Patent Application
20080101492
The disclosed embodiments relate to a method for tracking and compensating phase noise in an OFDM communication system.
In any wireless device phase tracking is necessary to compensate for the phase noise present in any local oscillator. The local oscillator doesn't have a perfectly constant frequency and any deviation from its nominal frequency induces a frequency offset. In a receiver or in a signal analyzer this results in two distinct effects: a rotation of the received complex symbols (causing a visible rotation on the constellation plot) due to close-to-carrier phase noise and a noise-like additive signal due to far-from-carrier phase noise. Any specification document of a local oscillator contains indications of phase noise values at different offset frequencies. Phase noise can accurately be measured and the resulting deviation from the nominal frequency can be simulated.
In today's OFDM systems tracking and compensating for the close-to-carrier phase noise is made possible by reserving some subcarriers of the subcarrier set for synchronisation purposes. The subcarriers used for synchronisation are called pilot tones. Each pilot tone carries a pilot symbol. Fortunately, the close-to-carrier phase noise present in the received signal affects all subcarriers in the same way. For this reason, the close-to-carrier phase noise component is often called ‘common phase noise’ (CPN). This is of importance, because it means that a value of the CPN obtained for one subcarrier can be applied to the other subcarriers. Note that the part of the phase noise due to far-from-carrier phase noise cannot be compensated for.
In classical Single Input/Single Output (SISO) OFDM systems the CPN is computed and removed after equalization (whereby the channel effect is removed). This is done by first correlating the pilot tones with their known ideal values, which yields an approximation of CPN for each symbol and subsequently applying the inverse rotation.
However, when considering tracking in a Multiple Input/Multiple Output (MIMO) communication, several other factors have to be taken into account:
Traditional MIMO systems often use at least shared or locked local oscillators. At the receiver side a local oscillator produces a signal that gets mixed with the RF signal to downconvert it from a radio frequency (RF) to baseband or to an intermediate frequency (IF). One downconversion is necessary per RF chain. Hence, if there are N RF chains, N downconversion operations are required. In traditional multichannel systems, the signal used to downconvert the N chains comes from the same local oscillator (LO). So, in this case, the LO is ‘shared’ among the N RF chains. If several LOs are used, another option to make them behave as if they were shared is to ‘lock’ them. In that case, a special mechanism is used to phase align the output signals from the different LOs. Even if they are mixed a same rotation can be observed on each combination of the different streams. This implies that the tracking scheme for a MIMO system is not so different from that of a SISO system: after the MIMO decoding (corresponding to equalization in SISO), and possibly spatial demapping, the tracking is carried out per stream, using the same ideal pilot values as in the transmitter.
However, in the above the hypothesis is assumed that the Local Oscillators are shared or locked, so the CPN components from different streams are obviously correlated. This is not the case anymore when samples are taken at different moments in time: the CPN components then are not ‘correlated’ anymore. Moreover, if in the transmitter the LOs are neither shared, the result is the same: different CPN components get mixed and it becomes impossible to track them. Hence, there is a need to overcome this problem.
The presently disclosed embodiments aim to provide a method for tracking and compensating phase noise in an OFDM communication system that is suitable for both SISO and MIMO systems and that overcomes the drawbacks of the prior art solution.
The embodiments presented relate to a method for tracking and compensating phase noise in an OFDM communication system, comprising the steps of:
In a preferred embodiment the distortion comprises the channel distortion introduced by the channel over which said applied signal is transmitted.
The method is advantageously applied to a multiple input/multiple output (MIMO) OFDM communication system. The distortion then comprises also the effect of a multiplication with a spatial mapping matrix. The spatial mapping can be performed by applying a direct mapping, spatial expansion, beamforming or by cyclic shift diversity.
Preferably the signal is prerotated by a value equal to the averaged offset value determined for a previously applied signal.
In a specific embodiment the OFDM communication system is a signal analyser.
In another aspect the embodiments relate to a method for MIMO communication wherein a downconversion from RF to baseband or IF frequency is applied and wherein a step of tracking and compensating phase noise is performed with the method as previously described, before carrying out a MIMO decoding operation.
A closer look at the MIMO receiver scheme shows that the signals with different CPNs only get mixed in the MIMO decoding block (if another method is used to decode than a per stream equalization) and in the spatial demapping block.
The solution according to the disclosed embodiments therefore proposes to perform the tracking before the MIMO decoding block. This means there are some important challenges to tackle. Firstly, the channel has to be taken into account as it has not been equalized yet. Further, in case the spatial mapping is performed by spatial expansion (i.e. by spreading the spatial streams to the transmit chain by multiplying them with a Hadamard mixing matrix) or by beam forming (i.e. predistorting the transmitted MIMO signal based on the channel characteristic in such a way that each stream is steered in the spatial domain to its proper destination user) one needs to cope with mixed pilot signals due to the spatial mapping.
Spatial mapping can be illustrated by the following short example. Assuming there are two input streams, denoted by a (complex symbols) vector [St1 St2]. The spatial mapping is a mapping method of the streams St1 and St2 to the transmit chains Tc1 and Tc2. The mapping is made of linear combinations of the inputs. The multiplication of the vector [St1 St2] by a “mixing’ matrix Qk can therefore be expressed as:
An important asset of the method disclosed herein is that the tracking is independent from the MIMO decoding algorithm used. For example, if the decoding algorithm is not linear, tracking would become difficult if it is performed after equalization. Moreover, if the local oscillators (LOs) in the transmitter are shared or locked, the tracking can still be carried out even if a real channel is placed in between (emulator or wireless channel) and even if a solution where a receiver would switch in time between the different received channels is used.
As already mentioned, the tracking is performed before the MIMO decoding, as shown in the block scheme of
In order to be able to reconstruct the pilot sequence, it is important to have a clear understanding of how the pilot tones are modified at various stages of the transmit/receive process.
The method assumes knowledge of the spatial mapping matrix Qk and of an estimate of the channel matrix. In practice both are available. Note that the matrix Qk simply becomes a scalar 1 when the OFDM system is a Single Input/Single Output (SISO) system. Further, as already mentioned, if a direct mapping is used, the matrix Qk is an identity matrix. A channel matrix estimate can be derived either blindly or by means of a known preamble field (if present), as is well known in OFDM communication systems.
The tracking algorithm itself can be described by the following steps:
In a specific embodiment the MIMO receiver system can be a signal analyser. This may occur when the method is applied on a test set-up wherein test equipment is used. This implies that the connections consist of wires. The channel matrix can then be considered diagonal. In this case the CPNs from the transmitter local oscillators (LOs) don't get mixed in the channel. At the receiver side one disposes of knowledge about the Qk matrix and the channel matrix H. So if the pilot ideal values are taken for each transmitter stream, one can construct ‘composite’ pilot values for each receiver stream by multiplying the pilot vector (vector corresponding to one subcarrier and containing Nstreams values, whereby Nstreams denotes the number of spatial streams at the input of the spatial mapping block) by the matrix Qk and subsequently multiplying the resulting vector by the channel matrix H.
The application field of the disclosed embodiments is not restricted to receivers only. It fits any application that needs the downconversion of an OFDM signal from RF to baseband or IF (intermediate frequency), as this downconversion implies the mixing of the RF signal with a carrier coming from one or several local oscillators.
In such equipment, the signal is being distorted to mimic a certain channel profile. Typically, OFDM signals are distorted in the frequency domain and at baseband. If such equipment has RF inputs, phase noise appears as a downconversion is required. Consequently, it is necessary to track the phase noise. The present method can be therefore applied.
