APPARATUS AND METHOD FOR RECEIVING DIGITAL VIDEO SIGNALS

Abstract

An apparatus is operable to receive a digital video signal transmitted over a channel and comprises an operational module configured to operate in a first mode of operation and in a second mode. The apparatus is configured to switch operation of the operational module from the first mode to the second mode in dependence of an estimate of an environment (condition) of the channel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the concept of a TDM-based power savings scheme;

FIG. 2 illustrates a hierarchical frame structure of DMB-T;

FIG. 3 illustrates a simplified block diagram of a DTT transceiver; and

FIG. 4 illustrates an implementation of a power reduction scheme in the receiver of the DTT transceiver of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiment of the present invention will be described in detail by way of following examples and with reference to the above-mentioned figures.

The above analysis on time-slicing and frame-slicing power reduction schemes show that a high burst data-rate, C_b in equation (1), is necessary for both time-slicing and frame-slicing schemes to achieve the required power saving efficiency. Also, to maintain a certain level of quality of service (QoS), the required high C_b should be always achievable independent of channel environment variations. Together, in practice, these requirements imply that the system architecture design and the choice of related algorithms should make the high-rate transmission workable under the worst channel conditions such as a fast fading environment (with larger Doppler frequency shift—a significant problem for mobile devices).

FIG. 3 depicts a simplified architecture 30 of a DTT transceiver. At the transmitter side, the MPEG TS 32 is first encoded by a Reed Solomon (RS) outer encoder 34. An outer interleaver 36 is deployed such that its receiving counterpart—outer de-interleaver 68—spreads the possibility of burst errors from the inner channel decoder 66.

After that, the bit streams will be encoded by an inner channel encoder 38 such as a convolutional coder, Turbo coder or Turbo-like coder. The coded bits are then sent to an inner interleaver 40. The resulting interleaved bit streams 42 are mapped to phase shift keying (PSK) or quadrature amplitude modulation (QAM) constellations (not shown). Finally, these constellation mapped symbols, are used to form the orthogonal frequency division multiplexing (OFDM) signal frames by an OFDM Modulator 44.

The transmitter also comprises a digital-to-analogue converter (DAC) 46 and a RF transmitter 48 for transmitting the transmission signal over a channel 50 to the receiver.

At the receiver side, the reverse operations of the transmitter are performed with some additional processing blocks such as automatic gain control (AGC), synchronisation and channel estimation for handling the noisy and multipath fading channel environments. As illustrated in FIG. 3, the receiver comprises an RF tuner 52, an analogue-to-digital converter (ADC) 54, a block 56 for carrier frequency, symbol timing synchronisation and channel estimation, automatic gain control 58, an OFDM demodulator 60, channel equalisation 62, an inner de-interleaver 64, an inner channel decoder 66, an outer de-interleaver 68 and an RS decoder 70.

If one considers P_ALL=P_RF+P_BB to be the overall power consumption of a regular DTT receiver (i.e., the device may not implement a TDM-based power reduction scheme), and P_RF and P_BB be the power consumed by the RF tuner 52 and the baseband processor (not shown), respectively. The required power consumption for a handheld device becomes:

$\begin{array}{cc}{P}_{\mathrm{HA}}\approx \frac{\left(S_b/C_b+t_s\right)\left(P_{\mathrm{RF}}+P_{\mathrm{BB}}\right)}{S_b/C_l}& \left(2\right)\end{array}$

Since DTV broadcasting is mainly in downlink transmission, it can be assumed that P_RF only varies with the AGC 58 control in adaptation to variations of actual channel 50 environment. In the baseband part, however, the situation is quite different. Processing complexity and, thus, required power consumption, P_BB, are usually design-dependent and become fixed after realisation. Thus, P_BB can be regarded as independent on the channel variations. Obviously, by default, the baseband processor would operate at its highest level of P_BB as the design and implementation of baseband demodulator and decoder need to take account of the worst channel condition. Taking into consideration the fact that P_RF and P_BB occupy almost an equal proportion of the overall power in a regular OFDM-based DTT system, it becomes necessary to reduce further P_BB such that the required power consumption, P_ALL, of a regular device, or, P_HA, of a handheld device is minimized. Following equation (2), when the required P_BB of a regular DMB-T device goes down from 800 mW to 500 mW, for example, P_HA will be down to 30 mW from 40 mW.

