WO1998032267A1

WO1998032267A1 - Ofdm receiver using pilot carriers

Info

Publication number: WO1998032267A1
Application number: PCT/GB1997/003549
Authority: WO
Inventors: Roger Paul Dealtry; Adrian Charles Turner
Original assignee: Nds Limited
Priority date: 1997-01-17
Filing date: 1997-12-24
Publication date: 1998-07-23
Also published as: GB9700947D0; AU5333398A

Abstract

This invention relates generally to receiving frequency division multiplex signals and more precisely to a method and apparatus of receiving orthogonal frequency division multiplex (OFDM) signals. OFDM signals comprise both data and reference carriers (also known as pilots). The pilots are used to compensate the data carriers for channel distortions and phase errors common to all carriers, which are introduced by the receiver itself. An interpolation technique is used.

Description

OFDM RECEIVER USING PILOT CARRIERS

The present invention relates to a method and apparatus for receiving frequency division multiplex signals and is particularly adapted to receiving orthogonal frequency division multiplex (OFDM) signals and discrete multitone (DMT) signals. OFDM signals include data carriers and reference carriers. The reference carriers are in the form of a sequence of pilot signals which are distributed in frequency and in time and are transmitted with a pattern of values known to the receiver. The pilot signals are used to calculate the data values between them and the data carriers are modified by reference to the pilot signals to compensate for channel distortions. This process is known as channel equalisation.

The present invention seeks to provide an improved method and apparatus to receive a frequency division multiplex signal.

According to the present invention there is provided apparatus to receive a frequency division multiplex signal which includes a sequence of symbols each of which has data carriers and reference carriers, the apparatus comprising an extractor to extract reference carriers from the multiplex signal, an interpolator to interpolate values interspersed between the extracted reference carriers, error indicating means to derive an indication from the reference carriers of a common phase error for each symbol and means to modify the interpolated values in response to the common phase error indication.

Further according to the present invention there is provided a method of receiving a frequency division multiplex signal which includes a sequence of symbols each of which has data carriers and reference carriers, the method comprising extracting reference carriers from the multiplex signal, interpolating values interspersed between the extracted reference carriers, deriving from the reference carriers an indication of a common phase error for each symbol and modifying the interpolated values in response to the common phase error indication.

The invention will now be described, by way of example with reference to the accompanying drawings in which:

Figure 1 shows a block diagram of an OFDM receiver according to the present invention,

Figure 2 is one example of the pattern of data carriers and pilot signals in an OFDM signal to be received by the receiver of Figure 1 , and

Figure 3 is block circuit diagram showing a time interpolator section of the receiver of Figure 1 ,

Referring to Figures 1 and 2, there is shown a receiver for receiving an OFDM signal at an input lead 10. The OFDM signal includes a succession of symbols which are indicated in Figure 2. Each symbol includes a set of frequency spaced data carriers indicated as x in Figure 2 together with scattered pilots or reference carrier signals, indicated as O, which are distributed amongst the data carriers. In addition the symbols each have continuous pilots or reference carrier signals O one of which is shown at the right hand margin of Figure 2.

The scattered pilots are distributed throughout the data carriers in a predetermined pattern and have values which are known at the receiver. The receiver has a circuit 11 to extract the data to be passed on to a block 14 at which the data carriers are modified to compensate for distortions in the transmission channel. The pilot or reference signals are also extracted at the circuit 11 and passed to a block 13. The block 13 uses the pilots to model the transmission channel and supplies signals to the block 14 which equalises the data received from the circuit 11.

The transmission channel introduces distortions which change across the carriers and with time. A problem is introduced by the receiver itself which adds a small phase error component which is common to all the carriers of one OFDM symbol and is known as the common phase error. Thus, as seen in Figure 2, the first symbol is formed by the carriers and pilots indicated at the top line and all subject to the common phase error δ

At the left margin of Figure 2, the first carrier is shown to have a channel phase shift φ., and a common phase error δ., The continuous pilot shown at the right margin of Figure 2 has a channel phase shift of oc, and a common phase error of δ . The second symbol in Figure 2 has a common phase error of δ₂ associated with all the carriers of the second symbol. The continuous pilot of the second symbol has a channel phase shift of α₂ and a common phase error of δ₂ . Succeeding symbols n have respective channel common phase errors of δ_n as indicated in Figure 2 for the first seven symbols in the sequence of symbols. Each continuous pilot signal has a corresponding channel phase shift α_n and common phase error δ_n as shown for the first seven symbols.

The scattered pilots are arranged in a pattern throughout the sequence of symbols such that the pattern repeats once every four symbols. This means that a pilot value has to be interpolated in time for those positions which do not have a transmitted pilot value. Each time interpolated pilot value is derived by reference to the repeating pattern of transmitted pilot values and consequently, although the data carriers of each symbol have the common phase error associated with that symbol, the interpolated pilot values will have a phase error which is made up of a combination of the common phase errors of other symbols. It is advantageous to maintain the correct common phase component in the interpolated pilot values.

