Jan De Nayerlaan, 5
B-2860 Sint-Katelijne-Waver
Belgium
www.denayer.be
Spread Spectrum (SS)
applications
ir. J. Meel
[email protected]
Studiedag Spread Spectrum
6 okt. ’
99
In the period of nov. 1997 - nov. 1999 a ‘ Spread Spectrum’ project was worked out at the
polytechnic ‘ DE NAYER instituut’ The goal of this project was the hardware/software
.
implementation of a Direct Sequence Spread Spectrum (CDMA) demonstrator in the 2.4 GHz
ISM band. A measurement environment (Vector Signal Analyzer, IQ-modulator, Bit Error Rate
Tester) was build out, resulting in a set of experiments based on this demonstrator. The project
results where communicated with SMO’ (Small and Medium Organisations) interested in Spread
s
Spectrum. These notes were used to introduce the SMO’ in the subject of Spread
s
Spectrum.This Spread Spectrum project was sponsered by:
Vlaams Instituut voor de bevordering van het Wetenschappelijk Technologisch onderzoek
in de industrie – (Flemisch Gouvernment)
Sirius Communications – Rotselaar - Belgium
V2 dec 99
CONTENTS
1. SPREAD SPECTRUM APPLICATIONS ..............................................................................3
1.1 WLAN IEEE 802.11 ...........................................................................................................3
1.1.1 Network Topology ......................................................................................................3
1.1.2 Physical Layer (Radio Technology) ...........................................................................4
1.2 GPS (GLOBAL POSITIONING SYSTEM) .................................................................................7
1.3 IS-95.................................................................................................................................13
1.3.1 Network Architecture ................................................................................................13
1.3.2 Forward Link Radio Transmission............................................................................14
1.4 W-CDMA..........................................................................................................................17
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 2
1. Spread Spectrum Applications
1.1 WLAN IEEE 802.11
IEEE 802.11 is the first internationally recognized standard for Wireless Local Area Networks
(WLAN), introducing the technology of mobile computing.
1.1.1 Network Topology
Ad-hoc Network
An Ad-hoc network or Independent Basic Service Set (IBSS) is a simple network where
communications are established between two or more wireless nodes or Stations ( STAs) in a
given coverage area without the use of an Access Point (AP) or server. The STAs recognize
each other and communicate directly with each other on a peer-to-peer level.
STA3 IBSS
STA1 STA2
Infrastructure Network
An Infrastructure network (or client/server network) is a more flexible configuration in which each
Basic Service Set (BSS) contains an Access Point (AP). The AP forms a bridge between the
wireless and wired LAN. The STAs do not communicate on a peer-to-peer basis. Instead, all
communications between STAs or between an STA and a wired network client go through the
AP. APs are not mobile and form part of the wired network infrastructure.
The Extended Service Set (ESS) consists of a series of BSSs (each containing an AP)
connected together by means of a Distribution System (DS). Although the DS could be any type
of network (including a wireless network), it is almost invariably an Ethernet LAN. Within an ESS,
STAs can roam from one BSS to another and communicate with any mobile or fixed client in a
manner which is completely transparant in the protocol stack above the MAC sublayer. The ESS
enables coverage to extend well beyond the range of the WLAN radio.
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 3
1.1.2 Physical Layer (Radio Technology)
Spreading and Modulation
IEEE 802.11 defines three variations of the Physical Layer: Infrared (IR) and two RF
transmissions in the unlicensed 2.4 GHz ISM-band, requiring spread spectrum modulation:
DSSS (Direct Sequence Spread Spectrum) and FHSS (Frequency Hopping Spread Spectrum).
Only the RF transmission has significant presence in the market.
