Multiple-Input Multiple-Output (MIMO) is one of the key elements of the air interface for high-speed wireless communications for many wireless communication technologies such as Long Term Evolution (LTE) and High Speed Packet Access (HSPA). MIMO can use the diversity in the channel to provide multiplexing gain by enabling the simultaneous transmission of multiple streams known as layers. Denoting the number of transmit antennas, receive antennas, and layers by NT, NR, and R, respectively, R is never greater than NT (and, often, smaller or equal to NR). One possible implementation of MIMO uses a precoder, often expressed mathematically as a left-multiplication of a layer signal vector (R×1) by a precoding matrix (NT×R), which is chosen from a pre-defined set of matrices, a so-called codebook exemplified in Table 1 and Table 2.
TABLE 1Codebook for LTE UL (2-TX)RIPMI010      1          2        ⁡      [                            1                                      1                      ]        1          2        ⁡      [                            1                          0                                      0                          1                      ]   1      1          2        ⁡      [                            1                                                  -            1                                ]  — 2      1          2        ⁡      [                            1                                      j                      ]  — 3      1          2        ⁡      [                            1                                                  -            j                                ]  — 4      1          2        ⁡      [                            1                                      0                      ]  — 5      1          2        ⁡      [                            0                                      1                      ]  —
TABLE 2Codebook for LTE UL (4-TX)RIPMI01230      1    2    ⁡      [                            1                                      1                                      1                                                  -            1                                ]        1    2    ⁡      [                            1                          0                                      1                          0                                      0                          1                                      0                                      -            j                                ]        1    2    ⁡      [                            1                          0                          0                                      1                          0                          0                                      0                          1                          0                                      0                          0                          1                      ]        1    2    ⁡      [                            1                          0                          0                          0                                      0                          1                          0                          0                                      0                          0                          1                          0                                      0                          0                          0                          1                      ]   1      1    2    ⁡      [                            1                                      1                                      j                                      j                      ]        1    2    ⁡      [                            1                          0                                      1                          0                                      0                          1                                      0                          j                      ]        1    2    ⁡      [                            1                          0                          0                                                  -            1                                    0                          0                                      0                          1                          0                                      0                          0                          1                      ]  — 2      1    2    ⁡      [                            1                                      1                                                  -            1                                                1                      ]        1    2    ⁡      [                            1                          0                                                  -            j                                    0                                      0                          1                                      0                          1                      ]        1    2    ⁡      [                            1                          0                          0                                      0                          1                          0                                      1                          0                          0                                      0                          0                          1                      ]  — 3      1    2    ⁡      [                            1                                      1                                                  -            j                                                            -            j                                ]        1    2    ⁡      [                            1                          0                                                  -            j                                    0                                      0                          1                                      0                                      -            1                                ]        1    2    ⁡      [                            1                          0                          0                                      0                          1                          0                                                  -            1                                    0                          0                                      0                          0                          1                      ]  — 4      1    2    ⁡      [                            1                                      j                                      1                                      j                      ]        1    2    ⁡      [                            1                          0                                                  -            1                                    0                                      0                          1                                      0                                      -            j                                ]        1    2    ⁡      [                            1                          0                          0                                      0                          1                          0                                      0                          0                          1                                      1                          0                          0                      ]  — 5      1    2    ⁡      [                            1                                      j                                      j                                      1                      ]        1    2    ⁡      [                            1                          0                                                  -            1                                    0                                      0                          1                                      0                          j                      ]        1    2    ⁡      [                            1                          0                          0                                      0                          1                          0                                      0                          0                          1                                                  -            1                                    0                          0                      ]  — 6      1    2    ⁡      [                            1                                      j                                                  -            1                                                            -            j                                ]        1    2    ⁡      [                            1                          0                                      j                          0                                      0                          1                                      0                          1                      ]        1    2    ⁡      [                            0                          1                          0                                      1                          0                          0                                      1                          0                          0                                      0                          0                          1                      ]  — 7      1    2    ⁡      [                            1                                      j                                                  -            j                                                            -            1                                ]        1    2    ⁡      [                            1                          0                                      j                          