Given a high target of C_b, operational module control algorithms which are usually of high complexity are most likely selected for achieving robust receiving under less-than-ideal channel conditions. The channel estimation algorithm, for example may need to be enhanced for fast fading channel conditions in a mobile environment. These enhanced algorithms, which are usually computationally expensive, are actually redundant in situations such as when the user is slowly moving (e.g., pedestrian) and even still.

A power reduction scheme is illustrated in FIG. 4. The receiver apparatus comprises an operational module configured to operate in a first mode and in a second mode, the apparatus being configured to switch operation of the module from the first mode to the second mode in dependence of an estimate of an environment (condition) of the channel. Examples of the operational modules of the receiver are the AGC 80, ADC 82, Channel Estimator 84 and Inner Channel Decoder 86 of FIG. 4. The apparatus may also have other operational modules depicted generally by 85, 87. An example of a first mode of operation for, e.g., ADC 82 is for the ADC 82 to operate with “normal” sampling resolution. The second mode of operation for ADC 82 is for the ADC 82 to operate with lower sampling resolution.

The receiver is configured to make a decision on whether to operate one or more of operational modules 80, 82, 84, 85, 86, 87 in the first or the second mode from a real-time assessment of the channel 50 conditions. One way of doing this is to monitor the error detection activities of the RS decoder (88 in FIG. 4), commonly adopted as the channel outer decoder in most DTT systems to estimate the channel environment or condition. Here, whether or not the receiver receives N error-free consecutive RS coding blocks (before error correction by RS, if any) is used as a decision criterion for assessing whether the channel environment is good or not good. If, at a time instant t, the receiver has received N or more than N consecutive error-free RS coding blocks, the current channel condition is assessed as “good”. Otherwise, the current channel condition is assessed as “not good”. Here, the value of N, which can be selected as a positive integer, controls the reliability of channel condition assessment. When N is selected small, the assessment result “the channel is not good” is more reliable than “the channel is good”. Correspondingly, when N is selected large, the assessment result “the channel is good” is more reliable than “the channel is not good”. When the receiver determines that the channel is in a “good” condition, the receiver switches one or more operational modules 80, 82, 84, 85, 86, 87 from the first mode of operation to the second mode of operation.

When continued monitoring of the channel is effected—i.e. an estimation of the channel environment is an ongoing process—the receiver is configured to toggle between first and second modes of operation in dependence of the continued estimation. Because of the reduction in complexity of the operational status of the receiver, embodiments of the receiver are configured to consume less electrical power when the module operates in the second mode of operation than when in the first mode of operation.

A detailed explanation of FIG. 4 is now given. Control variables M, N, P and k of FIG. 4 are defined as follows:

N is a predetermined minimum number of consecutive error-free RS coding blocks which are received at the receiver prior to a determination that the channel is in a “good” condition;

M is a predetermined maximum number of consecutive error-free RS coding blocks which are to be received in the second mode of operation prior to reverting to operation in the first mode of operation;

After operation in the second mode of operation, P is a predetermined minimum number of consecutive error-free RS coding blocks which are to be received in the first mode of operation prior to switching back to operation in the second mode of operation when the channel is in a “good” condition; and

k is a count of error-free RS coding blocks received in a channel “good” condition (i.e. after receipt of N error-free blocks described above) and is used to control the process flow.

Initially, k is set to zero, and any or all of operational modules 80, 82, 84, 85, 86, 87 are operated in the first mode of operation. RS decoder 88 monitors the signal received over channel 50 for N consecutive error-free RS coding blocks at decision step 90. Before N consecutive error-free RS coding blocks are detected, the condition of channel 50 is considered to be “not good”. As such, k is kept at zero at step 92 and the one or more operational modules 80, 82, 84, 85, 86, 87 are operated in the respective first modes of operation. Upon detection of the Nth consecutive error-free RS coding block at step 90, the apparatus determines that the channel is in a “good” condition, and switches operation of one or more of operational modules 80, 82, 84, 85, 86, 87 to the second mode of operation.

