Patent Application
20150304153
The present invention relates to the field of communications, and, more particularly, to wireless data communications and related methods.
The peak-to-average power ratio (“PAPR”), also known as peak-to-mean power ratio (“PMPR”) or peak or crest factor, may be an important characteristic of multi-carrier transmitted signals. The peak of the signal can be N times greater than the average power level, where N is the number of subcarriers. These large peaks may cause intermodulation distortion which can result in an increase in the error rate. These distortions are typically caused by the limitations inherent in a transmitting amplifier.
To prevent the transmitter amplifier from limiting (clipping), the average signal power is typically kept low enough to keep the signal relatively linear through the amplifier. To transmit a high power signal, a high power amplifier may be used which requires a large DC system power. A much higher power amplifier is typically used to transmit multi-carrier waveforms than for constant envelope waveforms. For example, using 64 carrier waveforms, a 40 dBm power amplifier would require about 15 dB of back off. Therefore, instead of operation at 40 dBm (10 watts) the amplifier is only capable of operating at 25 dBm (0.316 Watts). Thus to transmit at the desired 40 dBm, a 55 dBm (316 Watt) amplifier would be needed. In addition, such large power requirements may lead to associated increased space demands and heat dissipation requirements.
With the large amount of interest and activity with Orthogonal Frequency Division Modulation (OFDM), and in particular with IEEE 802.11a and 802.11g communication technology, the PAPR problem is exaggerated. The IEEE 802.11 standard with its use of complex waveforms may require highly linear RF amplifiers. Current IEEE 802.11 physical layer integrated circuits have not implemented PAPR reduction schemes. In particular, multi-tone OFDM typically requires greater than 10 dB power amplifier back-off because of a high peak-to-average power ratio. The net result of these factors may be increased DC power demand beyond that encountered with other IEEE 802.11 techniques. The effect may be less noticeable for short duty cycle signals, but can be significant for situations requiring continuous transmission of data.
OFDM, as mentioned above, is a method of transmitting data simultaneously over multiple equally-spaced and phase synchronized carrier frequencies, using Fourier transform processing for modulation and demodulation. The method has been proposed and adopted for many types of radio systems, such as wireless Local Area Networks (“LAN”) and digital audio and digital video broadcasting. OFDM offers many well-documented advantages for multi-carrier transmission at high data rates, particularly in mobile applications. Specifically, it has inherent resistance to dispersion in the propagation channel. Furthermore, when coding is added it is possible to exploit frequency diversity in frequency selective fading channels to obtain excellent performance under low signal-to-noise conditions. For these reasons, OFDM is often preferable to constant envelope modulation with adaptive equalization and is arguably less complex to implement.
The principal difficulty with OFDM, as alluded to above, is that when the sinusoidal signal of the N carriers add mostly constructively, the peak envelope power is as much as N times the mean envelope power. If the peak envelope power is subject to a design or regulatory limit then this has the effect of reducing the mean envelope power allowed under OFDM relative to that allowed under constant envelope modulation. If battery power is a constraint, as is typically the case with portable equipment such as mobile consumer appliances, and laptops, then the power amplifiers required to behave linearly up to the peak envelope power may be operated inefficiently with considerable back-off from compression. Digital hard limiting of the transmitted signal has been shown to alleviate the problem, but only at the cost of spectral sidelobe growth and consequential bit error performance degradation.
Various approaches are sometimes used to address the PAPR issues for OFDM packets. For example, U.S. Pat. No. 7,639,747 to Moffatt et al. (and assigned to Harris Corporation of Melbourne, Fla. the assignee of the present invention), which is hereby incorporated herein in its entirety by reference, describes a predictive signal producing method that levels transmitter output power in a multi-carrier communication system and results in approaching amplifier performance normally associated with constant carrier waveforms.
Another approach is set forth in U.S. Pat. No. 8,274,921 also to Moffatt at al. and assigned to Harris Corporation, which is hereby incorporated herein in its entirety by reference. This patent discloses a system which communicates data and includes a transmitter for transmitting a communications signal that carries communications data. It includes an efficient modulator for approximating the frequencies of sine/cosine basis waveforms using complex exponential functions and adding and subtracting the complex exponential functions to generate an OFDM communications signal as a plurality of N data subcarriers that carry communications data as data symbols. A receiver receives the OFDM communications signal and includes a demodulation circuit for demodulating the OFDM communications signal using logical shifts of multiples +/−2p based on complex exponential functions corresponding to sine/cosine basis waveform approximations to extract amplitude and phase values from a plurality of N data subcarriers as data symbols within the OFDM communications signal.
