A wideband communications receiver and a method for receiving data frames from a wireless device in a wireless local area network

FIELD OF THE INVENTION

The invention relates to a wideband communications receiver, especially a receiver in a wireless local area network and a method for receiving data frames from a wireless device in a wireless local area network.

BACKGROUND OF THE INVENTION

In general, wireless communications systems comprises a modulation of one or more baseband information signals onto one or more carrier signals, transmission of the resulting band-pass signal(s), and demodulation at a receiver to recover one or more of the baseband information signals. Such wireless communications systems are part of electronic devices, such as of pocket PCs, mobile phones, digital cameras, etc.

Such communications systems comprises wideband receivers, which employs typically heterodyne or super-heterodyne technique, which involves either down-converting or up-converting an input radio frequency signal (shortly called RF signal or carrier frequency signal) to some convenient intermediate frequency (shortly called IF) or directly to baseband (or near zero hertz) and the demodulating the IF signal or baseband signal. Previous designs of receivers included super-heterodyne architectures required IF filtering and power consumption are expensive. Direct conversion zero IF- receivers improve upon these parameters.

A further simple solution of a conventional direct converter is described in the US 6,230,000. The described direct down-converter improved more over a

conventional direct conversion design. In detail, the converter for converting a signal to baseband includes a commutating switch which serves to sample an RF waveform four times per period at the RF frequency. The samples are integrated over time to produce an average voltage at 0 degrees, 90 degrees, 180 degrees and 270 degrees. The average voltage at 0 degrees is the baseband in-phase signal, and the average voltage at 90 degrees is the baseband quadrature signal.

It is object of the invention to specify an extremely low-complexity and also low-performance and low-power wideband communications receiver in a wireless local area network and a method for receiving data frames from a wireless device.

The problem is solved by a wideband receiver comprising the features given in claim 1 and by a method comprising the features given in claim 12.

Advantageous embodiments of the invention are given in the respective dependent claims.

SUMMARY OF THE INVENTION

The invention comprises a new wideband communications receiver based upon a direct down-converter as described in the US 6,230,000. Particularly, the wideband communications receiver comprises an antenna interface module including a low noise amplifier connected to an antenna for receiving and amplifying a radio frequency signal. An analog front-end section including a radio frequency amplifier and said down-converter is established for down-conversion of the received amplified radio frequency signal directly to baseband into differential IQ signals for a baseband controller. Subsequently, an amplifier amplifies the differential IQ signals into saturation to connect it's directly with digital inputs of the baseband controller which processes the differential IQ signals bitwise. The advantage of such structured receiver is in the very simple design by prevention of a conventional analog-digital-converter. Furthermore, such structured receiver has a high exceptional performance in unique with minimal components. Therefore, such receiver is a very simple low-complexity, low-power and low-cost receiver.

In a preferred embodiment of the invention, said direct down-converter is a product detector which comprises a commutating switch having an input signal responsive to the amplified radio frequency signal in combination with capacitors to integrate portions of the input signal and having two outputs for producing the differential IQ signals. Such a direct down-converter is described in detail in the US 6,230,000. In detail, the described down-converter comprises one output is a zero degree output for producing the baseband in-phase signal and the other output is a 90 degree output for producing the baseband quadrature signal.

In a further embodiment, the operation rates of said commutating switch is controlled by a control input. The commutating switch is for example a four-position rotary switch revolving with said control input which equals four times a local oscillator frequency. Advantageously, in a very simple receiver with a simple direct down-converter the local oscillator frequency equals to the radio frequency signal (also called carrier frequency). In other words: The control input equals four times the radio frequency signal. Since the commutating switch is turning at exactly the radio frequency, each capacitor samples the signal once each revolution. That means each capacitor will track the RF's amplitude for exactly one-quarter of the cycle and will hold its value for the remainder of the cycle. The 0° and 180° capacitors differentially sum to provide the in-phase signal and the 90° and 270° capacitors sum to provide the quadrature signal.

