Practical Introduction to LTE Radio Planning

10 J. Salo et alThis is very rarely the case, and the secondstream must be transmitted at lower informationbit rate. This point is further illustrated inthe following subsection.4.3. Example of Measured MIMO Radio ChannelSpatial multiplexing transmission is feasibleonly at those time instants when the radio channelconditions are favorable. There exist awealth of literature on the theory of MIMO communicationsystems, and the interested readeris referred to the references. From radio planningpoint of view, the 2Tx×2Rx MIMO channelpower response λ at any given time instantcan be written, on a subcarrier, as 4 λ = λ 1 + λ 2 ,where λ 1 and λ 2 are the power responses ofthe first and second MIMO subchannels, respectively.Each of the subchannels carries one informationstream. As with a normal non-MIMOchannel, the powers of the subchannels experiencesignal fading. If the power of the weakersubchannel, say λ 2 , is very weak, the second informationstream should not be transmitted atthat time instant, since it will either be buriedin noise, or alternatively, from link adaptationpoint of view, it is more spectrally efficient totransmit one stream with higher-order modulationand/or less channel coding. In eithercase, usage of spatial multiplexing depends onthe instantaneous channel conditions which arereported to, and tracked, by the base station.When the second subchannel is weak, the channelis said to have "rank one", and spatial multiplexingis not used 5 .Fig. 7 presents an example of a narrowbandmeasurement of a 2Tx×2Rx channel inan urban environment at 2.1 GHz carrier frequrency.Two base station antenna configurationsare shown: i) two vertically polarized4 The symbol λ is used here for historical reasons, since theMIMO subchannel power is related to the eigenvalues of theMIMO radio channel.5 Mathematically, the rank of the 2Tx×2Rx channel is practicallyalways two, so this terminology is strictly speaking amisnomer.transmit antennas at 3 wavelength separation(||); ii) two cross-polarized transmit antennasat 0 deg and 90 deg angles (+). The dual-branchUE antenna is a realistic terminal microstrip antennaintegrated in the terminal chassis, whosebranch polarizations are not well-defined, as istypically the case for small integrated antennas.The figure shows the powers of the two MIMOsubchannels as a function of travelled distance.Roughly in the midpoint of the measurementroute the UE enters line-of-sight and the signalpower of the stronger stream (λ 1 ) increases.The second stream power behaves differentlyfor the two antenna setups. With the crosspolarizedantenna base station setup the secondstream (λ 2 ) is about 15 dB weaker than thestronger one. In contrast with the vertically polarizedantennas the second stream is about 30dB weaker. Thus, if the channel SINR is 20 dB,with the cross-polarized configuration the firststream SINR would be almost 20dB while thesecond stream would experience about SINR ≈5 dB which would still allow fairly high bit ratetransmission over the second MIMO subchannel.However, with the vertically polarized antennasthe second stream would be SINR ≈ −10dB, which would not support high-rate transmissionof the second stream.Fig. 8 illustrates the same measurementroutes using two different figures of merit,namely the total MIMO channel power responseand ratio of MIMO subchannel powers,1 λλ 2. From the upper subplot it can be seenthat the total power responses of the two antennasetups are slightly different. Verticallypolarized configuration has almost negligiblyhigher received power in the line-of-sight, whilethe cross-polarized configuration, in turn, hasslightly better power response in non-line-ofsight.From the lower subplot we note that thecross-polarized antenna setup has advantage interms of the second subchannel power in line-ofsightconditions; the second subchannel is about10 dB weaker than the first, compared to the 30or so dBs for the vertically polarized case.