Power saving can be effected by replacing complicated operation of the operational modules 80, 82, 84, 85, 86, 87 with simpler operational modes when the channel is found to be good for signal transmission. That is, in one example, the first mode of the operational module is a normal mode of operation and the second mode of the operational module is a simplified mode of operation. In FIG. 4, the receiver implements the power reduction scheme for any or all of operational modules 80, 82, 84, 85, 86, 87 in a DTT receiver as examples for illustrating the concept. If the channel condition is determined to be good, the AGC 80 gain of low-noise amplifier (LNA) in the RF tuner 52 is set to operate with a second mode gain which is less than a first mode gain; that is, to a lower but yet acceptable level such that lower power consumption can be achieved. In one example, the ADC module 82 is configured to operate with a second mode sampling resolution which is less than a first mode sampling resolution; the sampling resolution in the second (simplified) mode of operation is less than the sampling resolution in the first (normal) mode of operation. Similarly, as the inner channel decoding may involve a certain number of iterations (e.g., with Turbo decoder) and/or require certain data resolutions (e.g., with a soft-decision convolutional decoder), the power saving can be achieved by reducing the iteration number and/or the word-length when switching modes of operation from the first mode to the second mode.

When in the second mode of operation, the apparatus will revert operation of the one or more operational modules to the first mode of operation in either of two ways. First, if an error is detected in an RS coding block, decision step 90 determines that N consecutive error-free coding have not now been received. Count k is then reset to zero at step 92 and the one or more operational modules are then switched back to the first mode of operation.

Secondly, to prevent any possible misjudgement due to, for example, baseband processing delay, and in order to make the channel adaptation still robust when it is not possible to make a clear distinction between good or not good channel conditions, a regular return to the first mode of operation (i.e. a more sophisticated processing state), even when the current channel conditions are found good, is performed. After determination at step 90 that the channel 50 is in a “good” condition, the apparatus checks at step 94 whether the number of the presently-received block is equal to M. In other words, the apparatus determines whether the maximum number of consecutive RS coding blocks in the second mode (i.e. simplified mode) of operation has been exceeded (k is equal to M). If the apparatus determines at step 94 that M has not been exceeded, then the one or more of operational modules 80, 82, 84, 85, 86, 87 continue to operate in the second mode and count k is incremented by 1 at step 96.

If no error is detected in the received RS coding blocks, the apparatus controller loops around steps 80/82/84/85/86/87, 88, 90, 94, 96 incrementing k in each loop until the apparatus determines that count of the presently-received RS coding block means that the number M has been reached (that is, k is equal to M) and proceeds to revert operation of the one or more operational modules 80, 82, 84, 85, 86, 87 to the first (normal) mode of operation.

When operation is switched back to the first mode, predetermined count P defines the number of RS coding blocks to be received in this iteration of operation of the one or more operational modules 80, 82, 84, 85, 86, 87 in the first mode. At step 98, the apparatus determines whether the kth received coding block means count k=M+P. If not, count k is incremented by 1 at step 100 and operation of the one or more operational modules continues in the first mode of operation.

Upon detection that k equals M+P, k is reset (i.e. forced to zero) at step 92, and the one or more operational modules 80, 82, 84, 85, 86, 87 of the apparatus continue operation in the first mode. However, in the next pass through step 90, the apparatus detects that k has been reset to zero. If the channel condition remains “good” then receipt of N error-free coding blocks at step 90 is immediately detected and operation of the one or more operational modules is switched back to the second mode of operation.

Thus, by monitoring the parameter k, a balance between quality of service (QoS) with power savings can be effected. Further, continued toggling between first and second modes of operation is controlled by prescribing the maximum number of consecutive RS coding blocks that can be received in the second mode, and the minimum number of consecutive RS coding blocks that should be received in the first mode; that is, with reference to counts M and P. Therefore, it can be seen that such a signal processing algorithm can control the apparatus to toggle operation of the one or more operational modules between first and second modes in dependence of the received coding blocks, not only between good and bad channel conditions, but also in a regular pattern when the channel is friendly to transmission.

