WO2009107042A1

WO2009107042A1 - Frequency generation for an ultrawide band radio

Info

Publication number: WO2009107042A1
Application number: PCT/IB2009/050681
Authority: WO
Inventors: Dominique Brunel; Helen Waite
Original assignee: Nxp B.V.
Priority date: 2008-02-28
Filing date: 2009-02-19
Publication date: 2009-09-03

Abstract

A system for generating local oscillator frequencies for use in a transmitter or a receiver in an ultra wide band (UWB) wireless communications system. The UWB standard currently contemplated utilizes three bands of approximately500MHz each. To receive each of the bands, a receiver will typically require three different local oscillator frequencies. This disclosure describes hardware-efficient methods of generating these frequencies.

Description

Frequency Generation for an Ultra Wide Band Radio

FIELD OF THE INVENTION

An ultra wide band (UWB) signal is one that has an extremely wide bandwidth, generally greater than 500 MHz. In general, when referring to a signal, the bandwidth is the difference, in hertz, between the upper and lower cutoff frequencies of the signal. When referring to a filter, the bandwidth is the widest bandwidth signal the filter will send as an output. A common definition of a cutoff frequency is the frequency at which the amplitude of the signal is 3 dB lower than the peak value at the center frequency. This 3-dB frequency is commonly referred to as the half-power frequency, and thus, the bandwidth in this case is commonly referred to as the half- power bandwidth.

UWB communication enables very high data rates and a reduced transmitter power; it makes the signal more resistant to jamming, multipath, and other types of interference; and it enables multiple access to a common channel, among other features. An exemplary UWB system utilizes the unlicensed 3.1- to 10.6-GHz band designated for personal area networks, regulated in the United States by the FCC at 47 CFR § 15.

BACKGROUND OF THE INVENTION

An exemplary UWB system employs multi-band orthogonal frequency division multiplexing (MB-OFDM) in each of either three or seven frequency bands (see Fig. 1, discussed in further detail below), and quadrature (or quadriphase) phase shift keying (QPSK). OFDM is a method for multiplexing a plurality of information signals, in which the signals are transmitted over a number of orthogonal frequency bands. Here, each of the bands 1-13 is subdivided into multiple orthogonal sub-bands for OFDM. QPSK is a digital modulation scheme in which two carrier signals (one representing a real number, and the other representing an imaginary number) are modulated to each represent a binary symbol of 0 or 1 by shifting the phase of the respective carrier. Thus, a QPSK signal can encode a complex number in binary. Typically, the two carriers are orthogonal to one another, e.g., out of phase by 90°. In this case, one is referred to as an in-phase signal (I), and the other is a quadrature signal (Q). A spread spectrum system is one in which a signal occupies a bandwidth far in excess of the minimum bandwidth required to carry the information. The spreading of the signal is accomplished at the transmitter utilizing a spreading signal, which is independent of the information being sent. The receiver recovers the information by correlating the received signal with a local replica of the spreading signal. The most commonly used systems for spreading a signal are direct sequencing and frequency hopping. Frequency hopped spread spectrum is a scheme where the available channel bandwidth is divided into a plurality of nonoverlapping frequency bands, and the transmitted signal occupies one (or more) of the bands in any signaling interval, hopping between bands from one interval to the next. The hopping between bands is frequently done in a pseudorandom fashion, according to a time- frequency code (TFC) known by the receiver.

Fig. 1 shows the spectrum of an exemplary multi-band UWB scheme that divides the available spectrum of 3.1-10.6 GHz into 13 nonoverlapping bands, each with a bandwidth of 528 MHz. Information is transmitted over a plurality of the bands utilizing frequency hopping, with a hop in frequency every 312 ns. To improve performance, the time taken to hop from one frequency to the next should be very short relative to the hopping period of 312 ns. Generally, a change in frequency that occurs in a few nanoseconds is desired.

