Patent Application
20090135922
Ultrawideband wireless communications enable improved data throughput when compared with narrowband techniques. In particular, Orthogonal Frequency Division Modulation (OFDM) permits high data rates to be achieved over wireless networks because different parts of the data may be split up and simultaneously sent over separate carriers, each carrier operating at a different frequency. The data may then be re-combined at the receiver. The frequencies, typically closely spaced, are chosen so that the spectral distribution of each carrier becomes null at the other carriers' spectral peaks, and their signals will therefore not interfere with each other even though the frequencies are closely spaced. This frequency spacing is referred to as orthogonal frequencies.
By spreading the data across multiple carriers in this manner, high data rates may be achieved. For example, in a modulation technique such as quadrature phase shift keying (QPSK), each two bits of a 256-bit data string may be simultaneous transmitted on 128 carriers, taking only the time required to transmit 2 bits. However, simultaneously transmitting over multiple carriers in this manner, while greatly improving overall throughput, also significantly increases the power required in both the transmitter and receiver. Some low data-rate communication devices don't need the high bandwidth, but are battery powered and are very sensitive to power consumption. These types of devices make very inefficient use of power when they are forced to follow a standard OFDM-based communications protocol.
Some embodiments of the invention may be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings:
In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known circuits, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.
References to “one embodiment”, “an embodiment”, “example embodiment”, “various embodiments”, etc., indicate that the embodiment(s) of the invention so described may include particular features, structures, or characteristics, but not every embodiment necessarily includes the particular features, structures, or characteristics. Further, some embodiments may have some, all, or none of the features described for other embodiments.
In the following description and claims, the terms “coupled” and “connected,” along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Rather, in particular embodiments, “connected” is used to indicate that two or more elements are in direct physical or electrical contact with each other. “Coupled” is used to indicate that two or more elements co-operate or interact with each other, but they may or may not be in direct physical or electrical contact.
As used in the claims, unless otherwise specified the use of the ordinal adjectives “first”, “second”, “third”, etc., to describe a common element, merely indicate that different instances of like elements are being referred to, and are not intended to imply that the elements so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.
Various embodiments of the invention may be implemented in one or any combination of hardware, firmware, and software. The invention may also be implemented as instructions contained in or on a machine-readable medium, which may be read and executed by one or more processors to enable performance of the operations described herein. A machine-readable medium may include any mechanism for storing, transmitting, and/or receiving information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium may include a tangible storage medium, such as but not limited to read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; a flash memory device, etc. A machine-readable medium may also include a propagated signal which has been modulated to encode the instructions, such as but not limited to electromagnetic, optical, or acoustical carrier wave signals.
The term “wireless” and its derivatives may be used to describe circuits, devices, systems, methods, techniques, communications channels, etc., that communicate data by using modulated electromagnetic radiation through a non-solid medium. The term does not imply that the associated devices do not contain any wires, although in some embodiments they might not. The term “mobile station” is used to describe a wireless communications device that may be in motion while it is communicating. The term “base station” is used to describe a wireless communications device that is used to control communications with multiple mobile stations, and is generally (though not necessarily always) fixed in place while communicating.
Various embodiments of the invention may reduce the power required for OFDM communications by using only a subset of the carriers that are available, and/or by processing only part of the components of the signals that are received. This can be achieved in various ways, such as 1) changing the way the data is transmitted over the carriers, thereby reducing the power required to process the signals, 2) using single-rail operation, to eliminate the need for half of the analog-to-digital converters (ADC) and/or half the digital-to-analog converters (DAC). In addition, by using only the upper (or lower) half of the carriers, the remaining half may be treated as a separate carrier grouping, and may therefore be used as a separate band in multi-band OFDM communications.
Two relationships in OFDM communications are dictated by the nature of the technology:
(symbol duration)=1/(tone spacing) 1)
(sampling rate)=(tone spacing)×(number of sub-carriers) 2)
For example, in a standard promulgated for WiMedia, the symbol duration (the time to transmit a signal encoding a defined number of bits, e.g. 2 bits) is 242.2 ns, the tone spacing (the frequency separation between adjacent sub-carriers) is 4.125 MHz, the number of sub-carriers used is 128, and the sampling rate (how often the baseband signal must be sampled to re-create the original signals that were modulated onto the carriers) is 528 MHz. Assuming the tone spacing remains unchanged, the sampling rate is proportional to the number of sub-carriers being used. Reducing the number of sub-carriers therefore can cause a proportionate reduction in the sampling rate.
In signal processing circuitry, the sampling rate tends to control the clock frequency that is used to operate that circuitry. Since power consumption by CMOS circuitry is generally proportional to how often the transistor gates switch from one state to another, power consumption in signal processing circuitry may tend to be approximately proportional to the frequency of the clock driving that circuitry. Thus, reducing the number of carriers being used in an OFDM communication, as shown in
Examining
In some high-throughput devices, reducing the number of carriers or increasing symbol duration may not be advantageous. But in other devices, such as low throughput devices, and/or battery powered devices in which lower power consumption is more important than higher data rates, either or both of these techniques may be beneficial. In some networks, the base station may have to communicate with a mixture of mobile stations, containing devices that can benefit from these techniques and devices that cannot. Further, some network devices may use older technology that is unable to take advantage of these techniques, even if it would be beneficial. In such cases, the base station may need to be able to switch between: 1) standard carrier numbers and symbol durations, 2) reduced carrier numbers, 3) increased symbol durations, or 4) a combination of 2) and 3). Since these techniques can use the same hardware and the same protocols as conventional systems, the same system should be able to handle both these techniques and the more conventional techniques of legacy systems.