In another preferred embodiment, said local oscillator frequency is determined by a control voltage which is generated by a digital output of a digital- analog-converter (shortly called DAC), especially of a so called Sigma-Delta-DAC of the baseband controller with a subsequent filter, especially a low-pass filter. The integration of the local oscillator control loop in the baseband controller is one of the easiest ways to save even more cost and power. Especially, active components in the baseband controller are avoided.

Furthermore, the baseband controller comprises preferably at least one mean for phase, time and frequency tracking of the carrier frequency input by evaluation of signal preambles of received frames. In this case, further active components are also avoided in the baseband controller.

For amplifying the differential IQ output signals of the down-converter,

each of said two outputs of the down-converter for the differential IQ signals are connected with an amplifier. Through this, the amplified IQ signals can be directly connected with digital, over-sampling input pads of the baseband controller (also called baseband processor). An additional analog-digital-converter is avoided. This simplifies the hardware design of the receiver.

According to another feature of the invention, each detector's output is connected with a low pass filter for selecting a relevant band of the digitalized input signals for the baseband controller.

In an alternative embodiment of the invention, said antenna interface module comprises a plurality of antennas with a subsequent antenna switch for selecting one of the antennas. Advantageously, said antenna switch is connected with the baseband controller for determination the relevant antenna by a selecting signal which is generated by a selection function, e.g. a diversity selection function, of the baseband controller. This simplifies the hardware design of the receiver.

In summary, the invention describes a low-cost, high-performance and low-power approach of a wideband receiver in a flexible and simple manner.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a general block schematic diagram of architecture of a wideband receiver based on a direct down-conversion of the received radio frequency to baseband according to the invention. Fig. 2 shows a possible embodiment of architecture of a wideband receiver based on a direct down-conversion of the received radio frequency to baseband.

DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1 shows a general block schematic diagram of architecture of a

wideband receiver 1 with an antenna 2 which receives a radio frequency f _R p. The further described wideband receiver 1 could be a applicable for a so called WLAN- receiver designed to operate under the IEEE 802.1 Ib standard. Such a receiver could be part of consumer products like WLAN-enabled cordless headphones or telephones where range and high-data transfer rates are not an issue.

The received radio frequency signal f _RF is amplified in a low noise amplifier 3, which is connected to the antenna 2. The antenna 2 and the low noise amplifier 3 are components for example of an antenna interface module RFM (also called radio frequency section). The low noise amplifier 3 is for example a gain block of about 20 dB with less than about 3 dB noise figure.

An analog frond-end section AFE includes a standard band-pass filter 4 and a buffer amplifier 5 which saves constant output impedance of 50 ohm into a succeeding direct down-converter 6. The band-pass filter 4 operates with a bandwidth of 2400 MHz to 2500 MHz. The buffer amplifier 5 provides an excellent isolation of the direct down-converter 6 with the output impedance of 50 ohm. The direct down- converter 6 bases on a known direct down-converter described in the US 6,230,000. The direct down-converter 6 converts the amplified and filtered radio frequency signal f _RF down to baseband into differential IQ signals for a succeeding baseband controller 7. To processing the IQ signals of the direct down-converter 6, the IQ signals are amplified into saturation with separate amplifier 8.1 and 8.2. The baseband controller 7 processes the amplified IQ signals bitwise, e.g. dispreading, decoding, differential decoding, preamble detection, framing, etc. In the simplest way, this can be done directly by a baseband processor.

For determination the channel center of the direct down-converter 6, the converter 6 is controlled by a control input f _c , which is determined by a control voltage Uc which is generated by a digital output of the baseband processor 7. Said control input fc equals four times a local oscillator frequency f _L o of a local oscillator 9, wherein the local oscillator frequency fiχ _> equals to the radio frequency signal f _RF (also called carrier frequency). Fig. 2 shows in more detail a possible embodiment of architecture of a wideband receiver 1 based on a direct down-conversion of the received radio frequency f _RF to baseband.

