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Simone Morosi and Tiziano Bianchi

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Pulse Repetition and Cyclic Prefix Communication Techniques in Impulse Radio UWB Systems Simone Morosi and Tiziano Bianchi Electronics and Telecommunications ... – PowerPoint PPT presentation

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Title: Simone Morosi and Tiziano Bianchi


1
Università degli Studi di Firenze Dipartimento di
Elettronica e Telecomunicazioni
12 MCM of the COST 289 October 30-31 Firenze,
Italy
Pulse Repetition and Cyclic Prefix Communication
Techniques in Impulse Radio UWB Systems
  • Simone Morosi and Tiziano Bianchi
  • Electronics and Telecommunications Department,
    University of Florence
  • Via di Santa Marta 3, 50139 Firenze, ITALY
  • Tel 39 055 4796485 Fax 39 055 472858
  • e-mail morosi, bianchi_at_lenst.det.unifi.it

This work has been supported by Italian Research
Program (PRIN 2005) Situation and location aware
design solutions over heterogeneous wireless
networks.
LENST
Laboratorio di Elaborazione Numerica dei Segnali
e Telematica
2
Outline
  • Motivation
  • System model
  • Frequency Domain Detection
  • Comparison Criteria
  • Simulation Results
  • Conclusions

3
Motivation
  • Our goal
  • The comparison of two techniques for Impulse
    Radio UWB systems which are based on the pulse
    repetition according to the spreading factor
    value and the Cyclic Prefix insertion.
  • Both techniques cause a throughput loss and have
    to be compared both in terms of performance and
    capacity, i.e. the maximum data rate which is
    afforded.
  • The redundancy due to the CP approach is not
    considered as an overhead, but as an alternative
    to the processing gain Nf .
  • Our tool Frequency Domain Detection (FDD)
  • FDD has been proposed for UWB single user systems
    in Bia04 and Ishi04 and extended to high
    data-rate multiuser systems in Mor05
  • This approach is based on both the introduction
    of the cyclic prefix and the use of a frequency
    domain detector. This approach is well suited for
    the applications which are based on data-rate
    scalability and rely on data gathering.

Bia04 T. Bianchi and S. Morosi, Frequency
domain detection for ultra-wideband
communications in the indoor environment,
in Proc. of the IEEE Eighth International
Symposium on Spread Spectrum Techniques and
Applications, 2004, Aug.-Sept 2004. Ishi04 Y.
Ishiyama and T. Ohtsuki, Performance evaluation
of UWB-IR and DS-UWB with MMSE-frequency domain
equalization (FDE), in Proc. of the IEEE
GLOBECOM 04, vol. 5, Nov.-Dec. 2004. Mor05 S.
Morosi and T. Bianchi, Frequency Domain
Multiuser Detectors for Ultra-Wideband
Short-Range Communications, in Proc. of ICASSP
2005, Philadelphia, PA, USA, Mar. 2005.
4
Signal Structure (I)
  • tl indicates the delay of the l-th user with
    respect to the access point time reference
  • t(b) indicates the pulse shift that implements
    binary PPM
  • Tf and Tc are the frame and the chip periods
  • bl(i) 1 is the i-th binary symbol transmitted
    to the l-th user. The same bit is transmitted
    over Nf consecutive frame periods (TbNfTf ).
  • Nc chips fit exactly in one frame period (Tf
    NcTc ).
  • Each active user is associated with a
    time-hopping pseudo-random periodic pattern cl(m)

0
1
2
3
TH code 0,1,2,3
5
Signal Structure (II)
  • The transmitted signal can be represented more
    conveniently as

The discrete sequences pl(k) and ql(k) are
periodic with period Nw 2Nc Nf .
6
Downlink Model (I)
  • We consider a base station transmitting Nu
    signals synchronously to a set of Nu users Iu
    1, 2, . . . , Nu
  • The Received Signal can be expressed as
  • The function f(t) takes into account the effects
    of the channel, of the antennas, and of the
    matched filters of both transmitter and receiver.
  • The signal x(k) represents the digital
    counterpart of the UWB-IR TH-SS signal
  • The signal h(t) models thermal noise.

7
Downlink Model (II)
  • By assuming that channel characteristics are
    constant over the entire block of samples and by
    sampling r(t) with period Tw , the following
    digital transmission model is obtained
  • h(n) f (nTw) represents the equivalent
    discrete channel impulse response of the UWB-IR
    system.
  • e(n) h(nTw) represents a discrete time noise
    process.

8
Block Representation
  • The discrete signal xl(n) is divided in blocks of
    M samples
  • Low Data Rate scenario
  • MNM Nw , we need exactly NM blocks to transmit
    a single bit
  • High Data Rate scenario
  • M NbNw a group of Nb bits is transmitted over a
    block of M samples
  • Each block is extended by means of a cyclic
    prefix of length K.
  • If K Lc (Delay Spread), the channel does not
    cause any interference between adjacent blocks.

