Proposed IR-UWB Time Hopping PPM (TH-PPM) Transmitter

The biological signals obtained by bio-sensors in wireless sensor nodes are conditioned and digitized.

The digitized version of the signal are the information bits and serves as the input to the TH-PPM transmitter. The information bit are coded into 2-bit codeword for binary PPM and 8-bit codeword for 16-ary PPM. Each bit of the codeword is encoded in terms of position of a pulse p(t) of duration T_w (T_w= 64 ns) among N_w/2 (N_w= 32) positions within half of the symbol duration T_sym. The additional (N_w/2 - 1) waveform positions are used for time-hopping and to support multiple-BAN nodes for co-existence.

The architecture of the proposed IR-UWB TH-PPM transmitter is shown in Fig. 3.8. It con- sists of three blocks: Pulse Position Controller (PPC), six-segment PWLA SRRC pulse generator and up-conversion circuitry. The SRRC pulse generator contains two sub-blocks: controller and PWLA pulse generator. The inputs to the transmitter “CODEWORD” (1 or 0) and “PULSE POSITION”

{h₃, h₂, h₁, h₀} - specified by the “time-hopping sequence” come from the baseband processor which is not considered here. The transmitter is triggered by the signal “CODEWORD”. When CODE- WORD= 1, a pulse is transmitted and when CODEWORD= 0, no pulse is transmitted. A SRRC pulse is generated whenbb_trig= 1 and is followed by up-conversion to the carrier frequency (f_c) at 4 GHz for transmission.

PULSE POSITION CONTROLLER

CODEWORD h₃ h₂ h₁ h₀ PULSE POSITION

fh3; h₂; h₁; h₀g

Six-segment PWLA

PWLA Pulse Controller

sig₁ sig₂ sig₃

clk4

clk

SRRCp

bbtrig

clk64 From Baseband Processor

sin(2πf_ct) Antenna

Mod-4 Counter

Q₁Q₀

Mod-16 Counter Q₃Q₂Q₁Q₀

sig₄

LOCAL OSCILLATOR SRRC Pulse Generator

(PPC)

Generator

f_clk4=fclk/4 f_clk64=fclk/64

Proposed Transmitter fh₃; h₂; h₁; h₀g: 4-bit

time-hopping sequence CODEWORD: 0 or 1

Figure 3.8: Block Diagram of the proposed IR-UWB TH-PPM transmitter

bb_internal

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32Tw

clk64

CODEWORD

Figure 3.9: Timing Chart of the proposed PPC

3.5.1 PPC

The PPC generates the signal bb_trig to activate the pulse generator. The position of the pulse is at one of the sixteen positions defined by PULSE POSITION{h3, h2, h1, h0} when CODEWORD is high (indicated by bb_internal as shown in Fig. 3.9). For example, when CODEWORD is ‘1’ and {h₃, h₂, h₁, h₀}is ‘0111’, the signal “bb_internal” is high at the eighth position for a duration ofT_w (from 7T_w to 8T_w as shown in Fig. 3.9).

The advantage of the proposed PPC is that, the SRRC pulse generator is active only for a duration T_w/8 (bb_trig= 1) instead of the total allocated duration ofT_w (bb_internal= 1). This procedure ensures that the peak of the SRRC pulse is placed exactly at the middle of the durationT_w (Fig. 3.10).

clk4 CODEWORD

1 2 3 4 5 6 7 8 9 1011 1213 14 15 0

bbinternal

7Tw 8Tw

bbtrig

Tw/8 duration Tw/2

Figure 3.10: bbtrigg when CODEWORD is ‘1’

MOD-16 COUNTER BIT PULSE

clk64

PULSE POSITION fh3; h2; h1; h0g

clk4

bbinternal

MOD-16 COUNTER

bbtrig

Figure 3.11: Circuit diagram of the proposed PPC

Fig. 3.11 shows the proposed PPC that implements the following logic:

bb_internal= (Q₃h₃)(Q₂h₂)(Q₁h₁)(Q₀h₀) bb_trig =M₃M₂M₁M₀+M₃M₂(M₁+M₀) 3.5.2 Six-segment PWLA SRRC pulse generator

The pulse generator produces the PWLA pulse only when bb_trig is high (Fig. 3.12). The different segments are generated by switching on/off the appropriate current sources through the signalssig₁, sig₂,sig₃ and sig₄ (Fig. 3.12). The controller and the PWLA pulse generator are explained below.