Further benefits may be realized by carefully choosing the particular carriers that are selected in the technique of
If the number of carriers being used is to be reduced in the manner previously described, then the unused carriers may be assigned to another band. In the example shown in
At 330, the base station may assemble a second OFDM frame directed to those mobile stations selected at 320, using only the reduced subset of available carriers and/or using longer symbol durations. The base station may then lower the clock rate for it's DAC at 340, and where appropriate may also set an increased symbol duration for the frame at 350. Using these new parameters, the second frame may be processed for transmission at 360.
If a longer than normal symbol duration is being used, as determined at 430, the sampling clock rate may be lowered at 440 to save power when processing the frame. Similarly, if only a sub-set of available carriers are being used, as determined at 450, the sampling clock rate may be lowered at 460 to save power when processing the frame. If both a longer symbol duration and a reduced set of carriers are being use, then the cumulative reduction in sampling rate may be used. At 470, the portions of the frame that are directed to this mobile device may be processed, using the lowered sampling clock rate.
In the transmit chain 510, the data to be transmitted may be organized at 515. Such operations may comprise things such as, but not limited to: 1) dividing the data into frames, 2) adding, to the basic data, things that are needed for reliable and unambiguous communication, such as preambles, headers, checksums, etc., and 3) separating the data into segments for the different carriers and symbols. The data to be transmitted may be processed through an Inverse Fast Fourier Transform (IFFT) at 520 to produce a composite data stream representing the combined data for all the carriers. For example, a 128-point IFFT may be used if 128 carriers are to be used. At 525, a DAC may convert this digital composite data stream into an analog baseband signal. The analog baseband signal may then be modulated onto a radio frequency (RF) signal at 530, and transmitted through antenna 535.
In the receive chain 550, the RF signal received through antenna 535 may be demodulated into a baseband signal at 570, and the baseband signal converted into a composite digital data stream by the ADC at 565. At 560 a Fast Fourier Transform (FFT) may then be used to convert the composite data stream into the separate data streams that had been transmitted on each of the carriers. The resulting data may then be reorganized into its relevant parts at 550. This data reorganization may include things such as, but not limited to: 1) combing the contents of the different carriers and symbols into their original relationships, 2) removing preambles, headers, checksums, etc., and 3) combing the contents of frames, packets, etc. into their original order.
As previously mentioned, QPSK modulation may be used, thereby encoding 2 data bits for each carrier per symbol. This indicates that there is no symmetry assumed for the distribution of the carriers, and therefore, when they are transformed via IFFT, the time domain signal will be complex, having both real and imaginary parts. This typically leads to the so-called ‘dual-rail’ implementation that requires two ADC's and two DAC's, together with dual-rail (I-Q) modulation/demodulation in the radio circuitry. The properties of the FFT dictate that the time-domain signal will be real if its counterpart in the frequency domain is conjugate-symmetric. Therefore, conversely, it is possible to obtain real time-domain signals by making the frequency-domain carriers conjugate-symmetric, in which case half of the carriers are merely copied (after phase rotation) from the other half, carrying no further information.
By doing this, all the time-domain processing, including ADC/DAC and radio, becomes single-railed. This allows the radio to turn off half of the dual-rail circuitry at the expense of reducing the data rate by half. Half of the ADCs and DACs may therefore be turned off, as may the interpolation/decimation filters. ADCs and DACs typically consume a great deal of power (by comparison to the rest of the circuitry), and this therefore represents a significant power savings. In addition one rail of reconstruction and anti-aliasing filters can be turned off, as can be one channel of the I-Q modulation/demodulation circuitry.
Because not all devices will be able to handle these changes, compatibility with legacy devices should be maintained. By leaving the dual-rail circuitry in place, but shutting down the half that is not necessary for a given operation, a device may be able to handle both dual-rail and single-rail operation by simply providing or not providing power to the appropriate circuitry. In this context, ‘not providing power’ may mean one, some, or all of: 1) stopping the clock signal that causes the relevant digital circuitry to switch states, 2) removing operating voltage to the relevant circuitry, and 3) reducing the operating voltage level to the relevant circuitry.
Since the lower carriers are unused in this example, they can be redefined as a new band, labeled as Band X, to be used in a future transmission. Band X would be created by setting the upper carriers of Band 1 to zero, just the opposite to that shown in
The foregoing description is intended to be illustrative and not limiting. Variations will occur to those of skill in the art. Those variations are intended to be included in the various embodiments of the invention, which are limited only by the spirit and scope of the following claims.