The bands are defined as follows:

2904 + 528xband band = 1 - 4 f⁽band) = \

3168 + 528x£αm/ band = 5 - 13

where f_c is the center frequency of the band in MHz. Note the gap between bands 4 and 5; this is intended to avoid the 5 GHz band utilized by some wireless local area network (WLAN) systems.

According to this scheme, the bands are grouped into four band groups consisting of several bands each. The first three bands are grouped into Group A; bands 4 and 5 are grouped into Group B; bands 6-9 are grouped into Group C; and bands 10-13 are grouped into Group D.

Fig. 2 shows one example of frequency hopping, in bands 1-3 of Fig. 1. An information signal approximately 312.5 ns long is transmitted sequentially in the three bands. Each signal includes a header and data, and there is a guard interval of approximately 9.5 ns for switching time between bands. Various frequency hopping sequences are possible, and for a seven band radio, hopping would be on seven different frequencies.

In a multi-band UWB system utilizing frequency hopping, as described above, a hop in frequency occurs approximately every 312 ns. Therefore, there is a need for very fast switching between frequency bands, at a switching speed faster than achieved by conventional methods, which control the fixed frequency generated by a phase locked loop (PLL). For such a fast switching time, at both the transmitter and receiver there is a desire for switching between the bands to be accomplished on the order of a few nanoseconds.

As illustrated in Fig. 3, the IEEE P802.15 Working Group for Wireless Personal Area Networks has proposed a local oscillator (LO) frequency generator (in document IEEE 802.15-03/267r6.doc) for such a UWB system. This circuit utilizes one PLL 10 driven by an oscillator 12 to generate a fixed frequency of 4224 MHz. Divider 14 divides the frequency output by the PLL, providing a 528 MHz signal which is provided to a second divider 16, a single side band (SSB) mixer 20, and also acts as a sampling clock. Multiplexer 18 allows selection between the output of the divider 16 or the SSB mixer 20. The output of the multiplexer 18 is mixed with the 4224 MHz signal provided by PLL 10 at a second SSB mixer 22. Selecting between outputs of the multiplexer 18, and selecting side bands of the SSB mixers allows selection of the desired center frequency to be utilized by a frequency hopping system.

As illustrated in Fig. 4, a mixer is a nonlinear circuit or device that accepts as its input two signals, and multiplies them together, resulting in an output that includes (a) a signal equal in frequency to the sum of the frequencies of the input signals (the "upper side band"), (b) a signal equal in frequency to the difference between the frequencies of the input signals (the "lower side band"), and, if they are not filtered out, (c) the original input frequencies. The simplest form of a mixer is a Double Side Band (DSB) Mixer, and generates the two intermediate frequencies (Ifs) corresponding to the sum and difference of the incoming frequencies. In this case, the IFs are real (no I and Q).

A Single Side Band (SSB) Mixer is a combination of two DSB mixers driven in quadrature, in which it is possible to select the sum or difference by summing or subtracting the two outputs. The selected IF is real (no I and Q). The SSB mixer suppresses the original input frequencies and frequencies within one of the upper side band and the lower side band, resulting in an output ideally including a signal having frequencies of only the upper or lower side band. However, real world implementations of SSB mixers will result in some level of undesired, spurious emission, as illustrated in Fig 5. These spurious emissions are generally filtered out, requiring additional circuit components. A Complex Mixer (also called rotator or de- rotator) is the combination of two SSB mixers driven in quadrature (4 DSB mixers), combined in such a way that it generates a single complex IF (I and Q are generated).

SUMMARY OF THE INVENTION

According to exemplary embodiments of the present invention, local oscillator (LO) frequencies are generated. The generated LO frequencies may be utilized in a transmitter or receiver of a frequency hopped ultra wide band (UWB) wireless communication system. A first exemplary embodiment generates seven LO frequencies, and includes at least one frequency divider having a selectable divisor, a single SSB mixing stage, and two fixed frequency generators, which may be phase locked loops (PLLs). The system according to this embodiment is configured such that a frequency among the LO frequencies is selected by controlling a corresponding divisor and selecting from among an upper side band and a lower side band of the SSB mixing stage.

