Power Control in WCDMA–Background Bo Bernhardsson Dept. of Automatic Control Lund Institute of Technology Bo Bernhardsson: Power Control in WCDMA–Background
Power Control in WCDMA–Background
Bo Bernhardsson
Dept. of Automatic ControlLund Institute of Technology
Bo Bernhardsson: Power Control in WCDMA–Background
Contents
I The WCDMA SystemI OverviewI CDMA modulationI Power ControlI HandoverI xx
I Power Control - Analysis, next lecture
References:
I System overview: Links on home page
I Gunnarsson, Gustafsson, Control theory aspects of powercontrol in UMTS ,Control Engineering Practice 11 (2003),pp. 1113-1125
Bo Bernhardsson: Power Control in WCDMA–Background
IMT-2000
IMT-2000 global standard for third generation (3G) InternationalTelecommunication Union.In 1999 ITU approved five radio interfaces for IMT-2000 as apart of the ITU-R M.1457 Recommendation. The mostimportant are
I WCDMA Direct Spread
I CDMA Multi-Carrier, evolution of IS-95
I EDGE, a 2.5G, evolution of GSM
Bo Bernhardsson: Power Control in WCDMA–Background
UMTS
UMTS=Universal mobile telephony system
www.umts-forum.org
Bo Bernhardsson: Power Control in WCDMA–Background
License cost per country
Bo Bernhardsson: Power Control in WCDMA–Background
3GPP
The UMTS standard is developed in 3GPP (third generationpartnership project) and 3GPP2, see www.3gpp.org
“Collaboration” between operators, network providers, UE(=User Equipment) manufacturers.
Standardisation: Open interfaces. Critical parts for operabilitystandardized, other parts open for implementation,performance requirements. MANY test requirements.
Release 99, 4,5,6, etc.
Implementation often starts before standard finalized.
Bo Bernhardsson: Power Control in WCDMA–Background
Power Control and standardization
Power control critical, since it is the main form of resourceallocation
My personal view: Not sufficient requirements on power controlbehavior in standard to guarantee a well working system.
Interoperability issues
Egoistic behavior must be avoided.
Bo Bernhardsson: Power Control in WCDMA–Background
Network
Will focus on the air interface: between BTS=nodeB=basestation and UE=mobile unit
Bo Bernhardsson: Power Control in WCDMA–Background
Code Division Multiple Access
TDMA,FDMA: Good “orthogonality” between tranmitters. Canturn off radio when not used. Must turn on and off transmissioncorrectly, synchronization, reuse factor > 1CDMA: Can share the same frequency, flexibility by codeallocation, good interference immunity
Bo Bernhardsson: Power Control in WCDMA–Background
DL and UL, multiaccess
Downlink: Basestation transmits, UE receivesUplink: UE transmits, basestation receives
Bo Bernhardsson: Power Control in WCDMA–Background
DL vs UL
DLs from a common basestation are easily synchronized,harder to synchronize ULs well. Problem hence not symmetric,One-> Many, vs Many->One
Bo Bernhardsson: Power Control in WCDMA–Background
Power Control
Both UL and DL are power controlled
Power control commands for the UL are sent on the DLPower control commands for the DL are sent on the ULInterference couples all power control loops.“Orthogonality” would be goodWhat is the mechanism behind the coupling?Bo Bernhardsson: Power Control in WCDMA–Background
Spreading Sequence
Assume each symbol S (=I + jQ, in the figure only real partshown) is multiplied with a known code sequence.