Other approaches are described in U.S. Pat. No. 7,496,028 to Jung et al.; U.S. Pat. No. 7,301,891 to Park et al.; U.S. Pat. No. 6,925,128 to Corral; U.S. Pat. No. 8,442,137 to Moffatt et al.; U.S. Pat. No. 8,135,081 to Moffatt et al.; U.S. Pat. No. 7,903,749 to Moffatt; U.S. Pat. No. 7,822,136 to Moffatt et al.; U.S. Pat. No. 7,751,488 to Moffatt; and U.S. Pat. No. 7,649,951 to Moffatt, which are also hereby incorporated herein in their entireties by reference.
However, there may be a desire to further address the PAPR in some implementations.
A wireless communications device using a multi-carrier modulation communication signal may include a transmitter and a controller operable with the transmitter. The controller may be configured to reduce a peak-to-average power ratio (PAPR) associated with the multi-carrier modulation communication signal by at least: generating a plurality of intermediate multi-carrier modulation communication signals based upon respective different permutations of a Walsh transformation matrix; and selecting a given intermediate multi-carrier modulation communication signal for transmission as the multi-carrier modulation communication signal based upon a PAPR associated therewith.
More particularly, the controller may be further configured to generate at least one signaling symbol (which may be encrypted) for transmission with the multi-carrier modulation communication signal indicating the respective permutation of the Walsh transformation matrix associated therewith. The controller may select as the given intermediate multi-carrier signal the intermediate multi-carrier modulation communication signal having a lowest PAPR associated therewith, for example. Also by way of example, the controller may select the given intermediate multi-carrier modulation communication signal based upon a PAPR threshold.
The different permutations of the Walsh transformation matrix may comprise different row configurations of the Walsh transformation matrix. By way of example, the multi-carrier modulation communication signal may be an orthogonal frequency-division multiplexing (OFDM) signal. The wireless communications device may further include a quadrature amplitude modulation (QAM) modulator for generating a QAM signal constellation, and the controller may generate the plurality of intermediate multi-carrier modulation communication signals based upon applying the respective different permutations of the Walsh transformation matrix to the QAM signal constellation.
The controller may determine the respective different permutations of the Walsh transformation matrix based upon a transformation table. The controller may also pseudo-randomly generate the respective different permutations of the Walsh transformation matrix. The controller may iteratively generate the plurality of intermediate multi-carrier modulation communication signals based upon the respective different permutations of the Walsh transformation matrix. Also, the wireless communications device may further include an antenna coupled to the transmitter.
A related wireless communications device may include a receiver configured to receive a remote multi-carrier modulation communication signal from a remote wireless communications device, and a controller operable with the receiver to receive and decode the remote multi-carrier modulation communication signal from the remote wireless communications device based upon at least one remote signaling symbol associated with the remote multi-carrier modulation communication signal. The remote signaling symbol(s) indicates a respective Walsh transformation matrix permutation associated with the remote multi-carrier modulation communication signal from among a plurality of available Walsh transformation matrix permutations.
A related method is for using a wireless communications device to reduce a peak-to-average power ratio (PAPR) associated with a multi-carrier modulation communication signal. The method may include generating a plurality of intermediate multi-carrier modulation communication signals at the wireless communications device based upon respective different permutations of a Walsh transformation matrix, and selecting a given intermediate multi-carrier modulation communication signal for transmission from the wireless communications device as the multi-carrier modulation communication signal based upon a PAPR associated therewith.
The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.
As noted above, various techniques are sometimes used to mitigate PAPR. One approach requires side information from the transmitter to receive and process the signal. Furthermore, coding techniques may produce symbols with a low PAPR. Yet, this approach may have certain drawbacks, including spreading distortion caused by clipping large peaks over multiple symbols. This approach may also require additional overhead and access to the physical layer to implement.
Other techniques involve clipping and filtering, saturation, compression, or limiting of the signal. More particularly, such approaches may include hard clipping or soft clipping, peak windowing (i.e., clipping and then filtering), and companding (similar to speech companding). Yet, such techniques may result in compression due to the power amplifier, and they may not work well with higher order constellations (i.e., due to a small Euclidian distance between symbols). Furthermore, this may also degrade signal quality as a result of increased IBN (in band noise) and OBN (out of band noise), which thus increases BER (bit error rate).