DSSS FHSS
Spreading Spreading
1Mbps 11 Mcps 11 Msps 1 Mbps 1 Msps 1 Msps
dt 2-GFSK FH
dt DBPSK
Modulator Modulator
pn t
f hi
pnt
11 chip PN
FW
Barker code
RF (2.4 GHz)
RF (2.4 GHz)
Spreading
Spreading
I
2Mbps S 11 Msps 2 Mbps 1 Msps 1 Msps
4-GFSK FH
dt / pn t 11Mcps DQPSK dt
Modulator Modulator
P
Q f hi
pnt f RF pnt
PN
FW
code
11 chip
Barker RF (2.4 GHz)
RF (2.4 GHz)
DSSS
The DSSS physical layer uses an 11-bit Barker sequence to spread the data before it is
transmitted. This sequence gives a processing gain of 10.4 dB, meeting the minimum
requirements of FCC 15.247 and ETS 300 328.
The 11 Mcps baseband stream is modulated onto a carrier frequency (2.4 GHz ISM band, with
11 possible channels spaced with 5 MHz) using:
• DBPSK (Differential Binary Phase Shift Keying): data rate = 1 Mbps
• DQPSK (Differential Quaternary Phase Shift Keying): data rate = 2 Mbps
FHSS
In the FHSS physical layer the information is first modulated using:
• 2-GFSK (2-level Gaussian Frequency Shift Keying): data rate = 1 Mbps
• 4-GFSK (4-level Gaussian Frequency Shift Keying): data rate = 2 Mbps
Both modulations result in a symbol rate of 1 Msps.
The carrier frequency (2.4 GHz ISM band, with 79 possible channels spaced with 1 MHz) hops
from channel to channel in a prearranged pseudo-random manner (hop pattern). There are 78
different hop patterns (subdivided in 3 sets of 26 patterns). The FCC and ETS regulations
require a minimum hop rate of 2.5 hops/s or a channel dwell time of less than 400 ms.
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 4
Spectrum
The spectrum of the transmitted signals determines the network packing.
DSSS FHSS
11 Msps 1 Msps
2GFSK = +/- 100 kHz
small
frequency 4GFSK = +/- 75 kHz
deviation +/- 225 kHz
channel
f f
22 MHz @ - 35 dB 1 MHz @ - 20 dB
hop (> 6 MHz)
< 400 ms dwell time
> 2.5 hops/s
> 25 MHz
78 hop patterns
(3 sets of 26 patterns)
f f
2.4 GHz 2.4835 MHz 2.4 GHz 2.4835 MHz
11 channels - 5 MHz step 79 channels - 1 MHz step
DSSS
With a symbol rate of 11 Mbps the channel bandwidth of the main lobe is 22 MHz. There are 11
channels identified for DSSS systems, but there is a lot of overlap (only 5 MHz spacing). All
IEEE 802.11 DSSS compliant products utilize the same PN code. Since there is not a set of
codes available the DSSS network cannot employ CDMA. When multiple APs are located in
close proximity, it is recommended to use frequency seperations of at least 25 MHz. Therefore
the 2.4 GHz ISM band will accommodate 3 non-overlapping channels. Only 3 networks can
operate collocated.
FHSS
When the hop patterns are selected well, several APs can be located in close proximity with a
fairly low probability of collision on a given channel.
Up to 13 FHSS networks can be collocated before the interference is to high. This is based on
the probability of collisions where two of the nets choose the same one of 79 channels at the
same time. When the probability of collisions gets to high, network throughput suffers.
