Patent Publication Number: US-2010128817-A1

Title: Wireless receiver and method of saving power

Description:
TECHNICAL FIELD OF THE INVENTION  
     The invention relates to a wireless receiver and a method of saving power in a wireless receiver, and in particular to an ultra-wideband (UWB) receiver and method using a power-saving analogue-to-digital converter with switchable resolution. 
     BACKGROUND OF THE INVENTION 
     Ultra-wideband is a radio technology that transmits digital data across a very wide frequency range, 3.1 to 10.6 GHz. It makes use of ultra low transmission power, typically less than −41 dBm/MHz, so that the technology can literally hide under other transmission frequencies such as existing Wi-Fi, GSM and Bluetooth. This means that ultra-wideband can co-exist with other radio frequency technologies. However, this has the limitation of limiting communication to distances of typically 5 to 20 metres. 
     There are two approaches to UWB: the time-domain approach, which constructs a signal from pulse waveforms with UWB properties, and a frequency-domain modulation approach using conventional FFT-based Orthogonal Frequency Division Multiplexing (OFDM) over Multiple (frequency) Bands, giving MB-OFDM. Both UWB approaches give rise to spectral components covering a very wide bandwidth in the frequency spectrum, hence the term ultra-wideband, whereby the bandwidth occupies more than 20 per cent of the centre frequency, typically at least 500 MHz. 
     These properties of ultra-wideband, coupled with the very wide bandwidth, mean that UWB is an ideal technology for providing high-speed wireless communication in the home or office environment, whereby the communicating devices are within a range of 20 m of one another. 
       FIG. 1  shows the arrangement of frequency bands in a multi-band orthogonal frequency division multiplexing (MB-OFDM) system for ultra-wideband communication. The MB-OFDM system comprises fourteen sub-bands of 528 MHz each, and uses frequency hopping every 312 ns between sub-bands as an access method. Within each sub-band OFDM and QPSK or DCM coding is employed to transmit data. It is noted that the sub-band around 5 GHz, currently 5.1-5.8 GHz, is left blank to avoid interference with existing narrowband systems, for example 802.11a WLAN systems, security agency communication systems, or the aviation industry. 
     The fourteen sub-bands are organized into five band groups: four having three 528 MHz sub-bands, and one having two 528 MHz sub-bands. As shown in  FIG. 1 , the first band group comprises sub-band  1 , sub-band  2  and sub-band  3 . An example UWB system will employ frequency hopping between sub-bands of a band group, such that a first data symbol is transmitted in a first 312.5 ns duration time interval in a first frequency sub-band of a band group, a second data symbol is transmitted in a second 312.5 ns duration time interval in a second frequency sub-band of a band group, and a third data symbol is transmitted in a third 312.5 ns duration time interval in a third frequency sub-band of the band group. Therefore, during each time interval a data symbol is transmitted in a respective sub-band having a bandwidth of 528 MHz, for example sub-band  2  having a 528 MHz baseband signal centred at 3960 MHz. 
     The basic timing structure of a UWB system is a superframe. A superframe consists of 256 medium access slots (MAS), where each MAS has a defined duration, for example 256 μs. Each superframe starts with a Beacon Period, which lasts one or more contiguous MASs. The start of the first MAS in the beacon period is known as the “beacon period start”. 
     The technical properties of ultra-wideband mean that it is being deployed for applications in the field of data communications. For example, a wide variety of applications exist that focus on cable replacement in the following environments:
         communication between PCs and peripherals, i.e. external devices such as hard disc drives, CD writers, printers, scanner, etc.   home entertainment, such as televisions and devices that connect by wireless means, wireless speakers, etc.   communication between handheld devices and PCs, for example mobile phones and PDAs, digital cameras and MP3 players, etc.       

     Receivers for MB-OFDM UWB links require a very fast analogue-to-digital converter (ADC) with high resolution. A typical ADC for this link will capture a pair of (I,Q) samples at 528 MHz, with a resolution of six bits for each sample. The power consumption of the ADC increases with increasing sample rate. The power consumption of the ADC also increases exponentially with sample resolution, since the ADC will be a flash converter where power consumption is proportional to the number of comparators, and the number of comparators is proportional to 2 resolution . The power consumption of two 528 MHz ADCs with six bits of resolution will therefore be very high. 
     MB-OFDM UWB will be used in portable devices where low power consumption is very important. It is desirable that the UWB receiver within these devices operates continuously, so that it is able to receive frames transmitted by other devices. However, if the UWB receiver operates continuously, this will place an unacceptable load on the power supply, i.e. battery, of a portable device, because the receiver ADC will consume too much power. 
