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Applications: various Area Networks

- According to the applications, we define three “Area Networks”:
- Personal Area Network (PAN), for communications within a few meters. This is the typical Bluetooth or Zigbee application between between personal devices such as your cell phone, desktop, earpiece and so on;
- Local Area Network (LAN), for communications up 300 meters. Access points at the airport, coffee shops, wireless networking at home. Typical standard is IEEE802.11 (WiFi) or HyperLan in Europe. It is implemented by access points, but it does not support mobility;
- Wide Area Network (WAN), for cellular communications, implemented by towers. Mobility is fully supported, so you can move from one cell to the next without interruption. Currently it is implemented by Spread Spectrum Technology via CDMA, CDMA-2000, TD-SCDMA, EDGE and so on. The current technology, 3G, supports voice and data on separate networks. For current developments, 4G technology will be supporting both data and voice on the same network and the standard IEEE802.16 (WiMax) and Long Term Evolution (LTE) are the candidates

More Applications

- 1. WLAN (Wireless Local Area Network) standards and WiFi. In particular:
- IEEE 802.11a in Europe and North America
- HiperLAN /2 (High Performance LAN type 2) in Europe and North America
- MMAC (Mobile Multimedia Access Communication) in Japan
- 2. WMAN (Wireless Metropolitan Network) and WiMax
- IEEE 802.16
- 3. Digital Broadcasting
- Digital Audio and Video Broadcasting (DAB, DVB) in Europe
- 4. Ultra Wide Band (UWB) Modulation
- a very large bandwidth for a very short time.
- 5. Proposed for IEEE 802.20 (to come) for high mobility communications (cars, trains …)

IEEE 802.16 Standard

- IEEE 802.16 2004( http://www.ieee802.org/16/ ):
- Part 16: Air Interface for Fixed Broadband Wireless Access Systems
- From the Abstract:
- It specifies air interface for fixed Broadband Wireless Access (BWA) systems supporting multimedia services;
- MAC supports point to multipoint with optional mesh topology;
- multiple physical layer (PHY) each suited to a particular operational environment:

IEEE 802.16-2004 Standard

Table 1 (Section 1.3.4) Air Interface Nomenclature:

- WirelessMAN-SC, Single Carrier (SC), Line of Sight (LOS), 10-66GHz, TDD/FDD
- WirelessMAN-SCa, SC, 2-11GHz licensed bands,TDD/FDD
- WirelessMAN OFDM, 2-11GHZ licensed bands,TDD/FDD
- WirelessMAN-OFDMA, 2-11GHz licensed bands,TDD/FDD
- WirelessHUMAN 2-11GHz, unlicensed,TDD

MAN: Metropolitan Area Network

HUMAN: High Speed Unlicensed MAN

IEEE 802.16e 2005:

Part 16: Air Interface for Fixed and Mobile Broadband Wireless Access Systems

Amendment 2: Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands

and

Corrigendum 1

- Scope (Section 1.1):
- it enhances IEEE 802.16-2004 to support mobility at vehicular speed, for combined fixed and mobile Broadband Wireless Access;
- higher level handover between base stations;
- licensed bands below 6GHz.

IEEE 802.16-2004: Reference Model (Section 1.4), Figure 1

External Data

SAP=Service Access Point

By Layers:

CS-SAP

Service Specific Convergence Sublayer (CS)

Section 5

MAC-SAP

MAC

MAC Common Part Convergence Sublayer (CS)

Section 6

Security Sublayer

Section 7

PHY-SAP

Physical Layer

PHY

Section 8

IEEE802.16 Structure

data

Error CorrectionCoding

M-QAM mod

OFDM mod

TX

randomization

data

Error CorrectionDecoding

M-QAM dem

OFDM dem

RX

De-rand.

Choices:

OFDM and OFDMA (Orthogonal Frequency Division Multiple Access)

- Mobile WiMax is based on OFDMA;
- OFDMA allows for subchannellization of data in both uplink and downlink;
- Subchannels are just subsets of the OFDM carriers: they can use contiguous or randomly allocated frequencies;
- FUSC: Full Use of Subcarriers. Each subchannel has up to 48 subcarriers evenly distributed through the entire band;
- PUSC: Partial Use of Subcarriers. Each subchannel has subcarriers randomly allocated within clusters (14 subcarriers per cluster) .

An OFDM Symbol is made of

- Data Carriers: data
- Pilot Carriers: synchronization and estimation
- Null Carriers: guard frequency bands and DC (at the modulating carrier)

pilots

data

frequency

Guard band

channel

Guard band

IEEE 802.16, with N=256

Data (192)

0

0

Pilots (8)

12

13

Nulls (56)

24

38

24

63

24

88

12

100

101

IFFT

155

156

12

168

24

193

24

218

24

243

12

255

255

IEEE802.16 Implementation

- In addition to OFDM Modulator/Demodulator and Coding we need
- Time Synchronization: to detect when the packet begins
- Channel Estimation: needed in OFDM demodulator
- Channel Tracking: to track the time varying channel (for mobile only)

- In addition we need
- Frequency Offset Estimation: to compensate for phase errors and noise in the oscillators
- Offset tracking: to track synchronization errors

Basic Structure of the Receiver

Time Synchronization: detect the beginning of the packet and OFDM symbol

Channel Estimation: estimate the frequency response of the channel

Received Signal

Demodulated Data

WiMax Demodulator

Time Synchronization

In IEEE802.16 (256 carriers, 64 CP) Time and Frequency Synchronization are performed by the Preamble.