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 5
Comparison of DSSS and FHSS
DSSS FHSS
Spectral Density + reduced with processing gain + reduced with processing gain
Interference + continuous spread of the Tx power - only the average Tx power is spread,
Generation gives minimum interference this gives less interference reduction
Transmission + continuous, broadband - discontinuous, narrowband
Interference + narrowband interference in the - narrowband interference in the same
Susceptibility same channel is reduced by the PG channel is not reduced
+ narrowband interference in a different
channel has no influence
Multipath + rejection if the bandwith is wider - some of the narrowband channels are
than the coherence delay of the unusable
environment (outdoor applications) + hopping makes transmission on
- for a chiprate of 11 Mcps the chip usable channels possible
period is 91 ns, corresponding with a
wave distance of about 30 m (large
for indoor applications)
Modulation + BPSK and QPSK are very power - GFSK is less power efficient in
efficient narrowband operation
Higher Data Rates + the data rate can be increased by - a wider bandwidth is needed but not
increasing the clockrate and/or the available (it would cut the number of
modulation complexity (muli-level) channels to hop in)
Multiple Signals - only 3 collocated networks + up to 13 collocated networks
+ higher aggregate throughput - lower aggregate throughput
Synchronisation + self-synchronizing - many channels to search
Real Time (voice) + no timing constraints - if a channel is jammed, the next
- if a station is jammed, it is jammed available transmission time on a clear
until the jammer goes away channel may be 400 ms away
Implementation - complex baseband processing + simple analog limiter/discriminator
receiver
Power Consumption - more power consumption due to - more simple circuit
higher speed and more compex
processing
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 6
1.2 GPS (Global Positioning System)
GPS is a satellite navigation system, funded by and controlled by the U.S. Department of
Defense (DOD).
The GPS system consists of three building blocks: the Space Segment (SS), the User Segment
and the Control Segment (CS).
SS
(space segment)
CS US
(control segment) (user segment)
Space Segment (SS)
The Space Segment of the GPS system consists of the GPS satellites. These Space Vehicles
(SVs) send radio signals to the User Segment and the Control Segment.
The nominal GPS operational constellation consists of 24 satellites that orbit the earth in 12
hours. The satellite orbits have an altitude of 20.200km and an inclination of 55 degrees with
respect to the equatorial plane. There are six orbital planes (with nominally four SVs in each),
equally spaced (60 degrees apart). The satellite orbits repeat almost the same ground track once
each day (4 minutes earlier each day).
altitude
orbit
20.200 km
GPS
SS
GPS
US
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 7
Control Segment
Monitor stations measure signals from the SVs which are incorporated into orbital models for
each satellite. The models compute precise orbital data (ephimeris) and SV clock corrections for
each satellite. The Master Control station uploads ephimeris and clock data to the SVs. The
SVs then send subsets of the orbital ephimeris data to GPS receivers (User Segment).
User Segment
The GPS User Segment receivers convert SV signals into position, velocity and time estimates.
Four satellites are required to compute the four dimensions of X,Y,Z (position) and time.
Authorized users with cryptographic equipment and keys and specially equipped receivers use
the Precise Positioning System (PPS).
PPS Predictable Accuracy (95%):
• 22 meter horizontal accuracy
• 27.7 meter vertical accuracy
• 100 nanosecond time accuracy
Civil users worldwide use the Standard Positioning System (SPS) without charge or restrictions.
Most receivers are capable of receiving and using the SPS signal. The SPS accuracy is
intentionally degraded by the DOD by the use of Selective Availability.
SPS Predictable Accuracy (95%):
• 100 meter horizontal accuracy
• 156 meter vertical accuracy
• 340 nanoseconds time accuracy
GPS Satellite Signals
The SVs transmit two microwave carrier signals. The L1 frequency (1575.42 MHz) carries the
navigation message and the SPS code signals. The L2 frequency (1227.60 MHz) is used to
rneasure the ionospheric delay by PPS equipped receivers.
Three binary codes shift the L1 and/or L2 carrier phase.
• The C/A Code (Coarse Acquisition) modulates the L1 carrier phase. The C/A code is a
repeating 1.023 Mchip/s Pseudo Random Noise (PRN) Code. This noise-like code
modulates the L1 carrier signal, "spreading" the spectrum over a 1 MHz bandwidth. The C/A
code repeats every 1023 chips (one millisecond). This chip length Nc of 1023 chips results in
a processing gain of 30 dB. That’ why GPS receivers don’ need big satellite dishes to
s t
receive the GPS signal. There is a different C/A code PRN for each SV. GPS satellites are
identifed by their PRN number, the unique identifier for each pseudo-random-noise code.
