 by the increased inter-cell interference and hence the increased traffic as shown in Figure 12.1. In order to improve the throughput per user while traffic increases the reported CQI needs to be improved by physical layer optimization as shown in Figure 12.7. Use of MIMO can also improve the throughput per cell and per user as shown later in this section. In uplink direction the throughput per user depends on PUSCH SINR similarly as downlink throughput depends on CQI as shown in Figure 12.40. As can be seen from Figure 12.40 the uplink user throughput decreases exponentially as the average PUSCH SINR decreases. The low band cells provide extremely good uplink throughput in case the PUSCH SINR is good due to the excellent coverage compared to the high band cells. Also the PUSCH SINR is the worst for the low band cells due to the excessive overlap of some of the cells. In case of uplink throughput the performance can be greatly improved in case the PUSCH SINR can be improved as discussed later in this section. 12.7.1 MIMO Multi-stream Usage Optimization In the context of communication theory the term MIMO means a system that is able to transmit and receive multiple parallel information streams within the same spectrum, therefore increas- ing the spectral efficiency. The theoretical basis of MIMO, also called spatial multiplexing, dates back to 1950s [5] and was later popularized in the 1990s [6,7]. In radio communications, multi-stream communication requires multiple antennas at both ends of the radio link and the maximum number of parallel streams is equal to the minimum of the number of transmit and receive antennas. In this section the focus is on understanding, from the viewpoint of radio network planning, the factors that impact the throughput gain from spatial multiplexing; for details of multi- antenna communications the reader is referred to the extensive literature. It is, however, important to understand that spatial multiplexing is only one of the many ways to utilize multiple antennas at both link ends