The method will now be described with reference to Figure 2. using the

following convention:

V₄ describes the actual value of a carrier in symbol 4

V'₄ describes the estimated value of a carrier in symbol 4

For the fourth symbol represented in the diagram of figure 2 the interpolated

value V'₄ for the leftmost carrier comprising the channel phase shift and the

common phase component is:

V₄ = (φ₂ + φ₆) / 2 + (δ₂ + δ₆) /2 (1 )

whereas the required value is

V₄ = φ₄ + δ₄ (2)

By comparison of equations (1 ) and (2) we can equate (φ₂ + φ₆) / 2 and φ₄ if

we assume that linear interpolation models the channel sufficiently over four

symbols. The common phase components of each symbol are uncorrelated

and therefore the same model can not be relied upon to determine the

common phase component in symbol 4. The error in the interpolated value for

the common phase E₄ is represented by

E₄ = (δ₂ + δ₆) / 2 - δ₄ (3) If time interpolation is performed between the second and sixth symbols of

the continuous pilot shown on the far right of figure 2 in an identical manner, a

similar estimate K'₄ will be produced for the fourth symbol of that carrier

K'₄ =(α₂+ α₆)/2 + (δ₂+ δ_β)/2 (4)

Because the carrier under consideration in this case is a continuous pilot, the

absolute value comprising the channel phase shift and common phase

component K₄ is available

K₄ =α₄+ δ₄ (5)

and the common phase error term in the interpolator output for symbol 4 can

be evaluated by subtracting the values of equation (4) and (5) to give E₄

E₄ = (δ₂+ δ₆)/2-δ₄ (6)

Which is the value required to correct the interpolated common phase term of

equation (1). Referring now to Figure 3, there is shown in greater detail the circuit which constitutes part of the block 13 of Figure 1. The reference signals are supplied to a pilot signal separator 15. The separator 15 supplies the scattered pilots to a scattered pilot store 16 which stores the two most recent values for the transmitted pilots. A time interpolator 17 performs a linear interpolation between the values stored in the pilot store 16 to provide a time interpolated value for each possible pilot carrier. Thus one in four of the continual pilot values are stored in the scattered pilot store 16, and a continuous pilot store 18 and an interpolation error accumulator circuit 19 are used to provide error signals to correct the common phase components in the time interpolated pilot signals produced by the time interpolator 17.

The store 18 receives continuous pilot signals which have been extracted by the pilot signal separator 15 and is effective to store the continuous pilot signals for the last five symbols. The error accumulation circuit performs a sequence of comparisons between the continuous pilots in the store 18 and the interpolated estimates produced for each symbol by the time interpolator 17 operating on the continual pilots. For the purpose of comparison, the time interpolator supplies each time interpolation as an input to the error accumulation circuit 19.

As can be seen from Table 1 , 3 error terms E_n+1, E_n+2 and E_n+3 are calculated from the continuous pilot signals to derive an indication of error which is forwarded from the accumulator 19 to a common phase error restoration circuit 20. The circuit 20 restores the common phase error in respect of the interpolated pilots which are transmitted to the restoration circuit from the time interpolator 17 by way of a delay circuit 21 which imposes a delay to the pilots of one symbol. The accuracy of the error signal passed to the restoration circuit 20 is improved by averaging each of the 3 error signals across the symbol. Frequency interpolation of the pilots can now be carried out across the symbol maintaining the common phase error which will be equalised out along with the channel distortions. This adds little complexity and prevents errors propagating in time and subsequently in frequency.

TABLE 1

Position 1 φ_n + δ_n Absolute value available

Position 2 φ_{n + 1} + δ, n+1 -n + 1 = 3/4δ_{n + 1} + 1/4δ_n

Position 3 φ_{n + 2} + δ_{n + 2} -n + ₂ - (δ_{n +} ι + δ_{n + 4} )/2-δ_n

Position 4 φ_{n + 3} + δ_{n + 3} E_{n + 3} = 1/4δ_n+1 + 3/4δ_{n + 4} -δ_{n + :}

Claims

1. Apparatus to receive a frequency division multiplex signal which includes a sequence of symbols each of which has data carriers and reference carriers, the apparatus comprising an extractor to extract reference carriers from the multiplex signal, an interpolator to interpolate values interspersed between the extracted reference carriers, error indicating means to derive an indication from the reference carriers of a common phase error for each symbol and means to modify the interpolated values in response to the common phase error indication.

2. Apparatus as claimed in claim 1 , wherein there is further provided means to extract the data carriers and to modify the data carriers to compensate for distortions in the transmission of the multiplex signal.

3. Apparatus as claimed in claim 1 or 2, wherein the extractor is adapted to extract scattered reference carriers in the form of a sequence of pilot signals which are distributed in frequency and in time in the multiplex signal.

4. Apparatus as claimed in claim 3, wherein a scattered pilot store is provided to store a plurality of scattered pilot signal values and the interpolator performs a linear interpolation between the values stored in the scattered pilot signal store.

5. Apparatus as claimed in claim 4, wherein a continuous pilot store is provided to store a plurality of continuous pilot signal values, the error indicating means being adapted to compare the continuous pilot signal values with the output from the linear interpolator to derive the said indication of common phase error.

6. A method of receiving a frequency division multiplex signal which includes a sequence of symbols each of which has data carriers and reference carriers, the method comprising extracting reference carriers from the multiplex signal, interpolating values interspersed between the extracted reference carriers, deriving from the reference carriers an indication of a common phase error for each symbol and modifying the interpolated values in response to the common phase error indication.

7. A method as claimed in claim 6, wherein the data carriers are extracted and modified to compensate for distortions in the transmission of the multiplex signal, the modification being by means of the reference carriers.

8. A method as claimed in claim 7, in which scattered reference carriers are extracted, the scattered reference carriers being in the form of a sequence of pilot signals which are distributed in frequency and in time in the multiplex signal.

9. A method as claimed in claim 8, wherein a plurality of scattered pilot values are stored in a scattered pilot store and the interpolation is a linear interpolation between values stored in the scattered pilot store.

10. A method as claimed in claim 9, wherein a plurality of continuous pilot signal values are stored in a continuous pilot store, and the error indication is derived by comparing the continuous pilot signal values with the result of the linear interpolation.