This code-division-multiplexing technique allows the identification of the SVs even though
they all transmit at the same L1-band frequency. A low cross-correlation gives a minimum of
interference between the SV signals at the receiver side. The C/A code that modulates the
L1 carrier is the basis for the civil SPS.
• The P-Code (Precise) modulates both the L1 and L2 carrier phases. The P-Code is a very
long (seven days period = 6.19.1012 chips) 10.23 Mchip/s PRN code. In the Anti-Spoofing
(AS) mode of operation, the P-Code is encrypted into the Y-Code. The encrypted Y-Code
requires a classified AS Module for each receiver channel and is for use only by authorized
users with cryptographic keys. The P (Y)-Code is the basis for the PPS.
• The Navigation Message (NAV data) also modulates the L1-C/A code signal. The
Navigation Message is a 50 bps signal consisting of data bits that describe the GPS satellite
orbits, clock corrections, and other system parameters (1500 bits = 30 sec).
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 8
L1 carrier - 1575.42 MHz
x 154
satellite 90°
PRN ID
1.023 Mchip/s
÷ 10 C/A code
50 bps
÷ 20 NAV data L1 signal
10.23 MHz
10.23 Mchip/s
P(Y) code
L2 carrier - 1227.6 MHz
x 120 L2 signal
The Long code (P or Y code) is identical for each satellite.
The Short code or C/A code is a Gold code with the generator shown below.
SSRG [10,3] G1-code
1 2 3 4 5 6 7 8 9 10
C/A code
SSRG [10,9,8,6,3,2]
1 2 3 4 5 6 7 8 9 10
phase taps
G2i-code
(PRN 31)
The C/A code generator produces a different 1023 chip sequence for each phase tap setting.
The C/A codes are defined for 32 satellite identification numbers (PRN ID).
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 9
SV G2 phase First 10 chips
PRN ID Taps
1 2&6 1100100000
2 3 &7 1110010000
3 4&8 1111001000
4 5&9 1111100100
5 1&9 1001011011
6 2 &10 1100101101
7 1&8 1001011001
8 2&9 1100101100
9 3 &10 1110010110
10 2&3 1101000100
11 3&4 1110100010
12 5&6 1111101000
13 6&7 1111110100
14 7&8 1111111010
15 8&9 1111111101
16 9 &10 1111111110
17 1&4 1001101110
18 2&5 1100110111
19 3&6 1110011011
20 4&7 1111001101
21 5&8 1111100110
22 6&9 1111110011
23 1&3 1000110011
24 4&6 1111000110
25 5&7 1111100011
26 6&8 1111110001
27 7&9 1111111000
28 8 & 10 1111111100
29 1&6 1001010111
30 2&7 1100101011
31 3&8 1110010101
32 4&9 1111001010
Measuring the distance d between the SV and the RX is based on measuring the travel time t d of
the radio signal (L1/L2) send by the SV and the propagation speed c of the signal:
d = c.t d
The travel time td is measured by synchronizing the C/A code (or P(Y) code) of the receiver to
the C/A code in the signal received from the SV. The start time of this synchronized C/A code in
the receiver gives the Time Of Arrival (TOA) of the C/A code of the SV at the receiver. The start
time t1 of the C/A code in the SV is known (time information is included in the Navigation
Message). The travel time t d can be calculated from t 1 and TOA.
Because c = 3.108 m/s, the time must be measured very accurate:
d = 20.200 km → td = 67.333 µs
d = 300 m → td = 1 µs = chip period of C/A code
d = 30 m → td = 100 ns = chip period of P(Y) code
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 10
On the Space Vehicle (SV), timing is almost perfect because they have precise atomic clocks on
board. A low-cost GPS receiver cannot have an atomic-accuracy clock. The receiver clock time
tRX shows an offset toff from the SV’ GPS time tGPS:
s
t RX = t GPS − t off
Due to this inaccuracy the TOA is called the pseudo-range.
tGPS
GPS SV
(space vehicle)
L1/L2 signal
tGPS = tRX + toff
distance d toff
tRX
GPS RX
(receiver)
SV code received
SV code send
td = t2 - t1 RX code synchronized
tGPS t1 t2
tRX t1 - t off t2 - toff = TOA → pseudo-range
If the receiver clock is perfect, than all the satellite (SV) ranges would intersect at a single point
(which is the position of the receiver). Three perfect measurements can locate a point in 3-
dimensional space.
