TD-SCDMA Principle
Part B: Basic Concept of TD-SCDMA
Dr. Shihe Li
e-mail: [email protected]
Datang MobileCo
Contents
Part A: Basic concept of TD-SCDMA
• Review
• Smart Antenna
• Synchronous CDMA
• Software Radio and SDR
• Baton Handover
• Advantages
Basic CDMA concept
• Spreading code:
– Walsh code, it is an orthogonal code when synchronized to
distinguish user in a cell
– Long code (PN code), it is used to distinguish user in different cells
and to flat the spectrum is a carrier (as white noise)
• Frequency re-use factor = 1, with higher capacity
• CDMA is a self-interference system:
– Multiplex interference: between code channels, because
of Multi-path and Doppler
– Interference from other system and users in adjacent cell
• Near-far effect
• Capacity of a CDMA system is limited by total interference
level
Basic CDMA IS-95 concept
• IS-95, cdma2000, and WCDMA uses the same concept
• FDD CDMA system
– Based on the basic concept of CDMA
– CDMA + Rack receiver
– Fast power control (Strict up-link power control: to overcome nearfar effect)
– Macro-diversity and soft handover
– Variable rate voice coder to enhance capacity
• Basic physical layer technologies
– Channel code and interleaving
• Convolution coding and turbo coding
• Depth of interleaving
– Modulation: QPSK
– Orthogonal variable spreading factor for different services
Capacity in CDMA system:
General situation
•
•
•
•
•
•
Basic consumption:
– Ideal power control, received power
level from each active UE is the same:
Eb
– User distribution: uniform
When N users are active, then the
interference from the same cell will be (N1)Eb. The total interference from the
adjacent cells (1st and 2nd layers) will be the
same as that from the same cell.
Itotal = 2(N-1)Eb>>N0, N0is thermal noise
Required Eb/(Itotal +N0)≈ Eb/(Itotal )
≤lg(SF)-lg(2N)
N≤(10lg(SF)-Eb/ (Itotal + N0))/2
For QPSK modulation and R=1/3 coding,
Required Eb/(Itotal +N0)≈ 3.1dB(0.3) for
BER of 1x10-3
Or N≈SF/4
Soft-handover
•
Based on macro diversity
– 40% of users connect to 2 BSs
– 10% of users connect to 3 BSs
– Total 50% of down-link capacity has be
used of macro diversity
– Total 50% of system resources are used to
support this feature
•
Advantage of soft-handover
– No interrupt, in data services
•
Shortage of soft-handover
– Need more, available network and downlink capacity
– Between same carrier frequency cells
only
RNC
Soft-capacity
• Soft- capacity is one of the feature in IS-95/WCDMA
• Capacity for CDMA system is not a constant, which varied
with environment :
– Less interferences (less users in adjacent cell) will show higher
capacity
– Less required Q&S (higher BER) will provide more users.
• But for CDMA system, the limitation shall be the number
of spreading factor (SF), so-called soft-capacity can only
happen when true capacity much less than the limitation
number
Contents
Part A: Basic concept of TD-SCDMA
• Review
• Smart Antenna
• Synchronous CDMA
• Software Radio and SDR
• Baton Handover
• Advantages
Capacity in CDMA system:
General situation
•
•
•
•
Basic consumption:
– Ideal power control, received power
level from each active UE is the same:
Eb
– User distribution: uniform
When N users are active, then the
interference from the same cell will be (N1)Eb. The total interference from the
adjacent cells (1st and 2nd layers) will be the
same as that from the same cell.
If the antenna of BS with beamformed
pattern, then only a part of interference will
be inter demodulator.
Eb/N0≤lg(SF)+Eb/(N0+(2*(N-1)Eb)/m)~
lg(SF)-lg(2N/m)
N≤m(10lg(SF)-Eb/N0)/2
m means coverage anger/beam width
Basic concept of Antenna array
• Antenna is composed of multiple
array.
• In multi-path situation, each ray has
different phase and delay at each
antenna element
• If each user can be separated in
receiver for each element, then it may
combine main ray and may cancel the
other ray
• It means the total receiving beam
pattern is focused to the user selected.
Basic concept of Smart Antenna
• When the main incident ray
from a user k has an incident
anger of , then the received
signal at the i-th element will be
alejl , l means the l-th ray
• Then the total received signal
will be  wi(ejdicos  alejl )
–
–
–
–
Separate it by code
Find receiving weight W
Get DOA
Combine received signal (uplink beamforming
• Down-link beamforming

di
Calibration of Smart Antenna
• For receive beamforming,
no need calibration
• Each Rx and Tx link with
different transmission
coefficient, when one
want to get DOA, it has
to know the different.