Long Preamble: composed of 2 OFDM Symbols

Short Preamble:only the Second OFDM Symbol

First OFDM Symbol

Second OFDM Symbol

320 samples

320 samples

2 repetitions of a long pulse + CP

4 repetitions of a short pulse+CP

64

64

64

64

64

128

128

64

The standard specifies the Down Link preamble as QPSK for subcarriers between -100 and +100:

Using the periodicity of the FFT:

Short Preamble, to obtain the 4 repetitions, choose only subcarriers multiple of 4:

64

64

64

64

Add Cyclic Prefix:

64

64

64

64

64

Long Preamble: to obtain the 2 repetitions, choose only subcarriers multiple of 2 :

128

128

Add Cyclic Prefix:

64

128

128

CP

Several combinations for Up Link, Down Link and Multiple Antennas.

We can generate a number of preambles as follows:

With 2 Transmitting Antennas:

With 4 Transmitting Antennas:

Time Synchronization from Long Preamble

1. Coarse Time Synchronization using Signal Autocorrelation

Received signal:

preamble

OFDM Symbols

64

128

128

Compute Crosscorrelation Coefficient:

xcorr

Effect of Periodicity on Autocorrelation (no Multi Path). Let L=64.

Max starts at ….

Same signal

data

64

128

128

data

64

128

128

MAX when

Effect of Periodicity on Autocorrelation (no Multi Path):

… and ends at

Same signal

data

64

128

128

data

64

128

128

MAX when

Effect of Periodicity on Autocorrelation (with Multi Path of max length ):

Max starts at ….

Same signal

64

data

128

128

64

data

128

128

MAX when

Effect of Periodicity on Autocorrelation (with Multi Path of max length ):

and ends at

Same signal

64

data

128

128

64

data

128

128

MAX when

With Noise:

Then, at the maximum:

Information from Crosscorrelation coefficient:

Estimate of SNR

Estimate of Beginning of Data

Estimate of Channel Length

2. Fine Time Synchronization using Cross Correlation with Preamble

xcorr

Since the preamble is random (almost like white noise), it has a short autocorrelation:

64

128

128

128

… with dispersive channel

xcorr

Since the preamble is random, almost white, recall that the crosscorrelation yields the impulse response of the channel

64

128

128

128

However this expression is non causal.

It can be written as (change index ):

Which van be computed as the output of an FIR Filter with impulse response:

Taking the time delay into account we obtain:

Since the preamble is random, almost white, recall that the crosscorrelation yields the impulse response of the channel

64

128

128

128

Compare the two (non dispersive channel):

Autocorrelation of received data

Crosscorrelation with preamble

Synchronization with Dispersive Channel

Autocorrelation of received data

Crosscorrelation with preamble

Channel impulse response

Start of Data

Synchronization with Dispersive Channel

Let be the length of the channel impulse response

Channel impulse response

In order to determine the starting point, compute the energy on a sliding window and choose the point of maximum energy

xcorr

Maximum energy

L=max length of channel = length of CP

Channel Estimation

Recall that, at the receiver, we need the frequency response of the channel:

OFDM TX

OFDM RX

m-th data block

Transmitted:

Received:

channel freq. response

From the Preamble: at the beginning of the received packet. The transmitted signal in the preamble is known at the receiver: after time synchronization, we take the FFT of the received preamble

Estimated initial time

Received Preamble:

64

128

128

256 samples

FFT

Solve for using a Wiener Filter (due to noise):

noise covariance

Problem: when we cannot compute the corresponding

frequency response

if

Fact: by definition,

otherwise (ie DC, odd values, frequency guards)

Two solutions:

1. Compute the channel estimate

only for the frequencies k such that

and interpolate for the other frequencies. This might not yield good results and the channel estimate might be unreliable;

known

interpolate

2.

Recall the FFT and use the fact that we know the maximum length L of the channel impulse response

Since the preamble is such that either or

for the indices where we can write:

for

so that we have 100 equations and L=64 unknowns.

Channel Frequency Response Estimation:

1. Generate matrix

kF=[2,4,6,…,100, 156, …, 254]’; non-null frequencies (data and pilots)

n=[0,…,63]; time index for channel impulse response

V=exp(-j*(2*pi/256)*kF*n);

M=inv(V’*V+0.001*eye(64))*V’;

2. Generate vector z from received data y[n]:

Let n0 be the estimated beginning of the data, from time synchronization.

Then

y0=y(n0-256:n0-1); received preamble

Y0=fft(y0); decoded preamble

z=Y0(kF+1).*conj(Xp256(kF+1))/2; multiply by transmitted preamble

h=M*z; channel impulse response

3. Channel Frequency Response: H=fft(h, 256);

Example:

Spectrum of Received Signal

As expected, it does not match in the Frequency Guards

NOT TO SCALE

Estimated Frequency Response of Channel

Start after processing preamble

WiMax-2004 Demodulator

WiMax256.mdl

Standard OFDM Demod (256 carriers)

data

Error Correction Decoding

Ch.

Channel Tracking

In mobile applications, the channel changes and we need to track it.

IEEE802.16-2005 tracks the channel by embedding pilots within the data.

In the FUSC (Full Use of Sub Carriers) scheme, the pilots subcarriers are chosen within the non-null subcarriers as

with

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