**3.1 Channel measurement system using PN sequence**

In this section, we present an example of 2x2 MIMO channel measurement, utilizing a semisequential scheme. This semi-sequential scheme uses parallel receivers and a switch at the transmitter (Fig. 2). When a measurement process starts, the probing signal is firstly transmitted from the 1st transmit antenna (TX1) and the receive signal is sampled from 1st and 2nd receive antennas (RX1 and RX2) simultaneously. Thus the channel from TX1 to RX1 and RX2 can be measured at the same time. Then, a similar process is used to measure the channel from TX2 to RX1 and RX2. Strictly speaking, the semi-sequential MIMO channel sounder measures Single Input Multiple Output (SIMO) channels directly. The MIMO channel is obtained by combine the two SIMO channels, on the assumption that the MIMO channel doesn't change significantly in a single round of sequential measurement.

Fig. 2. Semi-sequential scheme of MIMO channel sounder [7].

Each SIMO channel can be measured by the algorithm introduced in Section 2.2 Then a combination of two SIMO channel construct the whole MIMO channel.
