MIMO Transmission schemes for LTE and HSPA Networks, 3G

SINR. How such a gain translates into a throughput increase then depends on e.g. where on the throughputcurve the link is operating. The qualitative behavior of the resulting throughput-vs-coverage curves isillustrated in Figure 3.ThroughputCTx rank one: 1x2Tx rank one: 2x2Tx rank one: 4x2Tx rank one: 8x2CoverageFigure 3. Throughput versus coverage curves illustrating how the array gain from the use ofbeamforming improves the throughput on the cell edge but not sufficiently close to the cell centerSpatial multiplexing in its simplest form entails transmitting one separately modulated symbol per antenna.The Alamouti code is a block code represented by a 2x2 codeword matrix of the forms s1c2s2 sc1C (2)where the rows correspond to the (transmit) antenna dimension and the columns may be another dimensionsuch as time or frequency and where c denotes complex conjugate.All the schemes described so far have in common that they fall under the framework of (linear) block basedprecoding. Such precoding can generally be described by an N T xL codeword matrix.Qx1 x2xLBqresqB~qimsqC (3)q1As seen, a codeword is obtained as a linear combination of the real and imaginary parts of the modulationsymbols. This kind of scheme is commonly referred to as linear dispersion coding and the Bq matrices areconsequently called dispersion matrices, to acknowledge the fact that they distribute each symbol over twodifferent dimensions. Each symbol sq corresponds to a certain symbol stream, also known as a layer. Thetransmission rank can be expressed as r = Q/L.It should be emphasized that the term precoding is a framework that encompasses a large number of widelydifferent transmission schemes. As such, it does not say much about the actual scheme being used and themeaning of precoding thus has to be understood from the context.The dispersion matrices can either be made dependent or independent of the channel. The Alamouti code isan example of when they are fixed while in beamforming they are adapted according to the channelconditions. Whether to use channel dependent or independent precoding depends on the availability of9