The receive radio frequency signal f _RF enters the receiver 1 through two antennas 2.1 and 2.2 and associated low noise amplifiers 3.1 and 3.2. Furthermore, an optional antenna switch 10 is established for selecting one of the antennas 2.1 or 2.2. The antenna selecting signal Sa is generated by a selection function, e.g. a diversity function or another possible selection function, integrated in the baseband controller 7.

Out-of band signals of the radio frequency signal f _RF are blocked by means of the standard band-pass filter 4. The band-pass filter 4 could be a well-known ceramic-filter. The signal path till here represents more or less a standard front-end section AFE. Then the RF signal f _RF travels through the buffer amplifier 5 whose task it is to guarantee a constant output impedance, e.g. of 50 ohm, into the direct down- converter 6. The output impedance of the amplifier 5 forms a first-order low pass filter with the capacitors Cl to C4 of the direct down-converter 6. The direct down-converter 6 converts the RF signal f _RF down to baseband into a differential IQ signal which is subsequently buffered, converted back to single-ended and passes through a low pass filter chain 11.1 and 11.2 that provides the necessary selectivity.

The direct down-converter 6 is a product detector which comprises in more detail a commutating switch SS having an input signal responsive to the amplified radio frequency signal f _RF in combination with the capacitors Cl to C4 to integrate portions of the input signal and having two outputs Al and A2 for producing the differential IQ signals. In detail, the described down-converter 6 comprises one output Al which is a zero degree output for producing the baseband in-phase signal I; the other output A2 is a 90 degree output for producing the baseband quadrature signal Q. The commutating switch SS is a four-position rotary switch. The operation rates of said commutating switch SS is controlled by the control input fc. The commutating switch SS revolves with said control input fc which equals four times the local oscillator frequency fiχ _> - The local oscillator frequency fiχ _> equals to the radio frequency signal f _RF (also called carrier frequency). In other words: The control input equals four times the radio frequency signal. Since the commutating switch SS is turning at exactly the radio frequency f _R p, each capacitor Cl to C4 samples the signal once each revolution. The 0° and 180° capacitors Cl and C2 differentially sum to provide the in-phase signal and the 90° and 270° capacitors C3 and C4 sum to provide

the quadrature signal.

The outputs Al and A2 of the IQ signals are amplified, and, in this example, limited and fed into the baseband controller 7 as digital signals. The baseband controller 7 performs the sampling task and then processes the resulting bit-stream digitally, e.g. despreading, CCK decoding, differential decoding, preamble detection, framing, etc.

The direct down-converter 6 needs a control frequency input f _c that determines the channel center. This is generated by a low-phase noise oscillator 9. The output frequency or control frequency fc of the local oscillator 9 is determined by the control voltage Uc which is generated by a Sigma-Delta-DAC in the baseband processor 7 in combination with subsequent filter 12, especially a low-pass filter. The soft- and hardware in the baseband processor 7 must provide the means to blindly scan through the available channels and select a channel and track phase and frequency offsets. This can be done for example by means of evaluation of the signal preamble of received frames (challenging) or additional hardware.

The described receiver 1 depicts one possible extremely simplified implementation that includes the following optional preferable features:

Additional optimization can probably be performed by leaving out conventional PLL divider and phase detector and instead closing the carrier oscillator control loop via the baseband controller 7. This structure saves even more cost and power.

Furthermore it could be possible to amplify the IQ output signal of the direct down-converter 6 into saturation by the amplifier 8.1 and 8.2 and connect it directly with digital, over-sampling input pads of the baseband processor 7.

LIST OF NUMERALS:

1 Receiver

2 Antenna

3 Low noise amplifier

4 Band-pass filter

5 Buffer amplifier

6 Direct down-converter

7 Baseband controller

8 Amplifier

9 Oscillator

10 Antenna selecting switch

11.1, 11.2 Low-pass filter

12 Low-pass filter fRF radio frequency signal fLO local oscillator frequency signal fc control input (frequency signal)

Uc control voltage