0                                      0                          1                                      0                                      -            1                                ]        1    2    ⁡      [                            0                          1                          0                                      1                          0                          0                                                  -            1                                    0                          0                                      0                          0                          1                      ]  — 8      1    2    ⁡      [                            1                                                  -            1                                                1                                      1                      ]        1    2    ⁡      [                            1                          0                                      0                          1                                      1                          0                                      0                          1                      ]        1    2    ⁡      [                            0                          1                          0                                      1                          0                          0                                      0                          0                          1                                      1                          0                          0                      ]  — 9      1    2    ⁡      [                            1                                                  -            1                                                j                                                  -            j                                ]        1    2    ⁡      [                            1                          0                                      0                          1                                      1                          0                                      0                                      -            1                                ]        1    2    ⁡      [                            0                          1                          0                                      1                          0                          0                                      0                          0                          1                                                  -            1                                    0                          0                      ]  — 10      1    2    ⁡      [                            1                                                  -            1                                                            -            1                                                            -            1                                ]        1    2    ⁡      [                            1                          0                                      0                          1                                                  -            1                                    0                                      0                          1                      ]        1    2    ⁡      [                            0                          1                          0                                      0                          0                          1                                      1                          0                          0                                      1                          0                          0                      ]  — 11      1    2    ⁡      [                            1                                                  -            1                                                                          -              j                        ⁢                                                  ⁢            j                                ]        1    2    ⁡      [                            1                          0                                      0                          1                                                  -            1                                    0                                      0                                      -            1                                ]        1    2    ⁡      [                            0                          1                          0                                      0                          0                          1                                      1                          0                          0                                                  -            1                                    0                          0                      ]  — 12      1    2    ⁡      [                            1                                                  -            j                                                1                                                  -            j                                ]        1    2    ⁡      [                            1                          0                                      0                          1                                      0                          1                                      1                          0                      ]  —— 13      1    2    ⁡      [                            1                                                  -            j                                                j                                                  -            1                                ]        1    2    ⁡      [                            1                          0                                      0                          1                                      0                                      -            1                                                1                          0                      ]  —— 14      1    2    ⁡      [                            1                                                  -            j                                                            -            1                                                j                      ]        1    2    ⁡      [                            1                          0                                      0                          1                                      0                          1                                                  -            1                                    0                      ]  —— 15      1    2    ⁡      [                            1                                                  -            j                                                            -            j                                                1                      ]        1    2    ⁡      [                            1                          0                                      0                          1                                      0                                      -            1                                                            -            1                                    0                      ]  —— 16      1    2    ⁡      [                            1                                      0                                      1                                      0                      ]  ——— 17      1    2    ⁡      [                            1                                      0                                                  -            1                                                0                      ]  ——— 18      1    2    ⁡      [                            1                                      0                                      j                                      0                      ]  ——— 19      1    2    ⁡      [                            1                                      0                                                  -            j                                                0                      ]  ——— 20      1    2    ⁡      [                            0                                      1                                      0                                      1                      ]  ——— 21      1    2    ⁡      [                            0                                      1                                      0                                                  -            1                                ]  ——— 22      1    2    ⁡      [                            0                                      1                                      0                                      j                      ]  ——— 23      1    2    ⁡      [                            0                                      1                                      0                                                  -            j                                ]  ———
Each precoding matrix is indexed by a rank indicator (RI) and a precoding matrix indicator (PMI). (Note that the r-th column vector of the precoding matrix represents the antenna spreading weight of the r-th layer.) The precoding matrix usually consists of linearly-independent columns, and thus R is referred to as the rank of codebook. One purpose of this kind of precoder is to match the precoding matrix with the channel state information (CSI) so as to increase the received signal power and also to some extent reduce inter-layer interference, thereby improving the signal-to-interference-plus-noise-ratio (SINR) of each layer. Consequently, the precoder selection requires the transmitter to know the channel properties and, generally speaking, the more accurate the CSI, the better the precoder matches.
