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WiMAX RF system design

In Electronic Infomation Category: W | on April 12,2011

Fixed WiMAX standard is based on orthogonal frequency division multiplexing (OFDM) technology, with 256 sub-carriers; The standard supports 1. 75 ~ 28 MHz multiple channels within the bandwidth, while supporting a variety of different modulation schemes, including BPSK, QPSK, 16QAM and TLV2252IDR datasheet and 64QAM.

1 to complete the main chip features

Superheterodyne TDD equipment used in this way to complete the design, in line with WiMAX RF chip set the standard before the introduction of the successful use of the IF chip SIGE company produced SE7051L10 and TLV2252IDR price and Texasinstruments TRF2436 RF chip to complete the design. IF frequency is fixed at 380 MHz, radio frequency 5. 725 ~ 5. 850 GHz band is optional.

1.1 SE7051L10

SE7051L10 primary completion is:

completed the launch time slot I, Q baseband signals on the 380MHz frequency to a fixed intermediate frequency signal;

completed in the receiving slot to receive a fixed 380 MHz IF signal down conversion to zero IF I, Q baseband signals;

complete synthesis of IF and TLV2252IDR suppliers and RF LO required functions; which IF LO frequency is fixed at 380 MHz; RF local oscillator frequency selectable, so that the system works in the desired channel within the work;

in the transmit and receive channels are built-in variable gain amplifiers, and Tx channel has 18 dB gain control range (step 6 dB), and 50 dB TX gain control range (step 1 dB), Rx channel has 50 dB of AGC range.

1.2 TRF2436

TRF2436 completion is:

time slot to complete the launch of fixed 380 MHz IF signal is upconverted to the desired RF channel frequency; completed in the receiving slot to receive the RF signal amplification and down conversion to a fixed 380 MHz IF signal; transceiver chip built-in switch, low-noise amplifier and switch control of the power amplifier; built-in RF local oscillator frequency doubler.

2 overall design

The SE7051L10 the same company with the TRF2432 sets of non-film, to be re-designed, mainly from the following considerations. Produce radio frequency chips SE7051L10

local oscillator, which synthesized frequency range 2 850 ~ 3 350MHz, if the system uses low local oscillator, require a minimum frequency of 2 672. 5MHz, SE7051L10 can not meet this requirement, the system can only use high local oscillator and high frequency of the vibration requirements of 3 052 ~ 3 115MHz; use will result in high local oscillator frequency and baseband spectrum image, the point system, since the receiver down-conversion will be launched on the frequency spectrum caused by the mirror flip the system without leaving any traces for demodulation; but as a CPE device, not with the standard base station couplet, the use of baseband I, Q signal is reversed connections, clever solution led to selection of high-vibration spectrum of the flip, with the standard signal source couplet, the system working properly.

SE7051L10 IF transceiver as a separate differential input and output, while the TRF2436 transceiver IF differential input and output is shared, in order to resolve this problem, use two single-ended double-throw switch, by sending and receiving switching control signal, the transceiver will SE7051L10 separate differential IF input and output requirements to switch to the TRF2436 IF differential input and output common to good effect.

As WiMAX CPE equipment, base stations to meet the requirements of different client devices, the system receives a fixed gain, do not have the AGC function of the received signal amplitude in order to ensure a constant, different CPE devices by dynamically adjusting the transmission power; therefore requested the WiMAX CPE equipment transmission channel with more than 50 dB of ALC control; Although SE7051L10 internal diameter of 1 dB step attenuator of 50 dB, but the frequency attenuation is too large, will affect the IF signal to noise ratio, thus affecting the system performance; The TRF2436 is for 802 . 11 system development, launch channel does not improve the system dynamic attenuator NC; as increasing the system launch dynamic, in the TRF2436 RF filter is added after a further 4 dB total path attenuation 28 dB attenuator NC.

Redesign SE7051L10 RF local oscillator of the loop filter to optimize the RF local oscillator phase noise, thereby improving the signal transmitting and receiving system relative vector error.

TRF2436 requirements of the local oscillator 100 differential input, local oscillator power level 0 dBm. Work by increasing the balance of this band - imbalances balun transform to solve the manifold, Manifold balanced balun output impedance 200 differential output impedance matching through the quarter wavelength impedance transformer to solve; the same time, through a monolithic amplifier output of the oscillator will SE7051L10 enlarged to 0 dBm, single-chip amplifier can help to improve the LO input and output isolation.