• Calibration: to get the
relative transmission
coefficient of each link
(both Rx and Tx link)
A
B
Rx
Tx
C
BB Processor
Calibration Method
•
Pilot antenna
Ri=PT*Cip*RiR RiP=PT*Cip*TiR
Ri/R1
Ti/T1
•
Calibration Network
– By using a network to replace pilot
antenna
– Advantage: Simplify, do not need
pre-calibration
– Shortage: Can design the network for
some special array, and can not calib.
the different between antennas
•
Self- calibration
– Sequentially trans by each antenna in
the array
– Shortage: more complex
BB
How to DL beamforming?
• Based on Received information
– Max of receiving signal, combine multipath with short delay (one
advantage of NB TDD)
– DOA estimation
– Interference cancellation (difficulty to use)
• Same as receiving beam
– By use the same receiving weight for transmission beamforming
– Can only be used in TDD
– Sensitive with location of UE
• Pencil beam based DOA
–
–
–
–
Synthesis a pencil beam
Advantage: not sensitive with UE location, for high speed moving
Shortage: can not use the energy in multi-path, less beamforming gain
Both TDD and FDD can use this method
Features of Smart Antenna
•
•
•
•
•
•
Increase receiving sensitivity
Increase EIRP
Reduce Interference
Increase Capacity of system
Reduce cost of BS
Increase reliability of BS
Contents
Part A: Basic concept of TD-SCDMA
• Review
• Smart Antenna
• Synchronous CDMA
• Software Radio and SDR
• Baton Handover
• Advantages
Up-link synchronization
•
•
•
Establish of synchronization
– Open loop estimate the distance to
BS.
– Send UpPTS
– Find error
– Adjustment Tx timing
Maintenance of synchronization
– Close loop
– Open loop
Coding tech: zero-interference window
(by LAS-CDMA)
– Limited number of code when need
wider ZIW
Up-link synchronization
•
How to synchronous (DL)
–
–
–
•
UE get 5ms received signal
Find DwPTS from frame structure
Based reference time in DwPTS to realize downlink synchronization
How to synchronous (UL)
–
Initial open-loop:
•
•
–
•
Send UpPTS by UE
Check error in BS receiving signal, and then goes in
close loop
UE received signal
Close loop: BS send physical layer signalling to
ask UE to adjust Tx timing
Maintenance of synchronization
–
–
•
•
5ms
Based midamble, BS will check the timing of
midamble for each user
Close loop: same
Error: one of eighth chip duration
Method to check timing: correlation with fingers
Sync error
BS received signal
Features of synchronous CDMA
• In multi-path condition, ideal
synchronization can not be reached
• One can do is synch for the main ray, and
takes the other ray as interference or noise
• The main feature is to sync all up-link
signals in the same time slot, simplify data
procession
Contents
Part A: Basic concept of TD-SCDMA
• Review
• Smart Antenna
• Synchronous CDMA
• Software Radio and SDR
• Baton Handover
• Advantages
Basic concept of SR
•
•
•
Multi- band and multi-standard in mobile
communication
Each standard ask different equipment design
Compose of a radio equipment:
–
–
–
–
•
•
•
Antenna and cable: wideband
RF transceiver: different bandwidth
BB digital signal processor
Ctrl and I/O: switch and MCU
The most active technology: in BB DSP
Software radio (SR): by using SW in general
DSP/FPGA to replace ASICs
Advantages of SR:
– Save developing time and cost, reduce risk
– Easy to up-grade of technology, upgrade SW
only
– Suitable for using new technologies
Antenna and
Feed cable
RF sub-system
BB digital signal
processing
Ctrl and interface
Key point in SR design
• BB DSP complexity: technologies to be used (Rack receiver or Joint
detection?)
• Present capacity of DSP/FPGS:
– For BS, the most powerful DSP is less than 1000MIPS, and FPGA is about
200gates
– For UE, DSP will be 3/400MIPS only (power consumption limitation)
• Sampling rate and accuracy:
– Single antenna or multiple antenna
– Bandwidth (chip-rate)
– This is why TD-SCDMA takes 1/3 of chip-rate for WCDMA
• Algorithm to be developed
– Suitable for DSP or FPGA?
– Complexity and accuracy?
• Low cost consideration: have to lower than developing and using ASIC
SR and SDR
• How to use software to define a radio
equipment in a reasonable bandwidth and
possible multiple standards?