Here spreading factor sf=4 is illustrated. (sf=4-512)High sf gives low data rate, but good noise protection.Chip rate 3.84 Mchips/sec (on I and Q each)
Bo Bernhardsson: Power Control in WCDMA–Background
Spreading Factors
For DL
I Voice high quality, sf=128, 60kbps over air, 12.2kbps userdata rate
I Video sf=32, 240kbps over air, 64kbps user rate
I Packed Data Service, sf=8, 960kbps over air, 384kbpsuser rate
Bo Bernhardsson: Power Control in WCDMA–Background
Spread Spectrum
Techniques developed in military in 40s and 50s to hide signalsbelow noise level, and to be robust against blocking interferers
Bo Bernhardsson: Power Control in WCDMA–Background
CDMA Transmitter
Example: Two signals from same transmitter
Note: Change s1and s2 to d1 and d2
Assume C1 and C2 “orthogonal”, i.e.s f∑
k=1
C1(k)C2(k) = 0,1s f
s f∑
k=1
C21 (k) = 1
Bo Bernhardsson: Power Control in WCDMA–Background
Receiver, ideal orthogonality case
Assume good RX-TX synchronization and no interchipinterference=“echos” in channel
Assume di(k) � di, i = 1, 2; k= 1, . . . s f
1s f
s f∑
k=1
c1(k) ⋅ (d1(k)c1(k) + d2(k)c2(k) + e(k)) = d1 +1s f
s f∑
k=1
e(k)︸ ︷︷ ︸
N(0,σ 2e /s f )
Bo Bernhardsson: Power Control in WCDMA–Background
Practical limitation
I In UL, transmissions from different UEs can not easily besynchronized with needed accuracy. It is also not possibleto find useful codes that are orthogonal after time shifts, i.e.
s f∑
k=1
c1(k)c2(k− τ ) = 0
I In both DL and UL, signals will often arrive with severalechos τ f
s f∑
k=1
c1(k)c1(k− τ f ) �= 1
Generates interference between signal streams1 chip = 78 meterRemedy: Use codes where correlation between delayedversions is smallBo Bernhardsson: Power Control in WCDMA–Background
Transmitter, one signal
Bo Bernhardsson: Power Control in WCDMA–Background
Transmitter
Bo Bernhardsson: Power Control in WCDMA–Background
Transmitter, three signals
At chip k send
u(k) = s(k)(c1(k)d1(k) + c2(k)d2(k) + c2(k)d3(k))
s(k) scrambling code, E(s∗(k)s(k− τ )) = 0; hs(k)h2 = 1c(k) channelisation coded1(k), d2(k), d3(k) data at chip k
Different transmitters have different scrambling codes
Bo Bernhardsson: Power Control in WCDMA–Background
(One finger) Receiver
Assume we want to receive data stream 1 and d1(k) = d1 fork = 1, . . . , s f . Ideally
d̂1(k) =1s f
s f∑
1
s∗(k)c∗1(k)u(k) = . . . = d1
With one echo of size α we instead get
d̂1(k) =1s f
s f∑
1
s∗(k)c∗1(k)(u(k) +α u(k−τ )) � d1 + α N(0,
1s f)
Suppression of echos.Suppression of other codessf = spreading gainMore advanced receivers exist with better suppressionBo Bernhardsson: Power Control in WCDMA–Background
Walsh codes
Used to create a tree of “channelisation” codes
Bo Bernhardsson: Power Control in WCDMA–Background
Channelisation Codes
Bo Bernhardsson: Power Control in WCDMA–Background
Code tree
Can allocate codes with different sf in a flexible way
Orthogonality between codes guaranteed if channelisationcodes belong to separate subtrees.
256 different codes with sf=256, etc.Bo Bernhardsson: Power Control in WCDMA–Background
Spreading Codes and Channelisationcodes
Transmitter j sends
u j(k) = s j(k)(c1(k)d1(k) + c2(k)d2(k) + c2(k)d3(k))
Spreading codes s j(k) with small correlation after time shifts,Gold codes, used to separate transmitters, (correlation 1/sf)
Channelisation codes=Walsh codes ck used to separateparallell data streams from the same transmitter (correlationbetween 0(ideally) and 1/sf (if many echos))
Receiver knows transmitter codes s j(k) and ci(k) (by clevermechanism not discussed here).