It should be pointed out that, here, the choice of the parameters N, M and P depends on actual design requirements, as these parameters are the determinant factors for balancing the required power saving efficiency and the QoS to be provided. If N is selected small, M large and P small, more power saving can be achieved, but at the price of lower QoS, and vice versa. In actual implementation, these parameters can be predefined or be hardware-reconfigurable.

Thus, it is possible to reduce further the required P_BB of a DTT receiver by making P_BB adaptive to the actual channel environment. Given a high target of C_b, those algorithms which are usually of high complexity are most likely selected for achieving robust receiving under very bad channel conditions. The channel estimation algorithm, for example may need to be enhanced for fast fading channel conditions in a mobile environment. These enhanced algorithms, which are usually computationally expensive, are actually redundant in most situations such as when the user is moving slowly (e.g., pedestrian) and even still.

The channel estimation 84, which is a significant component for achieving acceptable system performance in a mobile environment, is now discussed as a detailed example of demonstrating the effectiveness of the proposed power saving scheme. As discussed above, this module may be switched from enhanced to simplified functionality to reduce power consumption.

In the DMB-T system, the channel estimation is per signal frame based and is performed in the time domain using the PN sequence of each Frame Sync 20, please refer to China Patent Application No. 200410009944.1, publication date: May 18, 2005 (Patent 944). Suppose that the channel impulse response (CIR) at the Frame Sync 20 of the n^th signal frame has been estimated as ĥ(n,N₀,l). Here, N₀ denotes the relative position of Frame Sync 20 in a signal frame 8 and l denotes the index of CIR taps. Assume that the first path is the main path of the channel 50, the channel frequency response (CFR) estimation at the k^th subcarrier, Ĥ(n,N₀,k), over the Frame Sync 20 interval of the n^th signal frame 8 can be obtained by performing a discrete Fourier transform (DFT) on ĥ(n,N₀,l).

It can be seen that the CFR estimation achieved, Ĥ(n,N₀,k), can be used for equalising the Frame Body 22 of the n^th signal frame provided that the channel 50 is invariant over the duration of a signal frame 8. However, this may not be always true in practice, as indicated in Patent 944. When the channel 50 is timing-varying over the duration of a signal frame 8, the following enhanced channel estimation described in Patent 944 may apply.

Assume that the channel 50 undergoes a linear variation over the period of a signal frame 8, the k^th subcarrier's CFR, Ĥ(n,N₀,k), at the time instant of the i^th data symbol of the n^th Signal Frame Body 22 can be estimated by linear interpolation as:

Ĥ(n,i,k)=Ĥ_A(n,k)−a_iĤ_D(n,k)  (3)

where a_i is a linear function of i. Define:

Ĥ _A(n,k)(Ĥ(n,N₀,k)+{circumflex over (H)}(n−1,N₀,k))/2  (4)

and:

H _D(n,k)=(Ĥ(n,N₀,k)−{circumflex over (H)}(n−1,N₀,k))/2  (5)

And let X(n)=[X(n,1), X(n,2), . . . , X(n,N_b)] and Y(n)=[Y(n,1), Y(n,2), . . . , Y(n,N_b)] be transmitted and received data vectors of the n^th signal frame body 22, respectively. Also, let us define the diagonal matrices, A=diag (a₁, a₂, . . . a_Nb); U(n)=diag (Ĥ_A(n,1), Ĥ_A(n,2), . . . , Ĥ_A(n,N_b)); and V(n)=diag (Ĥ_B(n,1), Ĥ_B(n,2), . . . , Ĥ_B(n,N_b)) with Ĥ_B(n,k)=Ĥ_D(n,k)/Ĥ_A(n,k). The system transmission thus can be modelled in the frequency domain as:

Y(n)=(I−T(n))·U(n)·X(n)+Z(n)  (6)

where Z(n) is a white Gaussian noise vector, and, T(n)=WAW^HV(n) with W and W^H being the DFT and inverse DFT (IDFT) matrices, respectively. Thus, the equalised n^th signal frame body becomes:

{circumflex over (X)}(n)=U(n)⁻¹·(I−T(n))⁻¹·Y(n)  (7)

where I is an identity matrix. The actual implementation of equation (7) requires significant expense as it involves a very complicated matrix inversion operation, (I−T(n))⁻¹. The high complexity can be reduced by using the following approximation as:

$\begin{array}{cc}{\left(I-T\left(n\right)\right)}^{-1}\approx \sum _{i=0}^QT^i\left(n\right)& \left(8\right)\end{array}$

As a result, the simplified equalisation can be performed as:

$\begin{array}{cc}\hat{X}\left(n\right)\approx {U\left(n\right)}^{-1}·\left[Y\left(n\right)+\sum _{i=1}^QT^i\left(n\right)Y\left(n\right)\right]& \left(9\right)\end{array}$

Therefore, the receiver is configured to receive a signal frame of the transmitted signal, the signal frame comprising a frame body, and to perform, in the frequency domain, a simplified equalisation of the frame body. Thus, embodiments of the receiver perform the simplified equalisation of the frame body by performing an approximation of a matrix inversion operation.

Obviously, the above channel estimation and equalisation can be easily incorporated into the proposed power reduction scheme, as a trade-off between system performance and computational complexity can be easily made simply by choosing a suitable Q value (i.e. number of iterations of the “T” process). Receiver designers can choose Q=0 in a case where the channel is good and increase it to 1 or an even larger value for fast varying channels. Note that, from equations (8) and (9), with an increment of 1, an extra “T” process is required. Since the “T” process involves both IDFT and DFT operations, significant power savings are expected by reducing one “T” process in this case. That is, the receiver performs the approximation of the matrix inversion operation in an iterative process, a number of iterations of the iterative process being determined in dependence of the estimate of the channel environment. The receiver may also transition between enhanced and simplified functionality of the channel estimator module by variation of the number of iterations of the iterative process.

Embodiments of the receiver are configured to operate with simplified functionality by performing the simplified equalisation of the frame body instead of the normal equalisation. Significant power reductions may still be realised in such implementations.

It should be emphasised that, although this particular example demonstrates the RS decoder's error detection capability to assess the channel conditions in this invention, the concept can be extended to other scenarios where the RS coding is replaced by another error detection/correction mechanism such as a cyclic redundancy check (CRC) or even low-density parity-check (LDPC) code. As long as the replacement has error detection capability, the power reduction scheme presented in this example remains valid.

Further, it will be appreciated that the invention has been described by way of example only and variations in design detail may be made without departing from the spirit and scope of the invention.

Claims

1. Apparatus for receiving a digital video signal transmitted over a channel, the apparatus comprising an operational module configured to operate in a first mode and in a second mode, the apparatus being configured to switch operation of the module from the first mode to the second mode in dependence of an estimate of an environment of the channel.

2. Apparatus according to claim 1, wherein the apparatus further comprises an error detection module for detecting errors in the communications signal and the apparatus is configured to estimate the channel environment by monitoring the error detection module.

3. Apparatus according to claim 1, wherein the first mode of the operational module is a normal mode of operation and the second mode of the operational module is a simplified mode of operation.

4. Apparatus according to claim 3, wherein the apparatus is configured to toggle between first and second modes of operation in dependence of continued estimation of the channel environment.

5. Apparatus according to claim 4, wherein the apparatus keeps the operational module operating in the first mode or switches operation of the module from the second mode to the first mode when the environment of the channel is estimated not to be in a good condition.

6. Apparatus according to claim 4, wherein the apparatus switches operation of the operational module between the first and the second modes periodically when the environment of the channel is continuously estimated to be in a good condition.

7. Apparatus according to claim 4, wherein the apparatus is configured to toggle between first and second modes of operation in dependence of a count of coding blocks received at the apparatus.

8. Apparatus according to claim 3, wherein the operational module is an automatic gain control module configured to operate with a second mode gain which is less than a first mode gain.