In the case of 3GPP LTE UL, precoder selection is made by the receiver (NodeB) so that there is no need for feeding channel information back to the transmitter. The precoder selection includes not only rank selection, but also precoding matrix selection. It is also necessary for the receiver to obtain channel information, which can usually be facilitated by transmitting a known signal, in the case of LTE UL, the Demodulation Reference Signal (DM-RS) and the Sounding Reference Signal (SRS). Both DM-RS and SRS are defined in frequency domain and derived from a Zadoff-Chu sequence. However, since the DM-RS is precoded, while the SRS is not precoded, the channel information obtained from DM-RS is the equivalent channel that the R layers experience, not the physical channel that the NT antennas experience. Mathematically, letting the NR×NT physical channel matrix, the NT×R precoding matrix, and the NR×R equivalent channel be denoted by H, W and E, respectively, it follows thatE=HDW,  (1)where D is the NT×NT diagonal matrix whose diagonal elements represent a phase shift introduced by the transmitter chains. As will be seen later, the phase shift is not uniform and does not need to be constant. In detail, the i-th diagonal element is given as di=exp(jφi). As will be described below, the phase shift may result in significant performance loss, when the relative phase between the transmitter chains changes from SRS to PUSCH.
Using the above notation, the equivalent channels for PUSCH, DM-RS and SRS denoted by EPUSCH, EDMRS and ESRS can be expressed asEPUSCH=HW EDMRS=HW HSRS=HD.  (2)
Here it is assumed that there is no channel variation among PUSCH, DM-RS and SRS. Furthermore, D is set to the identity matrix for PUSCH and DM-RS without loss of generality due to the fact that we are only concerned with relative phase variations. Note that it is also assumed that PUSCH and DM-RS experience the same channel. In addition, note that HSRS in (2) is directly obtained from SRS, and based on HSRS the equivalent channel ESRS as a function of a hypothesized precoder W can be obtained as ESRS=HSRSW.
Precoder selection is preferably based on SRS, since precoder selection is more easily done with complete knowledge of the channel, i.e., the physical channel, HD in (2). Based on the physical channel estimated based on SRS, the best transmission mode is chosen by the receiver. The receiver then sends the chosen best channel to the transmitter. One of the criteria of selecting the transmission mode is to maximize the data throughput. For example, the effective SNR is calculated for each precoder, i.e., each selection of the rank and precoder matrix, the relevant throughput is calculated, and the precoder maximizing the throughput is selected. Consequently, it is easily understood that precoder selection is subject to inter-antenna imbalance variation between measurement period (SRS) and actual data transmission period (PUSCH).
Referring to FIG. 1, consider a generic User Equipment (UE) 100 with two transmit antennas 102, 104 for simplicity (although the following argument is equally applicable to a UE with more than two transmit antennas). Furthermore, throughout this document a UE 100 can be any one of a multitude of types of wireless or mobile communication devices. Furthermore, a UE as used herein, but not specifically shown in FIG. 1, may include a user interface for physical touch, sound and/or vision. Additionally, a UE inherently comprises one or more controller or microcontroller devices 112 and related circuitry adapted to perform, among other things, mode switching of the one or more transmitter (TX) branches 106, 108 within a transmitter architecture 110.
Denoting the absolute phases of transmitter branch #1 106 and #2 108 by φ1(t) and φ2(t), respectively, the relative phase (RP) is defined as δφ(t)=φ1(t)−φ2(t). Then the relative phase discontinuity (RPD) is defined as the difference of RP between two time instants t1 and t2, i.e., δφ(t1)−δφ(t2).
The RPD of a transmitter branch typically comprises a power-dependent term and a time-dependent term. The power-dependent term depends on the transmit power, whereas the time-dependent term varies with time. From the viewpoint of modelling, the power-dependent term can be given as a function of the current transmit power, whereas the time-dependent term can be given as an additive random process.