Still another approach is to use phase reduction carriers (PRC). That is, additional carriers are used to reduce unwanted peaks. However, this requires either additional frequency spectrum or a lower data rate (by using existing data carriers as the PRCs). Another “brute force” technique simply backs-off the transmit power level so that the input signal does not exceed the amplifier peak output level in order to avoid clipping. For example, a 10 dB larger power amplifier is required to maintain the same transmit power level for 10 dB backoff. However, this approach results in higher power consumption, as noted above.
Turning now to
In the illustrated example, the wireless communications device 30 is designated as a local device which transmits OFDM signals to, and receives OFDM signals from, a remote wireless communications device 35. The remote wireless communications device 35 may have similar components to those shown in the local wireless communications device 30, although in some embodiments one or both of the devices 30, 34 need not include both a transmitter and a receiver (e.g., a given device may be configured as a dedicated transmitting station or a dedicated receiving station, for example).
Beginning at Block 51 of the flow diagram 50 (
However, rather than just generating a single OFDM output signal, different permutations of the Walsh transformation matrix are applied to the QAM constellation to find the one with a lowest PAPR, and/or one that has a PAPR below a desired PARP threshold, at Block 53. The PAPR is iteratively determined by a PAPR threshold detector 43, which provides feedback to a Walsh permutator module 44 to change to a next permutation of the Walsh transformation matrix as appropriate. The controller 33 selects a given intermediate multi-carrier modulation communication signal for transmission as the multi-carrier modulation communication signal meeting the desired criteria, e.g., having a PAPR below the PAPR threshold or having the lowest PAPR from among the intermediate signals, for example. Considered alternatively, Walsh permutator 44 allows for multiple copies of the Walsh-spread signal to be generated, so that the copy with a lower (or lowest) PAPR may accordingly be selected for transmission to the remote wireless communications device 35 (Block 55).
Furthermore, the controller 33 may be further configured to generate one or more signaling symbols for transmission with the multi-carrier modulation communication signal, at Block 55, which indicate the respective permutation of the Walsh transformation matrix used to generate the multi-carrier modulation communication signal. For example, additional data bits may be sent in the signaling symbol, which allows the receiving device 35 to properly decode the sequence. In the illustrated example, each Walsh permutation is based upon different combinations or arrangements of rows in the Walsh transformation matrix, each of which will still provide an orthogonal transform. Thus, using the signaling symbols or bits to indicate the given order of rows in the Walsh transformation matrix used to generate the signal being transmitted, the receiving device 35 may accordingly determine the appropriate matrix arrangements for correctly decoding the received signal, as will be appreciated by those skilled in the art. However, it should be noted that other permutation approaches for arranging the matrix (e.g., different column configurations, etc.) may also be used. The method of
The foregoing will be further understood with reference to
The respective different permutations of the Walsh transformation matrix may be selected in various ways. One approach is to use a transformation table which dictates which row combination is to be used for which transformation. Another approach is to pseudo-randomly generate the respective different permutations (e.g., row combinations) of the Walsh transformation matrix. Other suitable approaches may also be used, as will be appreciated by those skilled in the art.
Empirical and derived probability results for an example embodiment of the above-described approach are set forth in the graph 60 of
F
sym=1−FWN
Solving for Fw (sampled-based ODE):
F
W=(1−Fsym)1/N
The OFDM symbol power CDF after k permutations is:
{circumflex over (F)}
sym
=F
sym
k=(1−FWN)k
After substitution, the OFDM symbol (after k permutations) into sample-based power CDF is:
As such, the OFDM sample complementary cumulative distribution function (CCDF) after “k” permutations is:
In the graph 60, the baseline (i.e., no PAPR reduction) empirical probability for a typical OFDM waveform (where k=1) is given by line 61, while the derived probability values for the same waveform are indicated by diamond shapes 62. A first set of empirical probability curves 63, 64, 65 corresponds to sixteen scrambling sequences (k=16) (with respective corresponding derived probability values indicated by circle shapes 66, 67, 68) having respective FFT sizes of 64, 128, and 256. A second set of empirical probability curves 70, 71, 72 corresponds to two-hundred fifty-six scrambling sequences (k=256) (with respective corresponding derived probability values indicated by square shapes 73, 74, 75) having respective FFT sizes of 64, 128, and 256.
It will therefore be appreciated that the above-described approach may advantageously improve PAPR for various QAM signal constellations, achieving up to approximately 5.1 dB of PAPR reduction in the example shown in
Many modifications and other embodiments of the invention will come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is understood that the invention is not to be limited to the specific embodiments disclosed, and that modifications and embodiments are intended to be included within the scope of the appended claim.