In a further embodiment, the first fixed frequency generator is configured to generate approximately 7392 MHz. The second fixed frequency generator is configured to generate a frequency of approximately 12,672 MHz. The mixing stage comprises a single side band (SSB) mixer.

In yet a further embodiment, the system includes two modes, with a first mode utilizing three frequencies and a second mode utilizing seven frequencies. The three frequencies are approximately 3432 MHz, 3960 MHz, and 4488 MHz. The seven frequencies are approximately 3432 MHz, 3960 MHz, 4488 MHz, 6336 MHz, 6864 MHz, 7392 MHz, and 7920 MHz.

In a second embodiment, the frequency generated by the first fixed frequency generator is double that of the first embodiment, at approximately 14,784 MHz. This is thereafter divided by two, resulting in in-phase (I) and quadrature (Q) signals, and the mixing stage is a complex mixer.

A third embodiment, for generating three LO frequencies, includes a first fixed frequency generator, a second fixed frequency generator, and a single mixing stage coupled to the first and second fixed frequency generators. The LO frequency is selected by selecting from among an output of the second fixed frequency generators, an upper side band of the mixing stage, and a lower side band of the mixing stage.

In a further embodiment, the first fixed frequency generator is at approximately 528 MHz, and the second fixed frequency generator is at approximately 3960 MHz. The single mixing stage is a SSB mixer configured to selectively output the upper side band and the lower side band.

In a fourth embodiment, the first and second frequency dividers are both double the frequencies of the third embodiment, at approximately 1056 MHz and 7920 MHz, and are both divided by two to generate I and Q signals, and the mixing stage is a complex mixer.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, together with the specification, illustrate exemplary embodiments of the present invention, and, together with the description, serve to explain the principles of the present invention.

Fig. 1 is a diagram illustrating the frequency bands utilized in an ultra wide band (UWB) communication system;

Fig. 2 is a diagram illustrating a frequency hopped spread spectrum system; Fig. 3 is a block diagram illustrating a frequency generator using multiple mixing stages as known in the prior art;

Fig. 4 is a diagram illustrating an exemplary double side band (DSB) mixer;

Fig. 5 is a set of graphs illustrating the output of an exemplary single side band (SSB) mixer;

Fig. 6 is a diagram illustrating the frequency bands utilized in a dual mode 3/7 -band UWB communication system according to an exemplary embodiment of the present invention;

Fig. 7 is a diagram illustrating the frequency bands utilized in a 3-band UWB communication system according to an exemplary embodiment of the present invention;

Fig. 8 is a block diagram illustrating a frequency generator for generating 3 or 7 local oscillator (LO) frequencies according to an exemplary embodiment of the present invention; Fig. 9 is a block diagram illustrating a frequency generator for generating 3 LO frequencies according to an exemplary embodiment of the present invention;

Fig. 10 is a block diagram illustrating a frequency generator for generating 3 or 7 complex LO frequencies according to an exemplary embodiment of the present invention; and

Fig. 11 is a block diagram illustrating a frequency generator for generating 3 complex LO frequencies according to an exemplary embodiment of the present invention. DETAILED DESCRIPTION OF THE DRAWINGS In the following detailed description, only certain exemplary embodiments of the present invention are shown and described, by way of illustration. As those skilled in the art would recognize, the invention may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Also, in the context of the present application, when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the another element or be indirectly connected or coupled to the another element with one or more intervening elements interposed therebetween. Like reference numerals designate like elements throughout the specification. Several exemplary embodiments of the invention pertain to the generation of the local oscillator (LO) frequencies utilized in an implementation of an ultra wide band (UWB) wireless communication system operating in the unlicensed 3.1- to 10.6-GHz band. A problem with prior implementations is that they utilize multiple SSB stages 20 and 22 as in Fig. 3. SSB mixers have an undesirable effect, in that they generate unwanted spurious frequencies in addition to the chosen frequency. Thus, a LO generator that generates the desired frequencies, while it reduces the number of SSB mixers, can result in less unwanted spurious signals.