With imperfect receiver clocks, a fourth measurement (done as a cross-check), will not intersect
with the first three. Since any offset from GPS time will effect the four measurements in an equal
way, the receiver must look to a single correction factor (timeoffset to) that it can substract from
all its timing measurements that would cause them all to intersect at a single point.
Making four satellite measurements gives accurate position and time information.
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 11
SV2
SV3
SV1 pr3 SV4
r3
pseudo-range
pr2 to
to
to to
pr1 r2
pr4
r1 RX r4
Position (X,Y,Z)
and
Time (t)
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 12
1.3 IS-95
IS-95 CDMA is a digital cellular radio system for mobile voice communication as well as many
new services like mobile fax and data transmission.
In the US, the initial standards were the Telecommunications Industry Association/ Electronic
Industry Association (TIA/EIA) Interim Standard 95 (IS-95) and related versions for base station
and mobile performance (IS-97 and IS-98, respectively).
The IS-95 system operates in the same frequency band as the analog cellular system AMPS
(Advanced Mobile Phone System).
1.3.1 Network Architecture
Mobile Station (MS)
The Mobile Station (MS) is the subscriber’ interface with the CDMA network. Both hand-held
s
MS units having a low-power radio transmitter and vehicle-mounted MS units are permitted. The
manufacturer assignes a unique 32-bit Electronic Serial Number (ESN) to each MS. It is a
permanent and private identification code of the mobile terminal.
Base Station Subsystem (BSS)
Each Base Station has a unique pilot PN-offset, a delay applied to a random number sequence
(PN Short Code) at the base station. This sequence is applied to forward direction transmissions
that enables the terminals in a cell to decode the desired signal and reject the signals from other
base stations. Pilot PN offsets ensure that the received signal from one cell does not correlate
with the signal from a nearby cell.
It is possible for adjacent cells to use the same CDMA radio channel frequency (f 1). Reusing the
same frequency in every cell eliminates the need for frequency planning in a CDMA system. Pilot
PN-offset planning must be done in stead.
In an area where the ranges of two cells overlap, there is an increased interference, but this only
reduces the number of users that can share the radio channel.
Base Transceiver Station (BTS)
The BTS comprises several base radio transceivers. Each transceiver consists of a transmitter
and a receiver which has a duplicated front end to match up with the two receiving antennas used
in the base antenna assembly.
Base Station Controller (BSC)
The BSC comprises control logic, data communication facilities and multiplexing and de-
multiplexing equipment. The BSC can control the radio power levels of the various transceivers in
the BTS, and also can autonomously control the mobile stations’radio transmitter power levels.
A single BSC can control several BTS radio equipment transmitters.
GPS Receiver
CDMA ‘ soft handover’(an MS establishes contact with a new base station before giving up its
radio link to the original base station) requires base stations to operate in synchronism with one
another. Therefore each base station contains a GPS receiver.
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 13
Mobile Switching Centre (MSC)
The MSC is a switching network that interconnects calls between Mobile Stations and between
Mobile Stations and the Public Switched Telephone Network (PSTN). The MSC is also needed
for ‘
automatic roaming’capabilities.
1.3.2 Forward Link Radio Transmission
The forward link is by convention the transmission from Base Station to Mobile Station (MS).
reverse link
forward link
MS
BTS
PN-offset ESN
Traffic Channel
The traffic channel can accept data rates of 9600 bps, 4800 bps, 2400 bps and 1200 bps
comming from a variable-bit-rate speech coder (QCELP = Qualcomm Code Excited Linear
Prediction). Check digits and tail bits (convolutional encoder tail sequence to drive the
convolutional encoder into a known state at the end of each frame) are included. The signals are
processed in frames of duration of 20 ms.
