Ultra Wideband Transceiver Design

Transcription

1 Ultra Wideband Transceiver Design By: Wafula Wanjala George For: Bachelor Of Science In Electrical & Electronic Engineering University Of Nairobi SUPERVISOR: Dr. Vitalice Oduol EXAMINER: Dr. M.K. Gakuru DATE:

2 Presentation Outline Introduction to UWB Project Objectives Types of UWB Transceivers IR-UWB: Transmitter Design IR-UWB: Channel Design IR-UWB: Receiver Design Simulation In MATLAB SIMULINK Applications Future Challenges 2

3 What is UWB? Governed by the FCC Definition: UWB is a modulated transmission with: more than 20% fractional bandwidth Or, at least 500 MHz of bandwidth. The UWB spectrum is between 3.1 and 10.6 GHz. Energy spectral density is limited to 41.3 dbm/mhz Bandwidth 3

4 UWB communications consists of very short pulses (Picoseconds) transmitted over a large spectrum at once Compared to narrowband RF and spread spectrum, UWB uses extremely low power, yet extremely wide bandwidth 4

5 Advantages of UWB Extremely difficult to intercept High Data rate transmission Less path loss Better immunity to multipath loss Availability of low cost transceiver Low transmit power Low interference Potential in localisation Low cost 5

6 Project Objectives To study Ultra-wideband Communications system To design a transceiver system based UWB To demonstrate that the UWB transceiver works. 6

7 Types of UWB Transceiver Impulse Radio (IR-UWB) Uses extremely short pulses with duration of the order of nanoseconds to transmit information Short Pulses have very large bandwidth of the order of a few GHz 7

8 Types of UWB Transceiver Advantages of Impulse Radio UWB With low duty cycle of pulses, the transmitter power can be small Carrier Modulation is not required No need of RF power amplifier Simple architecture Robust to multipath fading Offers great flexibility of the spectrum usage Adaptive transceiver design can be used for optimising system performance 8

9 Types of UWB Transceiver Multi Carrier UWB Whole GHz bandwidth is split into 528 MHz sub bands The fourth band is null to filter out the strong a interference Either single carrier or multi carrier modulation can be employed in each sub- band 9

10 Types of UWB Transceiver Advantages of Multi Carrier UWB Multi-Carrier OFDM enables high rate communication with inexpensive low power receivers It eliminates inter-symbol interference Disadvantages of Multi-Carrier UWB Complex architecture Consumes more power Up and down conversion is required High sensitivity to frequency, clock and phase inaccuracy Non-linear amplification destroys the orthogonality of OFDM 10

11 Transmitter Architecture The transmitter architecture that was employed is as follows It employs : Pulse generator Modulator High Pass Amplifier Bandpass Filter Digital Baseband Data Generation Antenna 11

12 Transmitter Architecture The transmitter architecture that was employed is as follows: SRDs Pulse Generator Step Recovery Diodes (SRD) or "snap off" diodes can be used to make very fast-rise time pulses. Conventional diodes conduct when forward biased and shut off when reverse biased. SRDs have a P-I-N structure, and charge is stored in the intrinsic layer when the SRD is forward biased. This allows the SRD to continue to conduct when the device is reverse biased. This reverse conduction continues until the charge is swept out of the intrinsic layer; with the charge gone, the diode abruptly stops conducting and "snaps off". The SRD presents low impedance during the forward/ reverse conduction and transitions to high impedance when it snaps off. BPSK Modulation Binary Phase Shift Modulation is a form of digital phase modulation. It involves changing the phase of the transmitted waveform instead of the frequency, these finite phase changes representing digital data. With BPSK, the carrier undergoes two changes in phase (two symbols) and can thus represent 1 binary bit of data per symbol. UWB Antenna A UWB antenna has some characteristics that are different compared to narrowband antennas: Wide bandwidth: in order to cover GHz band. Matching: with pulse generator over the whole band. Non-dispersive and low ringing: to preserve pulse shape. Possibility to be integrated: can be used in a tag-system. Directivity: in function of targeted applications 12

13 Transmitter Architecture A diamond dipole antenna was used for the transmitter architecture An ideal impulse UWB signal is as below, where Ai(t) is the amplitude of the pulse equal to Ep,where Ep is the energy per pulse, p(t) is the received pulse shape with normalized energy, and Tf is the frame repetition time. (A UWB frame is defined as the time interval in which one pulse is transmitted.) We also define Tp to be the duration of the pulse. Note that the pulse repetition rate is not necessarily equal to the inverse of the pulse width. In other words, the duty cycle of the transmitted signal is almost always less than 1. 13

14 Receiver Architecture The UWB receiver employs the following architecture The receiver consists of: RF Front End ADC/Clock distribution Digital Processing Receiver Topology- Digital Leading Edge Detection 14

15 Receiver Architecture RF Front End The RF Front End is composed of a series of broadband amplifiers, a bandlimiting filter, a broadband variable gain amplifier/variable attenuator, and a power divider. The LNA amplifies small input signals, is made of a low noise figure (NF), high transition frequency, with an amplification capability of 4 GHz, and provides at least a 60dB of total voltage gain. To limit the reflections, a 3 db attenuator was placed as an isolation device. The bandpass filter bandlimits the incoming signal and limits the input noise. The low pass filter that was used was a 3 rd order Butterworth design with a 600 MHz cut-off frequency whose work was to filter the carrier frequency to retrieve the base band pulses. ADC/CLOCK Distribution The RF Front block is followed by a limiter, the Receiver design is a Colonics Company design, in which the limiter is replaced by a MAX 108 by Maxim. The limiter acts as a power divider that sends the signal through a series of delay lines to the ADC bank. The bank of ADCs and clock distribution network which will be responsible for parallel sampling the received UWB pulse. Digital Processing It must be capable of handling multiple data streams from the ADCs and then demodulating the data in real time. In order to meet the design objectives, the digital processing must also be reconfigurable, in order to provide the flexibility demanded by a software radio design. A FPGA is used 15