Some exemplary embodiments employ multi-band orthogonal frequency division multiplexing (MB-OFDM), with frequency hopped spread spectrum, utilizing either three or seven frequency bands in the given range, and quadrature (or quadriphase) phase shift keying (QPSK). However, those skilled in the art will understand that the invention is not limited thereto.

A first exemplary embodiment is a multi-mode device, in which a first mode utilizes seven 528 MHz wide bands (illustrated in Fig. 6, the three bands in Group A, and the four bands in Group C from Fig. 1), and therefore requires frequency generation at 3432 MHz, 3960 MHz, 4488 MHz, 6336 Mhz, 6864 MHz, 7392 MHz, and 7920 MHz. The second mode only utilizes the three bands from Group A, and therefore requires frequency generation at 3432 MHz, 3960 MHz, and 4488 MHz only.

A second exemplary embodiment is a single mode device that utilizes bands 1-3, as illustrated in Fig. 7, in Group A from Fig 1.

In an exemplary embodiment, for frequency hopping, the signal hops from one band to another approximately every 312 ns. Therefore, there is a need for very fast switching between frequency bands, at a switching speed that faster than achieved with conventional methods, which control the fixed frequency generated by a phase locked loop (PLL). For such a fast switching time, at both the transmitter and receiver there is a desire for switching between the bands to be accomplished on the order of a few nanoseconds.

Fig. 8 is a block diagram showing a first exemplary embodiment, for use in a dual-mode UWB radio utilizing band groups A and C. This embodiment utilizes two phase-locked loops (PLLs) 100 and 102 as fixed frequency generators at 7392 MHz (PLL 100), and 12,672 MHz (PLL 102). In other embodiments, a delay locked loop (DLL), direct digital synthesis (DDS), or any other suitable method in the state of the art can be utilized for fixed frequency generation. Returning to Fig. 8, the 12,672 MHz PLL 102 is followed by two divider circuits: a K divider 106, and a divide-by-4 circuit 108. The dividers 106 and 108 are followed by a single sideband (SSB) mixer 104. The output of the SSB mixer 104 is fed through another divider 110.

As illustrated in Fig. 8, this architecture provides LO frequencies at the center of all seven bands. The output of the fixed frequency generator 100 is at 7392 MHz, and acts as the LO for band 8. The output of the SSB mixer 104 includes the LO for bands 6, 7, and 9, depending on the selection of the divisor in the 1/K divider 106, and the selection of the upper or lower side band in the SSB mixer 104. That is, band 6 (6336 MHz) is generated by setting K=3, and by selecting the lower side band; band 7 (6864 MHz) is generated by setting K=6, and by selecting the lower side band; and band 9 (7920 MHz) is selected by setting K=6, and by selecting the upper side band. Utilizing a similar selection scheme, bands 1, 2, and 3 are selectively generated at the output of the 1/2 divider 110. As discussed above, this embodiment enables a multi-mode system utilizing 7 bands, or utilizing 3 bands. In the second (three-band) mode, only the output of the 1/2 divider 110, with the LO frequency selected as above, is utilized.