A convolutional code, with constraint length K=9 and rate ½ protects each signal.
When the rate is less than 19200 bps, the transmitter repeats code bits (factor 1, 2, 4 or 8) to
bring the rate up to 19200 bps, corresponding to 384 bits in a frame of 20 ms.
An interleaver permutes the code bits in each frame. This will spread the influence of burst
errors, typical for wireless communications.
The baseband sequence is scrambled by the PN sequence derived from a Long Code
Generator (a PN sequence with length 242 – 1 at a rate of 1.2288 Mbps) and Long Code Mask (a
time-offset determined by the ESN of the MS for traffic channels). The Long Code period is:
2 42 − 1
= 3.5 x 10 6 s = 41.4 days
1.2288 Mcps
To match the rate of the Long Code sequence to the 19200 bps baseband rate, a decimator
extracts 1 bit out of 64 bits of the Long Code sequence.
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 14
Pilot 0 kbps 0 Mcps
W0
all ‘ s
0’
WALSH 0
Sync Convolutional Symbol 19.2 kbps Interleaver 19.2 kbps 1.2288 Mcps
Encoder Repetition 384 bits Ws
1.2 kbps R=1/2, K=9 4 (20 ms)
WALSH 32
Paging (p) Convolutional Symbol 19.2 kbps Interleaver 19.2 kbps 1.2288 Mcps
Encoder Repetition 384 bits Wp
9.6 kbps R=1/2, K=9 1 or 2 (20 ms)
4.8 kbps
1.2288 Mbps 19.2 kbps
WALSH p
Long Code Mask Long Code Decimate
(paging channel p) 42 bit PN 64:1
Traffic (ni ) Convolutional Symbol 19.2 kbps Interleaver 19.2 kbps 1.2288 Mcps
Encoder Repetition 384 bits Wti
9.6 kbps R=1/2, K=9 1 (20 ms)
1.2288 Mbps 19.2 kbps WALSH ni
Long Code Mask Long Code Decimate
(ESN i - MS) 42 bit PN 64:1
The baseband symbol stream is spread by multiplication with a Walsh sequence of length 64,
thus creating a baseband chip rate of 1.2288 Mcps. There are 64 orthogonal Walsh sequences
of length 64, certain of which are assigned to different users of the channel. All user’ s
transmissions occur synchronously from the base station, so these transmissions are also
synchronized at any individual subscriber’ receiver (synchronous CDMA). The use of a set of
s
orthogonal sequences thus allows perfect rejection of other-user interference associated with any
given transmission path within the cell.
The same baseband sequence is duplicated on the I and Q channels of an IQ-modulator. Then
they are spread with ‘ different’pilot sequences on the I and Q channels. This pilot sequence or
Short Code sequence has a length of 2 15 chips. A ‘ PN-offset’in the pilot sequences is assigned
to each base station and is synchronized to Universal Coordinated Time (UCT). To demodulate
a received signal, an MS synchronizes its receiver with the assigned base station and generates
I-channel and Q-channel pilot sequences with the value of ‘ PN-offset’assigned to the local base
station. Signals received from other base stations, with different values of ‘PN-offset’ appear as
,
low-level noise in the receiver of the MS, due to the correlation properties of the sequences.
There are 512 possible ‘ PN-offsets’ with offset i corresponding to a time delay of 64i chips (a
,
delay of 64 chips ≅ 52 µs ≅ 15km). Since the period of the sequence is 2 15 chips, there are 215 /
26 = 29 = 512 possible offsets.