The power-dependent RPD mainly comes from the power/configuration mode switching by which each transmitter branch switches the gain/bias state. The potential sources of the power-dependent RPD can be summarized as follows:
Power mode switching: Many state-of-the-art Power Amplifiers (PAs) switch the power mode according to the transmit power, in order to improve the power efficiency. Without extra design effort (or additional circuitry), the two transmitter branches 106, 108 tend to respond to the power mode switching differently, thereby resulting in RPD across the switching points.                Configuration mode switching: Depending on the transmit power, the RF/ABB switches the configuration modes characterized by gain switching, adaptive biasing, signal path switching etc. in order to reduce the power consumption. Without extra design effort (or additional circuitry), it is likely that the two transmitter branches experience different phase variation across the switching points. Therefore, the transmitter tends to experience non-negligible RPD in case of configuration mode switching.        AM-to-PM distortion: Since PAs are typically operated around the compression point to maximize the power efficiency; they may experience non-negligible AM-to-PM distortion without additional circuitry (e.g., digital pre-distortion). It is likely that the two PA devices have slightly different compression points, operate at slightly different power and operate under different loading conditions. This causes different distortion in the transmitter branches and consequently the transmitter tends to experience non-negligible RPD.        
When it comes to precoder selection, the RPD between the measurement and the relevant precoding are the most important considerations. Recalling that SRS is a natural choice for precoder selection, the importance of the RPD between the SRS transmission used for precoder selection and the subsequent PUSCH transmission applying the precoder is easily understood. The RPD may lead to non-optimal precoder selection, even when the wireless channel is perfectly known to the Node B. This may result in non-trivial performance loss, since the precoder selection typically relies on the phase information of the transmitter chains.
It follows that the time frame of interest is about a few (or a few tens of) subframes. It depends on the processing time (measurement and precoder selection) and the SRS periodicity. For example, if the processing time is 4 msec and the period of SRS transmission is 10 msec, a minimum of 8 msec and a maximum of 18 msec should be assumed as the time frame. Given such a time frame, the power-dependent term has a larger impact on RPD than the time-dependent term and thus there is a need to focus on the power-dependent term, i.e., how to cope with it, in some embodiments of this invention.
Denoting the current transmit power by P(t), the absolute phase is given asφ1(t)=f1(P(t))φ2(t)=f2(P(t)),  (1)where f1(x) and f2(x) represent the power dependence of absolute phase for the two transmitter branches. This is exemplified in FIG. 2. Defining the power dependence of RP as f1,2(x)=f1(x)−f2(x), the corresponding RP is given asδφ(t)=f1,2(P(t)).  (2).
This is further exemplified in FIG. 3. In other words, the Relative Phase (RP) is given as a function of the current transmit power. Similarly, the RPD between t1 and t2 is given asδφ(t1)−δφ(t2)=f1,2(P(t1)−f1,2(P(t2))  (2)
Therefore, the RPD is given as a function of the transmit powers of the two time instants. In other words, it is the transmit power change that gives rise to the RPD. For example, there exists no RPD, if the transmit power remains constant, i.e., P(t1)=P(t2). It is also found out that, given a certain level of transmit power change, the resulting RPD is determined by the power-dependence of the RP. For example, when the RP is independent of the transmit power, i.e., f1,2(P)=C (constant), there exists no RPD.
FIG. 4 shows an example of the relationship between transmit power and the RP. It is shown that RP changes abruptly across, for example, two transmit power levels of 0 dBm and 10 dBm—referred to as switching points (or switching point power levels) hereafter. In this example of FIG. 4, there are two switching points 402, 404 whose power levels are 0 dBm (a low default switching point power level) and 10 dBm (a high default switching point power level). There are three operation modes: the operation mode below 0 dBm is called Low-Power Mode (LPM) 406, the operation mode between 0 dBm and 10 dBm is called Mid-Power Mode (MPM) 408 and the operation mode above 10 dBm is called High-Power Mode (HPM) 410. Each mode has its own bias state and the phase of a bias state is not necessarily equal to those of other bias sates. This justifies the aforementioned RP change across the switching points. In other words, the transmit power change causes the UE or the controller 112 to mode switch the UE transmitter 110 and the mode switching in turn causes the RPD.
Every time a UE operates across a switching point 402, 404, the operation mode changes, thereby creating RPD. What is needed is a method of mode switching that can be used to alleviate or decrease the occurrence of RPD. Additionally, what is needed is a method of mode switching that eliminates unnecessary mode switches and thereby alleviates, decreases or reduces RPD. Furthermore, what is needed is a method of mode switching that can avoid unnecessary mode switching between SRS and PUSCH and thus mitigate the RPD between SRS and PUSCH.