Budget by sending and receiving channels, reasonably complete power amplifier and LNA design.

3 system workflow

System uses time division duplex work, when the transceiver baseband switch control signal is high, the system works in the hair slot, sent the baseband I, Q signals by the modulation, on the frequency, power amplifier and IF, filtered by the switch from the RF antenna to the receiver; at the receiving end, baseband transceiver control switch signal is low at this time high, the system working in closing slot, the received RF signal by the switch, LNA, the next frequency, the corresponding RF, IF filter, demodulated I, Q baseband signal to the baseband signal processing unit.

4 major technical indicators and targets to achieve assigned

4.1 transmission power to achieve

Because the system uses OFDM baseband modulation technique, OFDM is a wireless communication system, a key technology, is a multi-carrier transmission technology. Multi-carrier transmission technology in terms of the single carrier transmission technology has many advantages, such as anti-multipath interference, anti-burst noise and effectively overcome the frequency selective fading. But the major drawback of OFDM technology is a high peak to average power ratio (PAPR), high peak easily lead to nonlinear distortion; the same time, the higher the system such as 64QAM modulation, the linearity requirements on the system than high, the above problems, in the design and selection of devices, in order to ensure the system works in the linear region, all devices are required in the P1 dB 10 dB back work.

Amplifier design difficult are mainly final amplifier design, the system used SIRENZA final amplifier produced SZA5044, output P1 dB is 29 dBm, power back 10 dB, linear power output is 19 dBm, the final stage amplifier with a passive transceiver switch, harmonic suppression and low-pass filter MCX socket, and its insertion loss is the sum of 1. 6 dB, the socket output linear power output of 17. 4 dBm, to meet the equipment and technology indicator; the same time, SZA5044 a gain of 28 dB, to ensure that equipment specifications 16 dBm power output, SZA5044 input power requirements - 9 dBm, the RF amplifier stage before the switch, NC attenuator and filter insertion loss of the sum of 4. 4 dB, linear power output required TRF2436 - 4. 6 dBm, TRF2436 the output P1 dB is 22 dBm, linear power output of 12 dBm, to meet the technical requirements.

4.2 Realization of transmission channel ALC As the system design for point-

, AGC can not work the base station, base station receiver gain is relatively fixed, in order to ensure the normal communication, the base station receives uplink baseband test I, Q power level, with the standard I , Q power level compared to calculate the power error, sent to the client, through the open-loop control the client software, uplink transmission power; to ensure adequate dynamic, to adapt to the impact of the decline of the indicator requires the consumer side of the ALC control greater than 50 dB, the trail 1 dB.

SE7051L10 the system provided by the 30dB ALC ALC control range, the trail 1 dB; the same time, NC attenuator provides 28 dB of ALC control range, the trail 4 dB, in practice, the actual test a ALC control form the actual amount of attenuation from small to large order, the trail 1 dB, the companys 89,601 software by An Jielun measured transmit power level, while ensuring that the 50 dB dynamic range, the launch of the relative vector error of less than - 31 dB. In normal operation, the baseband software ALC control signal based on the current position of the form where the control and base station power measurement error, the client dynamically adjust transmit power to ensure the system work properly.

4.3 transmitter EVM indicators to achieve

Transmitter error vector is a measure of the relative integrated technical indicator of the transmitter, the baseband I, Q quadrature error, amplitude balance, phase noise local oscillator, mixer and power amplifier (PA) Linear Technology Index and system frequency deviation and other decisions. RF systems for this, I, Q quadrature error of the main PCB board through the I, Q signal traces to control strictly equal length; amplitude balance of the operational amplifier through the resistor to adjust the gain control; the use of the TRF2436 RF system as second mixers main chip, integrated mixer chip, can not control; launch EVM mainly by the decision of local oscillator phase noise, through the rational use of VCTCXO, optimize the loop filter and other measures to ensure the RF local oscillator phase noise to meet the - 88 dBc @ 1 kHz, - 90 dBc @ 10 kHz, thus ensuring the ultimate power of 0 dBm output TRF2436, the corresponding vector error of - 34. 5 dB; of the system, the amplifier determines the rational design of transmitter relative vector error.

5 Summary

As mentioned above, the system selects the final amplifier, the output power of 16 dBm, the relative vector error is 2% (- 34 dB), vector by calculating the relative error of the system - 32. 5dB, meet the technical requirements.

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