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 15
PN-offset I-Pilot Sequence
15 bit PN
pnI 1.2288 Mcps
0 Mcps
W0
I
Ws
1.2288 Mcps IQ 1.2288 Mcps
Wp mod
AMPS IS-95
Wti Q channel signal
1.2288 Mcps f RF
pnQ 1.2288 Mcps 30 kHz 1.23 MHz
= 41 x AMPS
PN-offset Q-Pilot Sequence RF (891 MHz) f
15 bit PN
869 MHz 894 MHz
25 MHz
The bandwidth of a CDMA signal is 1.23 MHz. The bandwidth of an AMPS channel (using the
same frequency band) is 30 kHz. Therefore the bandwidth of a CDMA signal corresponds to an
aggregate bandwidth of 41 AMPS channels.
Pilot Channel
The pilot channel uses WALSH 0, a sequence of all 0s (or 1s). The channel contains no
information, only the PN pilot sequence. It provides the MS with a beacon, timing and phase
reference (for coherent detection). The I and Q channels of the traffic channels (containing the
same information) can be despread independently to determine the amplitude of the channels.
The pilot sequence can be employed for channels sounding purposes to determine the
amplitudes and phases of various multipath components received at the MS (RAKE receiver).
Sync Channel
The sync channel uses WALSH 32, a sequence of 32 0s, followed by 32 1s. It provides the MS
with critical time synchronization data: system time (obtained from GPS), the PN-offset of the
pilot sequence and the rate of the base station paging channels (4.8 kbps or 9.6 kbps).
Paging Channel
A CDMA signal carries up to 7 paging channels and using WALSH 2 to WALSH 7. The paging
channels transmit information to terminals that do not have calls in progress.
Pilot
Sync
Paging
inactive
User 3
channels
User 2
User 1
1.23 MHz f 0 1 2 3 4 5 6 7 8 9 32 40 63
WALSH
code
Pilot Paging User 1 User 2 Sync User 3
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 16
1.4 W-CDMA
The target of the third-generation (3G) mobile communication systems (cellular) is the
introduction of multimedia capabilities. ETSI (European Telecommunications Standards
Institute) has been responsible for UMTS (Universal Mobile Telecommunications System)
standardization since the early 1990s. In January 1998 (historical milestone) the basic technology
for the UMTS Terrestrial Radio Access (UTRA) system was selected:
For the paired bands 1920 –1980 MHz and 2110-2170 MHz wideband CDMA (W-CDMA) shall
be used in frequency-division duplex (FDD) operation.
The bearer capability targets have been defined as:
• 384 kbps for full area coverage (→ Internet access)
• 2 Mbps for local coverage (→ video/picture transfer)
A variety of data services from low to very high bitrates must be supported.
Downlink Dedicated Physical Channel
The spreading and modulation of the downlink dedicated physical channel is illustrated in the
figure below.
3.840 Mcps/SF
= 30 ksps 3.840 Mcps
3.840 Mcps
SRRC FDD
α=0.22
I
downlink
SF = 128 3.840 Mcps
data +
control S Base 3.840 Msps
OVSF Scramble Code
/ Station QPSK
(SF = 4,8,16,32,64,128,256) Generator
60 kbps P
ID
SF = 128 3.840 Mcps
SRRC
α=0.22 Q
3.840 Mcps/SF 3.840 Mcps 3.840 Mcps
= 30 ksps fRF
RF (1965 MHz)
5 MHz
f
1920 MHz 1980 MHz 2110 MHz 2170 MHz
60 MHz 60 MHz
UMTS UMTS
Traffic data (voice) is multiplexed with control information (pilot bits, transmit power control bits,
rate information, … ). The serial-to-parallel converter maps the 60 kbps to the I and Q branch of
the QPSK modulator. This produces a 30 ksps symbol rate. The I and Q branches are then
spread to the 3.840 Mcps chip rate with the same Orthogonal Variable Spreading Factor (OVSF)
code. Since the spreaded bandwidth is the same for all users, multiple-rate transmission needs
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 17
multiple Spreading Factors (SF). The OVSF has an SF of 128 in this case (length of the
spreading code). This results in the relation:
2 (QPSK) x datarate x SF = chip rate
The OVSF code is the channelization code. Next a scrambling code is applied, which is unique
to the Base Station within the geographic area. Baseband filtering is done with a Square Root
Raised Cosine (SRRC) with roll-off 0.22.