     The problem of the ADC consuming too much power can be mitigated using two known strategies:
         1. The user might be required to switch on the receiver during periods when communication takes place, and then switch off the receiver following communication. This solution is undesirable because frames may be missed if the user does not switch on the receiver. Additionally power may be wasted if the user does not switch off the receiver.   2. The receiver can enter a “snooze” mode if no frames have been received for some time. The receiver can then wake up periodically to sense for frame activity. This solution is not ideal since the receiver may miss frames while it is “snoozing”.       

     It is an aim of the present invention to provide an improved wireless receiver, and a method of saving power in a wireless receiver. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a receiver for receiving a wireless communication signal, the receiver being operable in a first mode when the receiver is actively receiving a transmitted signal, and a second mode when the receiver is waiting to receive a transmitted signal. The receiver comprises an analogue to digital converter for converting a received analogue signal into a digital signal; and a receiver controller, adapted to control the operating resolution of the analogue to digital converter such that, in the first mode, the analogue to digital converter operates at a first resolution, and, in the second mode, the analogue to digital converter operates at a second resolution, wherein the second resolution is lower than the first resolution. 
     According to another aspect of the present invention, there is provided a method of saving power in a receiver having an analogue-to-digital converter for converting a received analogue signal into a digital signal. The method comprises the steps of operating the receiver in a first mode when the receiver is actively receiving a transmitted signal; and operating the receiver in a second mode when the receiver is waiting to receiving a transmitted signal. The analogue-to-digital converter has a first resolution when operating in the first mode, and a second resolution when operating in the second mode, the second resolution being lower than the first resolution. 
     The invention has the advantage of enabling the average power consumption of a UWB receiver to be reduced by placing the receiver in a low-power “listening” mode when it is not actively receiving a frame. In the “listening” mode, the RF and the detection correlator are active, and the ADC is placed in a low-power, low-resolution mode. Once the detection correlator detects the presence of a preamble symbol at the start of a frame, the receiver is powered-up and the ADC placed in a high-resolution mode. At the end of the frame (or at the end of the last frame of a burst of frames), the receiver may be returned to the “listening” mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the following drawings in which: 
         FIG. 1  shows the multi-band OFDM alliance (MBOA) approved frequency spectrum of a MB-OFDM system; 
         FIG. 2  is a block schematic diagram of a receiver according to the present invention. 
         FIG. 3  is a flow chart detailing the steps performed by the present invention. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE PRESENT INVENTION  
     The following description of the preferred embodiment is made in relation to a UWB receiver. However, it will be appreciated that the invention is also applicable to other types of wireless receivers, including UWB receivers adapted for use in standards other than the MB-OFDM standard. 
       FIG. 2  shows a block schematic diagram of a receiver  10  according to the present invention. 
     The receiver  10  comprises an antenna  12  which receives an RF signal  14 . An RF section  16  amplifies the received signal  14 , and outputs the amplified signal  18  to an analogue-to-digital converter  20 . 
     The receiver  10  further comprises a detection correlator  22 , a receiver controller  24 , and a receiver baseband  26 . 
     The receiver  10  has two operating modes: the “receiving” mode (or first mode) in which the receiver operates in a normal mode of operation (i.e. when receiving UWB signals from one or more other UWB devices), and a “listening” mode (or second mode) in which the receiver operates in a power saving mode according to the invention (for example when waiting to receive UWB signals from one or more other UWB devices). 
     While operating in the receiving mode, all sections of the receiver  10  are switched on, and the ADC  20  operates at a high resolution, for example a six bit resolution. The high-resolution output  38  is sent directly from the ADC  20  to the receiver baseband  26 . It will be appreciated that, although the preferred embodiment refers to the high resolution of the ADC  20  being six bits per sample, other resolutions may also be used. 
     According to the invention, the receiver controller  24  is adapted to place the receiver  10  in a listening mode to conserve power. For example, the receiver  10  can be placed in the listening mode at the end of a frame, or after the last frame of a burst of frames. 
     In the listening mode, the receiver baseband  26  and all other non-essential sections (not shown in  FIG. 2 ) are placed in a power-save mode, for example switched off, thereby reducing the power consumption of the receiver  10 . The RF section  16 , ADC  20 , detection correlator  22  and receiver controller  24  remain switched on, although the power consumption of these blocks may be reduced from normal operation. 
     However, according to the invention, the ADC  20  is configured to operate at a low resolution when the receiver is in the listening mode. For example, in a preferred embodiment, the low resolution is 1-bit for each sample. The power consumption of the ADC  20  is reduced when operating in the 1-bit resolution mode because fewer comparators will be active, thereby further reducing the power consumption of the receiver  10  as a whole. 
     The flow diagram of  FIG. 3  describes the basic principles regarding how the invention controls the operation of the receiver between a normal operating mode, i.e. the receiving mode, and the low power mode, i.e. the listening mode. 