Bo Bernhardsson: Power Control in WCDMA–Background
Summary
UL: Other UEs power become noise suppressed by 1/sfCodes from same UE are ideally orthogonal
DL: Codes from same base station ideally orthogonalCodes from different base stations suppressed 1/sf
In practice perfect orthogonality is not obtained, suppressionwill be between 0 and 1/sf
Bo Bernhardsson: Power Control in WCDMA–Background
Some more facts about the WCDMASystem
Frequency band:1920 MHz -1980 MHz and 2110 MHz - 2170MHz (Frequency Division Duplex) UL and DLMinimum frequency band required: 2x5MHzFrequency re-use: 1Carrier Spacing: 4.4MHz - 5.2 MHzVoice coding: AMR codecs (4.75 kHz - 12.2 kHz, GSMEFR=12.2 kHz) and SID (1.8 kHz)Channel coding: Convolutional coding, Turbo code for high ratedataReceiver sensitivity: Node B: -121dBm, Mobile -117dBm atBER of 10-3Mobile peak power: class 3 +24 dBm, class 4 +21 dBmModulation: QPSK
Bo Bernhardsson: Power Control in WCDMA–Background
Modulation scheme
QPSK = Quadrature phase shift keying
Bo Bernhardsson: Power Control in WCDMA–Background
Power Control Commands
BPSK = Binary phase shift keying
Bo Bernhardsson: Power Control in WCDMA–Background
More about the WCDMA System
Pulse shaping: Root raised cosine, roll-off = 0.22Chip rate: 3.84 McpsChannel raster: 200 kHzMaximum user data rate (Physical channel): 2.3Mbps(spreading factor 4, parallel codes (3 DL / 6 UL), 1/2 ratecoding), but interference limited.Maximum user data rate (Offered): 384 kbps (year 2002),higher rates ( 2 Mbps) in the near future. HSPDA will offerdata speeds up to 8-10 Mbps (and 20 Mbps for MIMO systems)Physical layer spreading factors: 4 ... 256 UL, 4 ... 512 DL
Bo Bernhardsson: Power Control in WCDMA–Background
More about the WCDMA System
Number of chips / slot: 2560 chipsNumber of slots / frame: 15Frame length: 10ms (38400 chips)Power control period: Time slot = 1500 Hz ratePower control step size: 0.5, 1, 1.5 and 2 dB (Variable)Power control range: UL 80dB, DL 30dBHandovers: Soft, Softer, (interfrequency: Hard)
Bo Bernhardsson: Power Control in WCDMA–Background
Base stations
Bo Bernhardsson: Power Control in WCDMA–Background
Soft Handover
Bo Bernhardsson: Power Control in WCDMA–Background
Power Control
DL PC: UE master, nodeB “slave”UL PC: nodeB master, UE slaveTPC commands 1500 times per second, up/down
Bo Bernhardsson: Power Control in WCDMA–Background
Inner and Outer loop power control
Not standardized, but most use the inner/outer loop controlstructure
Bo Bernhardsson: Power Control in WCDMA–Background
Assignment 4
Investigate (matlab) outer loop power control of the DL for asingle UE, with one service with block error rate targetBLERre f = p percent. The outer loop has block errors as input(a sequence of 1 and 0) and innerloop SNR-target as output.
We will not study the inner loop functionality at this point, so weassume the block error rate is given byB LER = normcdf(−k(t) ∗
√SNRtarnet) Here SNRtarnet is the
SNRtarget and k(t) depends on coding rate, basebandperformance, radio conditions, UE speed etc.
Bo Bernhardsson: Power Control in WCDMA–Background
Assignment 4
Assume one block arrives each 10ms with the block error rateBLER given by the formula above.
Try to find an outerloop controller that converges sufficientlyfast to follow these changes: k(t) = 2 for t<30 sec, k(t) = 2.5for 30<t<60, and k(t) = 1.5 for 60<t<90. How large variationswill your controller have for a stationary k(t) (E.g. k(t) = 2 for0<t<100),
Plot true BLER, BLER-estimate, and SNR-targets. Also handinthe matlab code.
Bo Bernhardsson: Power Control in WCDMA–Background