3.5.2.1 Controller

Fig. 3.13 shows the circuit diagram of the controller for generation of the pulseSRRC_pof duration

‘8T’ (as per Fig. 3.12). The outputs sig₁, sig₂, sig₃ and sig₄ are the inputs to the PWLA pulse generator.

sig

0 2T 3T 4T 5T 6T 7T 8T

SRRC

_p a b

c d

e f

-I₁

Currents! I₁ I₂ -I₂ -I₁ I₁ Time!

bb

_trig

Figure 3.12: Timing diagram of six-segment PWLA SRRC pulse generator

Q0 Q0

sig2

sig3 sig4

MOD-8 COUNTER

clk bbtrig

sig1

sig2

sig3

sig4

sig1

Figure 3.13: Circuit diagram of controller

VDD VDD

BP BP

B_N B_N

Charge Pump Circuits

CP1 CP2

sig1

sig2

sig3

sig4

I1 I2

SRRCp

VDD

bbtrig

VDD

V_DD BP

RG2

RG1

Bias Generator I0

Figure 3.14: Circuit diagram of PWLA Pulse Generator

3.5.2.2 PWLA pulse generator

Fig. 3.14 shows the proposed PWLA pulse generator. The pulse generator consists of two charge

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Figure 3.15: Up-conversion circuitry using double-balanced Gilbert Mixer

I₀ acts as the basic reference current source mirroring the currents I₁ and I₂. The current I₁ flows from the load to the first charge pump CP₁ when sig₂ is high (this generates the segments ‘a-b’ or

‘e-f’) as shown in Fig. 3.12. The same currentI₁ flows from the charge pump to load whensig₁ is low (this generates the segments ‘b-c’ or ‘f-g’). Similarly, the currentI₂ flows through the second charge pumpCP₂ to the load whensig₃ is low from 2T to 4T (this generates the segment ‘c-d’) and the same currentI₂ flows from load to charge pump whensig₄ is high from 4T to 6T for the segment ‘d-e’.

3.5.3 Up-conversion Circuitry

The single-ended baseband pulse SRRC(t) is next converted to differential baseband signals BB+

and BB- as shown in Fig. 3.15. The differential baseband signal (across BB+ and BB-) is up-converted by multiplying with a differential sinusoidal carrier at 4 GHz (across LO+ and LO-) that is generated by a cross-coupled LC-tank oscillator [337] shown in Fig. 3.15. The multiplier is designed with a double-balanced Gilbert Mixer [338]. The major advantage of double-balanced Gilbert mixer is that it provides good isolation between LO and RF ports and leads to reduced LO leakage to RF ports.

The differential RF output of the Gilbert mixer is filtered by a simple RC filter with -3 dB cut-off frequency at 4.5 GHz and fed to a balun for differential to single-ended conversion. The up-converted SRRC waveform is finally fed to the antenna for transmission.

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Figure 3.16: Simulated waveforms of (a) the six-segment baseband PWLA SRRC pulse “SRRC(t)” and (b) the corresponding up-converted RF pulse using double balanced Gilbert Mixer

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Figure 3.17: Normalized Spectral Density of the proposed six-segment PWLA SRRC pulse generator con- forming to the FCC transmit spectral mask [3] and the 15.6 transmit spectral mask [2]

Fig. 3.16(a) shows the baseband PWLA SRRC pulse and Fig. 3.16(b) shows the corresponding up-converted RF pulse using double-balanced Gilbert Mixer. Fig. 3.17 shows the normalized power spectral density of the up-converted PWLA SRRC pulse satisfying both the FCC transmit spectral mask [3] and the 15.6 transmit spectral mask [2].

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