16 Receiver Architecture Receiver Topology Digital Leading Edge Detection (DLED) Threshold detectors, also known as leading edge detection (LED) receivers, were some of the earliest and probably the simplest of all I-UWB receivers. The LED receiver sets a threshold at the receiver, and any incoming pulse that crosses the threshold is detected and demodulated. The problem with the threshold reception technique is that noise spikes, which happen to cross the threshold, will also be erroneously detected as a data pulse (known as a "false alarm" or "false detection"). To mitigate the problem of false detections, the receiver must continuously monitor the input noise signal and adaptively set a threshold such that only a small percentage of false detections will occur, similar to the constant false alarm rate (CFAR) used in radar. To operate properly, the LED receiver must use a device that is capable of responding to a very sharp change in received voltage in a very short time span 16

17 SIMULATION IN MATLAB The UWB transceiver was simulated as shown The simulation has : Transmitter Channel Receiver 17

18 SIMULATION IN MATLAB Transmitter Bernoulli Binary Generator It generates random binary numbers using a Bernoulli distribution. The Bernoulli distribution with parameter p produces zero with probability p and one with probability Buffer block It redistributes the input samples to a new frame size. Buffering to a larger frame size yields an output with a slower frame rate than the input. BPSK Modulator Baseband It modulates using binary phase shift keying method. The output is a baseband representation of the modulated signal. Transmitter Scatter Plot Scope It displays scatter plots of a modulated signal, to reveal the modulation characteristics, such as pulse shaping or channel distortions of the signal. The parameter settings of this scope were set at -1.5 and 1.5 on the x-axis and y-axis respectively. Pad Block It extends or crops the dimensions of the input by padding or truncating along its columns, rows, columns and rows, or any dimension(s) you specify. It was modified as pad signal field was placed at the beginning and the padding being done along the columns with specified output rows at a value of one. IFFT Block It computes the inverse fast Fourier transform (IFFT) of each channel of a P-by-N or length-p input. 18

19 SIMULATION IN MATLAB Transmitter Unbuffer Block The Unbuffer block unbuffers an Mi-by-N frame-based input into a 1-by-N samplebased output. That is, inputs are unbuffered row-wise so that each matrix row becomes an independent time-sample in the output. Transmitter Spectrum Scope The Spectrum Scope block computes and displays the periodogram of the input. The input can be a sample-based or frame-based vector or a frame-based matrix. The spectrum scope is used to display the transmitted UWB signal as shown. 19

20 SIMULATION IN MATLAB UWB Channel A Additive White Gaussian Noise Channel is used to model the propagation properties of a UWB signal The AWGN Channel block adds white Gaussian noise to a real or complex input signal. When the input signal is real, this block adds real Gaussian noise and produces a real output signal. When the input signal is complex, this block adds complex Gaussian noise and produces a complex output signal. The AWGN model is set up to allow variations in the SNR, which was the parameters within the model were set up with the SNR equal to 30, number of bits per symbol equal to 2, input signal power equal to one watt, and symbol period equal to 2 nanoseconds Shannon s Channel Capacity theorem C = B log2 (1 + S\N) where C = Maximum Channel Capacity (bits\sec) B = Channel Bandwidth ( Hertz) S = Signal Power (Watts) N = Noise Power (Watts) 20

21 SIMULATION IN MATLAB UWB Receiver The receiver architecture had the following blocks: FFT Block It computes the fast Fourier transform (FFT) of each channel of a P-by-N or length-p input, u. The parameter settings were set at table lookup in the twiddle factor computation field and speed in the optimise table field. Its functionality is to get the fast fourier transform of the received signal for proper demodulation of the signal. The selector It generates as output selected or reordered elements of an input vector, matrix, or multidimensional signal. The Input type parameter was set to the type of signal Matrix. The parameter dialog box and the block's appearance changed to reflect the type of input that was selected. To Frame Conversion It passes the input through to the output and sets the output sampling mode to the value of the Sampling mode of output signal parameter, which can be either Framebased or Sample-based. BPSK Demodulator Baseband It demodulates a signal that was modulated using the binary phase shift keying method. The input is a baseband representation of the modulated signal. The BPSK Demodulator parameter output type was set at bit and the constellation ordering was set to gray code. 21

22 SIMULATION IN MATLAB The received signal plus the AGWN is as shown below Discussion and Analysis The transceiver design is as implemented herein. It is from the simulation results of the spectrum scope at the transmitter and receiver side that they showed the actual functioning of the transceiver. The waveform from the transmitter is the one actually received at the receiver side plus an AWGN, as theory dictated. The simulated and theoretical discussions agree, thus the project being a success 22

23 UWB APPLICATIONS The following application illustrates UWB applications in Wireless Personal Area Network (WPAN) 23

24 UWB APPLICATIONS 24

25 Future Challenges Have to co-exist with the present and possible future standards in the 3.1 to 10.6 GHz Robustness and long term viability of this technology needs to be determined Feasibility for high level silicon integration in order to yield low cost and low power solution needs to be determined. 25

26 Summary Defined and discussed the benefits of UWB Technology Design of the UWB Transceiver FPGA is used for demodulation Simulation of the transceiver in MATLAB was demonstrated UWB Applications was discussed 26

27 EUREKA 27