9
Frequency Domain Detection (I)
  • Any circulant matrix can be diagonalized by using
    a DFT.
  • We can express the channel matrix as
  • WM is an MM Fourier transform matrix and ?H is a
    MM diagonal matrix whose entries represent the
    channel frequency response.
  • The received vector after cyclic prefix removal
    can be expressed as a function of the bits of all
    active users, the TH sequences, and the channel
    frequency responses.

10
Frequency Domain Detection (II)
  • The decision variables can be expressed as
  • Low Data Rate
  • High Data Rate
  • Minimum Mean Square Error (MMSE) detection has
    been considered, due its good tradeoff between
    performance and complexity.

where s2e is the noise variance and s2b
indicates the power of transmitted symbols. This
solution avoids noise amplification at the
detector when the SNR is low.
11
How to compare the systems
  • If we assume that the CP size K has been fixed,
    the minimum block size required by FDD is M K.
  • Since UWB allows for redundancy in terms of pulse
    repetition, we set the block size as small as
    possible and compensate for the loss of
    throughput by shortening the pulse repetition
    factor Nf.
  • The block size is set to M K. In order to have
    the same rate of the original system, the
    repetition factor of the FD system is set to NCPf
    Nf/2.
  • This choice does not impose any relationship
    between M and the number of samples NCPw
    2Nc NCPf that are associated with a single bit.

12
Complexity considerations
  • Choosing K Lc - 1 (classical FD receiver) gives
    optimum performance at a cost of a high
    complexity, i.e., too long CP.
  • Also the rake has to face an analogous
    inconvenient with suboptimum implementation
    (partial RAKE, selective Rake, ..)
  • We proposed also a reduced complexity FD
    receiver, in which only a subset of the total
    channel paths is taken into account in this
    system K (and hence M) is reduced.
  • This solution is the FD counterpart of the
    partial RAKE and permits a smaller length of the
    CP and, therefore, a smaller size FFT. The
    drawback of this solution is the introduction of
    an increased ISI term (due to the last replicas
    of the channel which are no more contained into a
    single block of samples).
  • Nonetheless the MMSE detector can be redisegned
    by considering the increased ISI this solution
    is defined partial-FD (P-FD) receiver.

13
Channel Model
  • The channel model has been simulated relying on
    the model proposed by Cassioli et al in
  • Dajana Cassioli, Moe Z.Win, and Andreas
    F.Molisch,The ultra-wide bandwidth indoor
    channel From statistical model to simulations,
    IEEE J. Select. Areas Commun., vol. 20, no. 6,
    pp. 12471257, Aug. 2002.
  • A slow fading scenario has been assumed, so that
    the channel coefficients could be approximated as
    constant over a single block of samples.
  • Only the small scale fading statistics have been
    considered, assuming no shadowing and a reference
    pathloss of 0 dB.
  • A constant power delay profile has been assumed,
    setting the power ratio between the line-of-sight
    replica and the reflected ones as 0.4 and
    choosing a decaying constant corresponding to a
    rms delay spread of about 50 ns, a typical value
    for indoor environments.

14
Working Conditions
  • The considered an UWB-IR scenario consists of an
    Access Point (AP) transmitting to a variable
    number of Mobile Terminals (MTs).
  • All the communications from the AP have been
    assumed synchronous.
  • The information bits are modulated (2-PPM) with
    Tw 2 ns pulse duration
  • High Data-Rate (HR) system
  • Nf 4, Nc 4 15.6 Mbit/s.
  • Medium Data-Rate (HR) system
  • Nf 16, Nc 4 3.9 Mbit/s.
  • Low Data-Rate (LR) system
  • Nf 128, Nc 32 243.6 kbit/s.
  • The digital channel model has LRAKE 100
    sample-spaced resolvable replicas.

15
LR Single User
Both receivers achieve good performance unless a
very low number of fingers or a very short CP is
considered.
Nf 64, Nc 16, single data flow
16
LR 100 load
It is important to consider an enough long CP in
order to prevent from ISI detrimental effects.
In particular, for high values of Eb/N0 the
MMSE is not able to suppress the effects of the
ISI caused by short CP and its performance tend
to converge to the values of the RAKE receiver
error floor.
Nf 64, Nc 16, 16 data flows
17
MR 50 load
Nf 16, Nc 4, Eb/N0 15 dB
Nf 16, Nc 4, 2 data flows
The same trend can be seen for a half loaded high
rate system. Note that the PFDMMSE Equalizer has
interesting results the definition of the new
equalization law permits to avoid the degradation
caused by the ISI.
18
HR Single User
Nf 4, Nc 4, single data flow
If an enough long CP is used the FD MMSE
equalizer greatly overcomes the system based on
the pulse repetition and the use of the RAKE
receiver.
19
Conclusions
  • We compared UWB-IR systems based on the pulse
    repetition according to the spreading Factor
    value and the use of the RAKE receiver and on the
    Cyclic Prefix insertion and Frequency Domain MMSE
    Equalization.
  • Both systems admit sub-optimal implementation.
  • Both systems have been considered in different
    scenarios characterized by services with
    different rate and system with variable load.
  • The simulation results show that the system which
    is based on the Cyclic Prefix insertion and the
    adoption of the Frequency Domain MMSE is more
    suitable for high data rate highly loaded systems.
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