Different physical channels in the same cell use different channelization codes. Several downlink
physical channels can be transmitted in parallel on one connection using a ‘ grouped'
channelization code (with lower SF and thus less transmission quality) in order to achieve higher
channel bit rates.
Orthogonal Variable Spreading Factor (OVSF) Codes
The OVSF codes preserve mutual transmit orthogonality between different downlink physical
channels, even if they use different Spreading Factors and thus offer different channel bit rates.
The use of OVSF codes is thus a key factor in the high degree of service flexibility of the W-
CDMA air interface.
Let CN be a matrix of size NxN and denote the set of N binary spreading codes of N chip length,
CN(i) is the row vector of N elements and N = 2 n. The matrix CN is generated from C N/2:
C N / 2 (1)C N / 2 (1)
.
C N / 2 (1)C N / 2 (1)
.
C N (1)
C (2) C N / 2 (2)C N / 2 (2)
.
CN = N = C (2)C (2)
M
N/2 . N/ 2
M
C N (N )
C N / 2 (N / 2)C N / 2 (N / 2)
.
C (N / 2)C (N / 2)
N/ 2 . N/ 2
These variable length codes can be generated from a tree structure as shown in the figure below.
Starting from C1(1) = 1, a set of 2 n spreading codes with the length of 2 n chips are generated at
the nth layer. The generated codes from the same layer constitute a set of Walsh functions and
they are orthogonal, although the rows of C N are not in the same order of H N. Any two codes of
different layers are also orthogonal except for the case that one of the two codes is a ‘ mother’
code of the other. For example C2(2) is a mother code for C 4(3), C4(4), C8(5), C8(6), C8(7), C8(8),
… , so these codes are not orthogonal against C 2(2). A mother code is mapped on all the codes
in the sub-tree produced by that code (they all start with that code). In other words, a code can be
used in a channel if and only if no other code on the path from the specific code to the root of the
tree or the sub-tree produced by the specific code is used in the same channel.
For example if C8(1) is assigned to a user, all the codes {C 16(1), C16(2), C32(1), … , C32(4), … }
generated from this code cannot be assigned to other users requesting lower rates; in addition,
mother codes {C2(1), C4(1)} of C8(1) cannot be assigned to users requesting higher rates. The
OVSF code C8(1) has a Spreading Factor (SF) of 8. With a given (fixed) symbol rate of 3.840
Msps, this results in a data rate:
2bits/symbol(QPSK) x 3.840 Msps / (SF = 8 ) = 960 kbps
C8(1) is utilizing 12.5% of the available code space (channel capacity).
The OVSF code C4(2) gives a datarate of 2.048 Mbps and uses 25% of the channel capacity.
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 18
These restrictions are imposed in order to maintain orthogonality.
SF = 4 SF = 8 SF = 16
limited set of bitrates: 1.920 Mbps 960 kbps 480 kbps
(flexibility)
C8(1) = 1 1 1 1 1 1 1 1
C4(1) = 1 1 1 1
C8(2) = 1 1 1 1 0 0 0 0
C2(1) = 1 1
C8(3) = 1 1 0 0 1 1 0 0
C4(2) = 1 1 0 0
C8(4) = 1 1 0 0 0 0 1 1
C1(1) = 1
C8(5) = 1 0 1 0 1 0 1 0
C4(3) = 1 0 1 0
C8(6) = 1 0 1 0 0 1 0 1
C2(2) = 1 0
C8(7) = 1 0 0 1 1 0 0 1
C4(4) = 1 0 0 1
code-tree C8(8) = 1 0 0 1 0 1 1 0
SF = 64 SF = 8 SF = 32 SF = 4 SF = 16
120 kbps 960 kbps 240 kbps 1.920 Mbps 480 kbps
code
0 255
code-tree
DE NAYER (ir. J. Meel) IWT HOBU-fonds Spread Spectrum 19
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