MESP) and demonstrate its efficacy by simulation results for MIMO OFDM multi-user mobile radio downlinks.

It is expected that in the forthcoming 3/sup rd/ generation (3G) mobile radio systems the downlinks will have to support much higher data rates than the uplinks. A promising measure to enable high downlink data rates consists in improving the carrier-to-interference ratios at the mobile receivers by the utilization or adaptive mufti-element transmit antennas. However, for the efficient operation of such antennas information about the downlink radio channels is required at the base stations. In the case of the 3G partial standard time division(TD)-CDMA designated for the time division duplexing (TDD) radio frequency bands, such information-thanks to the reciprocity theorem-is readily available from the uplink channel estimators. However, the 3G partial standard W(wideband)-CDMA designated for the frequency division duplexing (FDD) radio frequency bands suffers from the inherent disadvantage that, due to the frequency difference between uplink and downlink, the results of the uplink channel estimation cannot be directly used as the channel information required for adjusting the multi-element transmit antennas. Nevertheless, as long as the frequency difference between uplink and downlink is not too large, approaches to an at least sub-optimum adjustment of the downlink transmit antennas based on uplink channel estimates exist. In the paper such approaches are presented and evaluated by simulations.

The UTRA TDD signalling mode is considered, which uses a combination of the multiple access schemes FDMA, TDMA and CDMA, which is termed TD-CDMA. In CDMA cellular mobile systems both intersymbol interference (ISI) and multiple access interference (MAI) cause degradations to the system performance. In TD-CDMA a joint detection CDMA (JD-CDMA) processor performs unbiased signal estimation to eliminate both ISI and MAI within the reference cell. This paper reports an extension of the previously reported receiver processor to achieve intercell interference cancellation to reduce the interference from the adjacent cells.

Time division CDMA (TD-CDMA) is one of the air interfaces selected in the 3/sup rd/ Generation Partnership Project (3GPP) for 3/sup rd/ generation mobile radio systems (3G systems). In many investigations it has been shown that the uplink performance of TD-CDMA, or, more generally, of time slotted CDMA air interfaces, can be considerably enhanced by utilizing multi-element adaptive receiving antennas, for which mature algorithms of spatial-temporal signal processing are available. However, the application of adaptive multi-element transmit antennas in the downlink of such systems has not yet been extensively studied. In this paper, first a transmission model which allows the inclusion and consideration of adaptive transmit antennas in the TD-CDMA downlink is developed. Then, based on this model, an algorithm for implementing transmit antenna adaptivity is proposed; the crux of this algorithm is the determination of the antenna weights of the array feeding network based on an energy criterion. Finally, the efficiency of this algorithm is illustrated by simulation results. Special attention is paid to the support of higher data rates by CDMA code pooling.

The multiple access schemes TDMA and CDMA are compared and evaluated with respect to the demands on third generation mobile radio systems such as high capacity and flexibility. Further, an approach to evolve GSM towards third generation mobile radio systems by introducing an optional CDMA component is presented.

In code division multiple access (CDMA) systems transmitting over time-varying multipath channels, both intersymbol interference (ISI) and multiple access interference (MAI) arise. The conventional suboptimum receiver consisting of a bank of matched filters is often inefficient because interference is treated as noise. The optimum multi-user detector is too complex to be implemented at present. Four suboptimum detection techniques based on zero forcing (ZF) and minimum mean square error (MMSE) equalization with and without decision feedback (DF) are presented and compared. They combat both ISI and MAI. The computational complexity of all four equalizers is essentially the same and they are independent of the size of the data symbol alphabet. It is shown that all four equalizers considerably outperform the conventional receiver. The MMSE equalizers are superior to the corresponding ZF equalizers. Furthermore, the equalizers with DF perform better than the corresponding equalizers without DF. An algorithm termed channel sorting is introduced to reduce the impairing effect of error propagation on the equalizers with DF.>

This special issue covers the topics: power control, multiuser receiver and joint detection, network aspects, adaptive equalisation, coding aspects, broadband CDMA systems, and optical systems.>

A low-cost linear algorithm for unbiased separation of not necessarily orthogonal signals is proposed. A new view on the suitability of signals is opened which increases the variety of candidate signals for e.g. CDMA applications. The proposed algorithm can be advantageously implemented by a modified matched filter bank.>

Data estimation in the uplink of a synchronous mobile radio system applying code-division multiple access (CDMA) is considered. In mobile radio systems applying CDMA, multipath propagation leads to intersymbol interference (ISI) and together with time variance, to cross interference between the signals of different users regardless of whether the user codes are chosen orthogonal or not. A linear unbiased data estimation algorithm is presented that eliminates both ISI and cross interference perfectly by jointly detecting the different user signals, leading to unbiased estimates of the transmitted data symbols. By theoretical analysis and simulation, the performance of the linear unbiased data estimation algorithm is examined under the assumption that the radio channel impulse responses are known at the receiver. The price to be paid for the interference elimination are SNR degradations, which are calculated for typical mobile radio situations in urban areas. The resulting average uncoded bit error probabilities lead to the conclusion that systems applying the linear unbiased data estimation algorithm are well suited for mobile radio applications.>