• Wide-band antenna design
• Different SW for Ctrl and I/O
• Multi-use RF sub-system
– Enough bandwidth
– Meet different spec
– Digital IF for different signal BW
– High dynamic range
• BB DSP
– Capacity for multiple standard
– Speed to change operation mode
• SDR is a future tech
Antenna and
Feed cable
RF sub-system
BB digital signal
processing
Ctrl and interface
t
Contents
Part A: Basic concept of TD-SCDMA
• Review
• Smart Antenna
• Synchronous CDMA
• Software Radio and SDR
• Baton Handover
• Advantages
Why do not use Soft-handover
• Points to considering
– Advantage of Soft-handover
– All code channel is used for
services, have no more DL
channel for macro-diversity
– Overcome shortages of Soft
handover
– Do not need macro-diversity for
capacity
– Difficult to use two receivers for
macro diversity
• Possibility
– Smart antenna for UE location
– Synchronous CDMA
RNC
Baton Handover
•
Measurement:
– DwPTS only
– List strength of measured pilot and report to BS until one reaches HO threshold
•
Find target BS:
– Send UpPTS
– Enter close loop synch with the target BS
– Record new timing for UL synch with the target BS
•
Open-loop synch with target BS
– Find the change of DL timing of target BS
– Renew UL timing
•
Excuse HO
–
–
–
–
System decision
DL transmission by two BS
Switch to target BS
Excuse measurement
Contents
Part A: Basic concept of TD-SCDMA
• Review
• Smart Antenna
• Synchronous CDMA
• Software Radio and SDR
• Baton Handover
• Advantages
Narrow Bandwidth
• Flexible for allocation
– Especial for private network
• Suitable for software radio
5MHz
– Smart antanna
– Joint detection
– Synchronous CDMA
1.6MHz
5MHz
Single carrier frequency
•
•
•
Wave Propagation depends on frequency,
environment, time, and etc. In mobile
communication, it is varied.
In TDD, up-link and down-link takes the
same carrier frequency, the wave
propagation performance will be nearly
the same if the TDD period is short
enough.
Advantages of one carrier frequency:
–
–
BS can get the up-link propagation from
the received signal from one user
Then BS can assume down-link has the
same performance as up-link, and some
technologies are easy to use such as
•
•
–
Beamforming for smart antenna
Pre-rack receiver
For UE, the open-loop power control will
has much higher accuracy
Wave Propagation
Fading;
Multi-path;
Doppler;
etc.
Highest spectrum efficiency
• Spectrum efficiency is one of most important issue in 3G
• TD-SCDMA can use all code channel, reaches wolsh limitation.
• For data services, TD-SCDMA can provide 3x16 pairs of symmetric
channels
– When voice coding is 12.2kbps, 3GPP asks to provide one voice channel
by using of 2 code channels, the spectrum efficiency is
3x8/1.6=15/MHz/cell
– When voice coding is 8kbps, the spectrum efficiency is
3x16/1.6=30/MHz/cell
– For WCDMA, (3 for 12.2kbps and 6 for 8kbps coding rate) its spreading
efficient is the lowest one in 3G standards.
• For data services, the calculation and definition is varied, but TDSCDMA is always the highest one.
• The spectrum efficiency will double when smart antenna is used in
FDD modes
Asymmetric Services
•
•
•
•
The main service for 3G: Mobile internet
From UMTS, 3G services will be 5:1
asymmetric by 2005, and 10:1 by 2010
TDD is the best solution for asymmetric
services
Problem is how to overcome the interference
between cells when providing different
asymmetric services?
f
Down-link
Up-link
– Multiple carriers, in each carrier is equal.
– Smart antenna + DCA
•
The last one should be the best solution, but
need developing the tech in our team
t
All environment
• In traditional understanding, TDD can only use in micro- or
pico-cell environment
• TD-SCDMA has overcome the shortage in TDD mode, by using
of smart antenna technology, it can be used in all environment
system
Sensitivity
WCDMA -117dBm(voice）
-99dBm(384kbps)
TDSCDMA
Max comm. Operation level
distance
Max cell
radius
3.7km
2.1km
2.5km
1.4km
-113dBm(Voice） 4.5km
-110dBm(384kbps) 3.8/5.9 km
-111dBm(Voice）
-93dBm(384kbps)
-107dBm(Voice） 3.0km
-104dBm(384kbps) 2.6/4km
Low cost solution
• Cost for 3G equipment
– CN/RNC/UE will be nearly the same for each standard
– Cost for Node_B is varied with technology
• Smart antenna reduce cost
– ACU and etc. will be the same cost
– The most expensive part is linear power amplifier
– Smart antenna can use a group small PA to replace the
expensive HPA
• Now BS for IS-95 and WCDMA is much
expensive than GSM, but we’ll provide BS with
lower price than GSM.
IPR issue
• IS-95 and cdma2000, main IPR belongs to Qualcomm
• WCDMA, 200 companies own 2,3000 patents
• TD-SCDMA, we own essential IPR, mainly in the field:
–
–
–
–
–
–
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Frame structure
Up-link synchronization
Smart antenna, basic TDD SA, calibration, antenna structure
Cell search
Baton handover
Frequency tranking
Etc.
• We’ll check other patent clamed by other
companies in coming monthes
Continue
Part C: Physical Layer Tech. in TD-SCDMA
• Frame structure
• Spreading and Modulation
• SA + JD
• Cell Search and Random Access
• Power Ctrl and Synchronization Ctrl