     Upon receiving an indication to enter the listening mode, for example an end of frame signal or after the last frame of a burst of frames, step  301 , the receiver is adapted to place the ADC  20  in a low resolution mode of operation, step  303 . Preferably, the low resolution mode of operation is a 1-bit mode of operation. While in the listening mode, the receiver is adapted to detect the presence of a preamble signal based on the 1-bit output of the ADC  20 , step  305 . If no preamble is detected, the receiver remains in the listening mode, and the ADC  20  in the 1-bit mode. However, upon detection of a preamble signal, the ADC is set to the high resolution mode once more, step  307 . 
     Thus, whilst in the listening mode, if a frame is transmitted by another device it will be amplified by the receiver RF section  16  and then passed to-the ADC  20 . The output of the ADC is then passed to the detection correlator  22 . The detection correlator  22  is able to reliably detect the presence (or absence) of a frame preamble symbol, based on the information from a 1-bit sample of the received signal  30 . When the receiver controller  24  receives a “preamble present” signal  32  from the detection correlator  22 , the receiver controller  24  will immediately wake up all sections of the receiver  10  and place the ADC  20  into the high-resolution mode, so that the receiver  10  is placed in the “receiving mode”, and thus able to receive the incoming frame. This is achieved by sending a “power-save” signal  34  to the receiver baseband  26 , instructing the baseband  26  to exit power-save mode and power up, and sending a “resolution control” signal  36  to the ADC  20 , instructing the ADC to operate at a high resolution. 
     At the end of the frame or at the end of the last frame in a frame burst, the receiver controller  24  sends a power-save signal  34  to the receiver Baseband  26 , instructing the receiver  10  to enter the power-save mode, and sends a resolution-control signal to the ADC  20 , instructing the ADC  20  to operate at the lower resolution. Thereafter, the receiver  10  re-enters the listening mode. 
     The listening or power-save mode may be achieved, for example, by clock-gating the digital logic within the receiver baseband  26 . Clock-gating temporarily disables clocks to registers within inactive digital logic. This significantly reduces the power consumption of the inactive digital logic. However, a person skilled in the art will appreciate that many other possible methods for achieving a power-save mode can be provided without departing from the scope of the present invention. 
     As can be seen from the above, the invention relies on the detection correlator  22  being able to reliably detect the presence of a preamble symbol in the received signal, and thereafter the receiver  10  being able to switch to receiving mode relatively quickly so that as little information is lost as possible. The invention is made possible by the fact that the preamble found in a UWB signal is relatively long. For example, the standard MB-OFDM signal includes 24 preamble symbols. However, it is possible to correctly receive a frame using fewer of these preamble symbols, say only 18 of the preamble symbols. In such a scenario, the receiver  10  can therefore afford to lose the first 6 preamble symbols of each packet. If the detection correlator  22  detects the presence of the first preamble symbol then the remainder of the receiver  10  must power-up within 5 preamble symbols (i.e. 1.5625 μs). 
     This enables the preamble to be detected reliably using just 1-bit resolution. For example, the 1-bit detection correlator sums how often the sign of the samples in a sliding window (equal to the length of the preamble symbol) coincides with the sign of the expected preamble signal. When this sum is compared to a pre-computed threshold, a reliable decision on the presence or absence of an MB-OFDM preamble can be made. The threshold is chosen (by modelling the statistics of the detection correlator) to achieve certain false alarm and missed frame probabilities. 
     The above mentioned characteristics of the preamble enable the receiver to be placed in the receiving mode without losing any data. 
     The ADC  20  can rapidly switch between high and low resolutions, as this merely involves powering up the necessary extra comparators, for switching to the high resolution, or powering down the unnecessary comparators, for switching to the low resolution. In addition, the receiver baseband  26  can also switch rapidly between an operational mode and a power-save mode, for example by employing clock-gating as mentioned above. 
     Further modifications to the receiver  10  will be apparent to the person skilled in the art. For example, the preamble-detection function of the detection correlator  22  could be replaced by an RF power detection circuit. Such a circuit would infer the presence of a transmitted signal by detecting radio-frequency power in the received signal. This is a sub-optimal solution, as the RF power detection circuit will not be able to distinguish between preamble symbols and other radio communications. However, such a system would employ the same inventive concept as the present invention, and is therefore to be considered as falling within the scope of the invention as defined in the appended claims. 
     In addition to changing the resolution of the ADC during the listening mode; the receiver can also be adapted to change the resolution of the ADC during the receiving mode, for example depending on the quality of the signal being received. The ADC  20  may be configured to operate at a lower resolution, such as 3-bit, 4-bit or 5-bit, when a high quality signal is being received, and to operate at a 6-bit mode of operation when a poor quality signal is being received. 
     It is noted that the invention may also be used with non-wireless systems where a power saving mode by reducing the accuracy of the ADC is advantageous. 
     It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word “comprising” does not exclude the presence of elements or steps other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims shall not be construed so as to limit their scope.