At the beginning of this chapter, background knowledge on MIMO, CoMP and OFDM was provided, which will be needed through the rest of this work. Afterwards, the JT CoMP system model was described and a signal model with an imperfect precoder was derived.
The precoder mismatch with respect to the channel was captured by an additive equivalent channel error, which can be due to any of the impairments. Based on the signal model, the power of the users’ self-signal and inter-user interference was analyzed and closed-form expressions were derived for the mean SIR, both for the Rayleigh fading channel as well
2.6 Summary of Chapter 2
as for the SIR upper bound. It was shown that for zero-forcing precoding, the mean SIR is inversely proportional to the channel MSE, and that it grows with the number of base stations and drops with the number of jointly served users. Furthermore, it was shown that appropriate user selection can improve robustness against imperfect precoding and enhance the SIR. In practical systems, a scheduler should combine the users in a cooperation cluster in sets whose channel vectors are nearly orthogonal. In this way, transmission becomes more stable against impairments and performance becomes close to optimal despite for simple zero-forcing precoding. Clustering and using power constraints are also important, so that the total transmit power level remains practical.
3 Inter-site-distance limitations for CoMP
In this chapter, we investigate on the maximum allowed inter-site distance (ISD) for per-forming joint transmission coordinated multi-point (JT CoMP) between macro-cell base stations. As a key metric, we use the excess delay measured at the 95% point of the cu-mulative power delay profile (PDP) resulting from transmission of multiple base stations.
As propagation distances in JT CoMP are typically larger than in single-cell transmis-sion, distant-dependant channel modeling is essential. Therefore, we consider Greenstein’s statistical propagation model [75], which we here extend for broadband multi-cell transmis-sions. We extract all model parameters from 2.65 GHz multi-cell measurements performed in outdoor field trial, parametrize the model at a fixed ISD and validate it by simulations.
We also investigate the impact of antenna downtilt and find that when a larger downtilt is used, the root mean square (RMS) delay spread and 95% excess delay become smaller.
Then, we consider larger ISDs and indicate how the delay parameters grow. It is found that, based on the extended Greenstein’s model, the short cyclic prefix in Long Term Evolution – Advanced (LTE-A) is hardly violated for realistic ISD at 2.65 GHz.
Previous related work and objectives
The dependence of channel delay spread from the path gain and thus the propagation distance has been addressed in previous works on channel modeling. In [76], it has been found that in urban scenarios, the delay spread as well as it’s standard deviation increase with the distance and with deep shadow fading. An overview of the state of the art in cooperative multiple-input multiple-output (MIMO) channel modeling can be also found in [77].
In this chapter, we investigate the impact of base station (BS) cooperation on the overall delay statistics in a realistic deployment scenario. We seek an answer to the question what distance is allowed between cooperative base stations without violating a given cyclic prefix (CP). As a starting point, we use a distance-dependent statistical propagation model in [75], which describes also the correlation between path gain and delay spread. We parametrize this model based on coherent multi-cell channel measurements from the LTE-A field trial in Berlin, Germany [78–80]. Therefore, we extract the distance dependencies of path loss, shadow fading and delay spread as well as their correlation at a carrier frequency of 2.65 GHz in an urban macro-cell scenario.
We prove that Greenstein’s model predicts the delay statistics properly at least at larger distances from the BS. Close to the BS, however, we observe 3D effects, leading to
signif-icant deviations from the predicted statistics if the downtilt is set so that the main beam touches the ground at 0.9 times the ISD. With a downtilt of 0.33 times the ISD, as recom-mended by Third Generation Partnership Project (3GPP) for LTE-A (see [81]), the overall delay statistics is mostly due to the overlap region covered jointly between adjacent sites, where Greenstein’s model is more appropriate. Thus we can increase the ISD and predict the delay statistics by using our parametrized propagation model. Our results indicate that BS cooperation in is feasible for distances up to 1.7 km between the sites at 2.65 GHz without violating the short CP in LTE-A.
The rest of this chapter is organized as follows. In Section 3.1, the problem of inter-symbol interference (ISI) in JT CoMP using orthogonal frequency division multiplex-ing (OFDM) is briefly described. Section 3.2 reviews Greenstein’s model and extends it for broadband multi-cell transmissions. In Section 3.3, our measurement setup and the parameter extraction routines are described. Section 3.4 presents the extracted parameters and discusses the scope of the model by comparing prediction and measurement results.
Furthermore, the parametrized model is used for predicting the delay statistics in larger cells. Finally, conclusions are discussed in Section 3.5.
3.1 Inter-symbol-interference in JT CoMP using OFDM
It is known that OFDM is well-suited for transmission over frequency-selective channels, as it divides the available bandwidth in a number of orthogonal sub-channels, where each of them observes frequency-flat channel fading and can be processed separately. However, the CP needed for subcarrier based equalization reduces the spectrum efficiency. It is therefore chosen longer than the largest multipath delay in the targeted propagation environment, in order to avoid ISI due to the channel echoes of the previously transmitted OFDM symbol.
Figure 3.1 shows shows how the CP protects the transmission from ISI between consecutive OFDM symbols.
Figure 3.1: The cyclic prefix "absorbs" multipath echoes from the previous OFDM symbol and protects from inter-symbol interference. It also relaxes time synchroniza-tion requirements.
In practical systems, the beginning of the OFDM symbol is one of the first parameters a terminal needs to estimate in order to establish data exchange with a base station.
Accurate time synchronization allows for removing correctly the CP and applying the fast