Because this architecture generates all seven frequencies utilizing only a single mixing stage, it reduces the generation of spurious signals, thus reducing the need for further filtering of the LO signals. It further allows very fast switching between the different frequencies required, because, as the PLLs 100 and 102 are fixed, PLL settling is not required. The SSB mixer 104 may take complex signals as inputs, but only generates a 'real' output signal. In some applications, however, a complex LO may be desired. In another embodiment, as illustrated in Fig. 10, a complex mixer 304 may be utilized in place of the SSB mixer 104. As is well-known in the art, a complex mixer is similar to an SSB mixer, but generates a complex signal: an in-phase (I) and a quadrature (Q) signal. The use of a complex mixer is useful as it automatically generates the I and Q LO signals needed for the QPSK modulator needed in the transmitter, and for the IQ ZIF receiver. The complex mixer 304 requires two complex inputs. Dividers such as 306, 308, and 310 result in complex signals, but fixed frequency generators 300 and 302 generally do not. The exemplary embodiment of Fig. 10 would double the frequency of VCOl from that of the embodiment in Fig. 8, to about 14.8 GHz, and add a divide-by-two divider 312 after VCOl 300. This divider 312 would output the I and Q signals at 7392 MHz. Thus, when utilizing a complex mixer 304, the outputs, including an I and Q signal, would reduce or eliminate the need for the subsequent generation of a quadrature signal in a separate block after the LO generation of the block diagram of the embodiment of Fig. 8.

Fig. 9 is a block diagram illustrating a second exemplary embodiment, for use in a single mode UWB radio utilizing band group A. According to this embodiment, two PLLs 200 and 202 act as fixed frequency generators at 528 MHz and 3960 MHz, respectively. In other embodiments, a DLL, a DDS, or any other suitable method in the state of the art may be utilized for fixed frequency generation. The fixed frequencies output from PLLs 200 and 202 are fed into an SSB mixer 204.

As illustrated in Fig. 9, this architecture provides for LO frequencies at the center of the three bands in group A. The output of the fixed frequency generator 202 is at 3960 MHz, and acts as the LO for band 2. By selecting the upper or lower side band of the SSB mixer, 204, the LO for bands 1 and 3 can be chosen.

In a further embodiment, in a case where all three frequencies were required at the same time (not required under the latest thinking for a UWB radio, but still conceivable), then this approach could provide all three frequencies together, by replacing the SSB mixer 204 with a conventional mixer, and taking both upper and lower sidebands at the output for bands one and three, and band two is directly available from the frequency generator.

While the present invention has been described in connection with certain exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, and equivalents thereof.

Claims

CLAIMS:

1. A system for generating a plurality of local oscillator (LO) signals for frequency hopped ultra wide band wireless communications, the system comprising: a first frequency generator for generating a first signal having a first frequency; - a second frequency generator for generating a second signal having a second frequency; at least one frequency divider having a selectable divisor, the frequency divider for dividing the first frequency of the first signal to generate a third signal having a third frequency; and - a single mixing stage for mixing the third signal with the second signal or a signal derived from the second signal, wherein the system is configured such that a LO signal among the plurality of LO signals is selected by selecting a corresponding divisor for the at least one frequency divider and selecting an upper side band or a lower side band output of the single mixing stage.

2. The system of claim 1, wherein the first frequency is approximately 7392 MHz; the second frequency is approximately 12,672 MHz; and - the single mixing stage comprises a single side band mixer, or wherein the first frequency is approximately 14,784 MHz; the second frequency is approximately 12,672 MHz; and the single mixing stage comprises a complex mixer.

3. The system of claim 2, wherein the system is configured to operate in at least one of a first mode utilizing three frequencies or a second mode utilizing seven frequencies, wherein the three frequencies comprise approximately 3432 MHz, 3960 MHz, and 4488 MHz; and wherein the seven frequencies comprise approximately 3432 MHz, 3960 MHz, 4488 MHz, 6336 MHz, 6864 MHz, 7392 MHz, and 7920 MHz.

4. The system of claim 1- 3, wherein each of the first frequency generator and the second frequency generator comprises a phase locked loop.

5. A system for generating three local oscillator (LO) signals for frequency hopped ultra wide band wireless communications, the system comprising: a first frequency generator for generating a first signal having a first frequency; a second frequency generator for generating a second signal having a second frequency; and a single mixing stage coupled to the first frequency generator and to the second frequency generator, wherein the system is configured such that a frequency among the three frequencies is selected by selecting an output of the second fixed frequency generator, an upper side band output of the mixing stage, or a lower side band output of the mixing stage.

