The Impulse Radio Waveform

1
The Impulse Radio Waveform Characteristics and
BER Analysis
Joydeep Acharya
2
Introduction to UWB

• Some facts
• UWB is the generic name given to radio systems
  with very large bandwidth
• 2 definitions under consideration by FCC
  when the 3 dB bandwidth of a radio
• signal becomes 25 or more of the signals
  center frequency, or if the RF
• bandwidth of the signal is greater than
  1.5GHz.
• is the upper 3 dB and is the lower 3 dB
  cut-off frequency of the signal spectrum

3
Introduction to UWB- Contd

• No carrier modulation is done. Instead UWB
  implementations can directly modulate a
• short burst of RF energy (impulse) that has a
  very sharp rise and fall time, thus resulting
• in a waveform that occupies several GHz of
  bandwidth.
• Hence UWB technology is a baseband and
  impulse technology

4
Different types of UWB signal waveforms

• A simple Gaussian monocycle.
• Impulse with Direct Sequence Spread Spectrum
• Impulse with time hopping format and pulse
  position data modulation
• (The Impulse
  Radio)

5
The Impulse Radio
A typical pulse train looks like
where

• w(t) transmitted monocycle waveform
• k index for denoting the transmitter
  dependent quantities
• j index denoting the monocycle number
• Tf the pulse repetition rate
• Cj(k) Pseudorandom hopping code
• Tc Unit shift for the hopping code
• Time shift for data modulation
• Ns No. of monocycles per data symbol

6
Relations amongst the different parameters

• Each Pseudorandom code element is periodic with
  period NP . i.e.
• Each code element is an integer in the range
• It is also assumed that
• So the overall PPM waveform is periodic with
  period
• Since there are hops per data symbol the
  symbol rate is
• Further it is assumed that

7
Frequency domain picture and Sequence design

• So pulse energy is spread over 1/D in frequency
• There is need to produce a uniform psd (whiten
  the spectrum)
• This involves proper sequence design of the PPM
  waveform

8
Sequence Design (Contd)
The PSD of is given
by Where This is because the pulse train
is periodic with period

• Observations
• The term is dependent on the time
  hopping sequence
• If is an integer multiple of ,
  is periodic with period
• A design criterion may be

9
Typical Values of some parameters
(Pulse repetition rate)
100ns (Chip rate)
1ns (Time shift due to
data modulation) 0.156ns (Period of the
UWB pulse) 0.7ns (rms
delay spread of UWB channel) 7-10ns
(Doppler frequency of UWB channel)
1Hz The no. of pulses per symbol and the
bit rate are related by
500 for 20 Kbps
10 for 1 Mbps
10
Receiver Signal Processing
The received signal can be modeled as

where
No. of active users
Attenuation of transmitter ks signal
Delay of transmitter ks signal
The received bit waveform for the first user
(reference user) is The optimal receiver for
this signal in AWGN is a symbol duration
correlator employing The one-pulse template
signal is
11
Receiver Signal Processing (Contd)
The Multiple Access Case


where

i.e. AWGN interference due to other users
Important Assumption The above expression is
assumed to be zero-mean Gaussian
process Hence

Where is a measure of SNR and
is given by
12
Receiver Signal Processing (Contd)
is given by is given
by Noise due to AWGN is Noise due
to the interference by other users is
13
Receiver Signal Processing (Contd)
Where It turns out that Where Hence
the effective SNR can be expressed as

14
Receiver Signal Processing (Contd)
Discussions The parameter can be optimized
by minimizing the function
log10Perror
RS 20 Kbps
-2.4
-2.5
-2.6
RS 10 Kbps
-2.7
-2.8
RS 5 Kbps
-2.9
2000
4000
6000
8000
10000
NU No. of active users
15
Receiver Signal Processing (Contd)
The Multipath Case If we consider dominant
paths for the user, the received signal
is Therefore a RAKE receiver could be
employed for combining the dominant
paths Assumption The receiver is locked to the
first users time hopping code and knows the
delays of the selected paths
16
UWB A Broader Outlook

• Challenges
• Noise and interference due to other signals in
  the same bandwidth
• Efficient pulse generation
• Pulse reception (with dispersion)
• Wideband antenna design
• Development of efficient coding techniques in
  order to control spectral characteristics
• of the signal, allowing optimum usage of
  bandwidth as per FCC regulations
• Synchronization (esp. for the time hopping case)
• FCC compliance

17
Work done so far

• Investigation into the nature of ultra wideband
  antenna
• Analytical studies on UWB propagation, channel
  modeling, BER studies
• Implementation of simple UWB transceiver
  prototypes
• Development of localizers for positioning
  systems

18
Scopes for future work

• Not only for Impulse Radio but for any UWB
  Technology
• More rigorous analysis of the UWB channel
• Synchronization issues in UWB
• Studies of issues like power control in UWB
• Development of the MAC layer for UWB
• Silicon implementation of more complex UWB
  transceiver designs

19
References
1 R.A. Scholtz, Multiple access with
Time-Hopping Impulse Modulation in Proc.
MILCOM, Oct. 1993 2 Moe Z. Win, R. A. Scholtz,
Impulse Radio How it works, IEEE Comm.
Letters Vol. 2, No. 1, Jan. 1998 3 Ali
Taha, Keith M. Chugg, Multipath Diversity
Reception of Wireless Multiple access
Time-Hopping Digital Impulse Radio , 2002 IEEE
Conf. on Ultra Wideband Systems and
Technologies 4 I ODonnell, M. Chen, S. Wang,
R.W. Brodersen, UWB Hardware Design, Panel
Session, MURI/Intel UWB Workshop From Research
To Reality, Oct. 3-4, 2002 5 I ODonnell, M.
Chen, S. Wang, R.W. Brodersen, An Integrated
Low-Power Ultra Wideband Transceiver
Architecture for Low-Rate Indoor Wireless
Systems, IEEE CAS Workshop on Wireless
Communications and Networking, Sept. 5-6, 2002
6 S. Ghassemzadeh ,Characterization of
Ultra-Wideband Channel at 5 GHz, Intel UWB
Forum, Portland, Oct. 2001 7
http//www.aetherwire.com 8 http//bwrc.eecs.be
rkeley.edu/Research/UWB/default.htm 9
http//ultra.usc.edu 10http//www.winlab.rutgers
.edu/msridhar/MARCH7.ppt
20
Thank You