9. Apparatus according to claim 3, wherein the operational module is an analogue to digital converter module configured to operate with a second mode sampling resolution which is less than a first mode sampling resolution.

10. Apparatus according to claim 3, wherein the operational module is a decoder module configured, when operating in the second mode, to operate with a number of iterations and/or word length which is less than a number of iterations and/or word length when operating in the first mode.

11. Apparatus according to claim 3, wherein the operational module is a channel estimator module configured to operate with enhanced functionality when operating in the first mode, and to operate with simplified functionality when operating in the second mode.

12. Apparatus according to claim 11, wherein the apparatus is configured to receive a signal frame of the transmitted signal, the signal frame comprising a frame body, and to perform, in the frequency domain, a simplified equalisation of the frame body.

13. Apparatus according to claim 12, wherein the apparatus is configured to perform the simplified equalisation of the frame body by performing an approximation of a matrix inversion operation.

14. Apparatus according to claims 13, wherein the apparatus is configured to perform the approximation of the matrix inversion operation in an iterative process, a number of iterations of the iterative process being determined in dependence of the estimate of the channel environment.

15. Apparatus according to claim 14, wherein the apparatus is configured to transition between enhanced and simplified functionality of the channel estimator module by variation of the number of iterations of the iterative process.

16. Apparatus according to claim 12, wherein the apparatus is configured to operate with simplified functionality of the channel estimator by performing the simplified equalisation of the frame body.

17. A method for receiving, at an apparatus, a digital video signal transmitted over a channel, the apparatus comprising an operational module configured to operate in a first mode and in a second mode, and switching operation of the module from the first mode to the second mode in dependence of an estimate of an environment of the channel.

18. A method according to claim 17, wherein the apparatus comprises an error detection module for detecting errors in the communications signal, the method further comprising monitoring the error detection module to estimate the channel environment.

19. A method according to claim 17, comprising operating the first mode of the operational module as a normal mode and the second mode of the operational module as a simplified mode of operation.

20. A method according to claim 19, comprising toggling operation of the operational module between first and second modes in dependence of continued estimation of the channel environment.

21. A method according to claim 20, comprising keeping the operational module operating in the first mode or switching operation of the module from the second mode to the first mode when the environment of the channel is estimated not to be in a good condition.

22. A method according to claim 20, comprising switching operation of the operational module between the first and the second modes periodically when the environment of the channel is continuously estimated to be in a good condition.

23. A method according to claim 20, comprising toggling between first and second modes in dependence of a count of coding blocks received at the apparatus.

24. A method according to claim 19, wherein the operational module is an automatic gain control module and the method comprises operating the automatic gain control module with a second mode gain which is less than a first mode gain.

25. A method according to claim 19, wherein the operational module is an analogue to digital converter module and the method comprises operating the analogue to digital converter with a second mode sampling resolution which is less than a first mode sampling resolution.

26. A method according to claim 19, wherein the operational module is a decoder module and the method comprises, when operating in the second mode, operating the decoder module with a number of iterations and/or word length which is less than a number of iterations and/or word length when operating in the first mode.

27. A method according to claim 19, wherein the operational module is a channel estimator module and the method comprises operating the channel estimate module with enhanced functionality when operating in the first mode and with simplified functionality when operating in the second mode.

28. A method according to claim 27, further comprising receiving, at the apparatus, a signal frame of the transmitted signal, the signal frame comprising a frame body, and performing, in the frequency domain, a simplified equalisation of the frame body.

29. A method according to claim 28, comprising performing the simplified equalisation of the frame body by performing an approximation of a matrix inversion operation.

30. A method according to claim 29, comprising performing the approximation of the matrix inversion operation in an iterative process, a number of iterations of the iterative process being determined in dependence of the estimate of the channel environment.

31. A method according to claim 30, wherein the apparatus transitions between enhanced and simplified functionality of the channel estimator module by varying the number of iterations of the iterative process.

32. A method according to claim 28, wherein the apparatus is configured to operate with simplified functionality of the channel estimator by performing the simplified equalisation of the frame body.