Modeling and simulation of complete systems are proposed as an alternative to measurements performed with functional setups. In a case study, a complex digital spread spectrum transmission system is considered. Simulation results are compared to measurement results obtained with the functional setup. The effort in terms of costs and time for both methods is evaluated with respect to the quality of predictions obtained in each case. It is shown that the simulation approach is more flexible, cheaper and less time-consuming and that the results are sufficiently accurate. The lessons learned in this case study can be generalized and especially applied to raise efficiency when designing complex mobile radio systems.>

In this paper, data estimation in the uplink of a synchronous mobile radio system applying CDMA is considered. In mobile radio systems applying CDMA, multipath propagation leads on the one hand to ISI and on the other hand, together with time-variance, to cross interference between the signals of different users, regardless whether the user codes are chosen orthogonal or not. A linear unbiased data estimation algorithm is presented which cancels both ISI and cross interference perfectly by jointly detecting the different user signals, thus leading to unbiased estimates of the transmitted data symbols. This algorithm is less complex than MLSE performed e.g. by the Viterbi algorithm and furthermore, the complexity does not depend on the kind of the applied linear modulation scheme, which opens a simple way to offer services with different data rates in one system. By theoretical analysis and simulation, the performance of the unbiased data estimation algorithm is examined under the assumption that the radio channel impulse responses are known at the receiver. The price to be paid for the interference cancellation is SNR-degradations, which are calculated for typical mobile radio situations in urban areas. The resulting average uncoded bit error probabilities lead to the conclusion that systems applying the unbiased data estimation algorithm are well-suited for mobile radio applications.>

A novel signal generation concept for continuous phase modulations (CPMs) with modulation index 1/2 based on real impulses is presented. With this concept, bandpass CPM signals can be generated directly in one step instead of the two consecutive steps, namely, the generation of the complex envelope and the modulation of the carrier by the complex envelope, which are necessary in conventional signal generators. Mathematical expressions for both the real impulses and the bandpass CPM signals are derived and a simple modulator structure is discussed. Examples for the real impulses are given. Among these are the well-known CPM schemes of minimum shift keying (MSK), sinusoidal frequency shift keying (SFSK), and Gaussian minimum shift keying (GMSK). As an example, the validity of the novel signal generation concept is shown for the latter CPM scheme.>

Certain radio systems that have a narrowbeam directive antenna at one station and a low-gain antenna at the other station present a problem in discriminating between mainlobe and sidelobe reception. A discrimination method is presented that takes advantage of the fact that a broadband signal transmitted or received over a directive antenna can suffer distortions that depend on the aspect angle under which this antenna appears from the other station. This method can be applied if the bandwidth of the transmitted waveforms is sufficiently large, which is the case, for example, in spread-spectrum systems for communication, identification, or navigation. The application of the method is illustrated by means of an example.>

A device which can be used to suppress strong constantenvelope interfering signals at the input of spread-spectrum receivers is described and analyzed. Unlike similar circuits known from literature, this device is not restricted to the suppression of narrow-band jamming signals; it can also cope with broad-band interfering signals bearing any kind of angle modulation. The theoretical results are confirmed by measurements.

In spread-spectrum communication systems, a basic task which has to be performed is the synchronization of the receiver pseudonoise (PN) signal with the pseudonoise signal contained in the input. In this paper, a synchronization system employing a surface acoustic wave tapped delay line (SAW TDL) matched filter for both initial synchronization (acquisition) and tracking is presented. The periodic correlation peaks at the SAW TDL output repeatedly correct the epoch of the local PN signal clock phase and the code generator initial condition to their correct values. As the correlation impulses are distorted and attenuated due to the effects of message modulation, Doppler frequency shifts and unavoidable interferences, a modulation canceller and a differentiating device are employed to improve synchronization performance. Formulas for estimating the performance of a system incorporating these ideas are given. A hardware implementation of the suggested system has been built and tested. Experimental results obtained with the system are presented.

For a non-coherent detection circuit with a broad-band pre-detection filter and narrow-band post-detection-filter, equations are obtained for the probability of detection of a pulsed sinusoidal signal embedded in white noise. The equations can be used to calculate the detection probability directly from envelope form, noise power at the input of the circuit and the impulse response of the postdetection filter. The application of the equations is illustrated with reference to a sliding correlator acquisition circuit for PN-code generators.