6. The system of claim 5, wherein: the first frequency is approximately 528 MHz; the second frequency is approximately 3960 MHz; and the single mixing stage comprises a single side band mixer, configured to selectively output the upper side band output or the lower side band output.

7. The system of claim 5 or 6, wherein the system is configured to concurrently generate three frequencies, the three frequencies comprising: the lower side band output of the mixing stage; the output of the second fixed frequency generator; and - the upper side band output of the mixing stage.

8. The system of claim 5, 6 or 7 further comprising: a first frequency divider coupled between the first frequency generator and the mixing stage; and a second frequency divider coupled between the second frequency generator and the mixing stage, wherein: the first frequency is approximately 1056 MHz; the second frequency is approximately 7920 MHz; and - the single mixing stage comprises a complex mixer.

9. The system of any of claims 5-8, wherein the mixing stage is a dual side band mixer, and wherein the system is adapted to provide the three local oscillator frequencies simultaneously.

10. A method for generating local oscillator frequencies for a frequency hopped ultra wide band wireless communications system, the method comprising: generating a first signal having a first frequency and a second signal having a second frequency; - selecting a divisor for a first frequency divider for dividing the first frequency of the first signal; selecting an upper side band or a lower side band of a single side band mixer; dividing the first frequency of the first signal by the divisor utilizing the first frequency divider, whereby the first frequency divider outputs a third signal having a third frequency; mixing the third signal with the second signal utilizing the single side band mixer, resulting in a mixed signal having a fourth frequency; dividing the mixed frequency by utilizing a second frequency divider, whereby the second frequency divider outputs a fifth signal having a fifth frequency; and selecting the local oscillator signal from among the second signal, the mixed signal, and the fifth signal.

11. The method of claim 10, wherein: the first fixed frequency is approximately 12,672 MHz; the second fixed frequency is approximately 7392 MHz; the divisor is selected from among values of 8, 12, and 24; the second frequency divider divides the fourth frequency by two.

12. The method of claim 10 or 11, further comprising: selecting a first local oscillator frequency, wherein the divisor is selected to be 24, the lower side band output of the single side band mixer is selected, and the local oscillator signal is selected to be the fifth signal; selecting a second local oscillator frequency, wherein the divisor is selected to be 24, the upper side band output of the single side band mixer is selected, and the local oscillator signal is selected to be the fifth signal; and selecting a third local oscillator frequency, wherein the divisor is selected to be 8, the upper side band of the single side band mixer is selected, and the local oscillator signal is selected to be the fifth signal.

13. The method of any of claims 10-12, further comprising: selecting a fourth local oscillator frequency, wherein the divisor is selected to be 12, the lower side band of the single side band mixer is selected, and the local oscillator signal is selected to be the mixed signal; selecting a fifth local oscillator frequency, wherein the divisor is selected to be 24, the lower side band of the single side band mixer is selected, and the local oscillator signal is selected to be the mixed signal; - selecting a sixth local oscillator frequency, wherein the local oscillator signal is elected to be the second signal; and selecting a seventh local oscillator frequency, wherein the divisor is selected to be 24, the upper side band of the single side band mixer is selected, and the local oscillator signal is selected to be the mixed signal.

14. The method of any of claims 10-13, wherein: the first local oscillator frequency is approximately 3432 MHz; the second local oscillator frequency is approximately 3960 MHz; and the third local oscillator frequency is approximately 4488 MHz.

15. The method of claim 13 , wherein: the first local oscillator frequency is approximately 3432 MHz; the second local oscillator frequency is approximately 3960 MHz; the third local oscillator frequency is approximately 4488 MHz; the fourth local oscillator frequency is approximately 6336 MHz; the fifth local oscillator frequency is approximately 6864 MHz; the sixth local oscillator frequency is approximately 7392 MHz; and the seventh local oscillator frequency is approximately 7920 MHz.