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


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


UMTS

UMTS=Universal mobile telephony system

www.umts-forum.org


License cost per country


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.


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.


Network

Will focus on the air interface: between BTS=nodeB=basestation and UE=mobile unit


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


DL and UL, multiaccess

Downlink: Basestation transmits, UE receivesUplink: UE transmits, basestation receives


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


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)


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


Spread Spectrum

Techniques developed in military in 40s and 50s to hide signalsbelow noise level, and to be robust against blocking interferers


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


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 )


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


Transmitter


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


(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


Channelisation Codes


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).


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


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


Modulation scheme

QPSK = Quadrature phase shift keying


Power Control Commands

BPSK = Binary phase shift keying


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


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)


Base stations


Soft Handover


Power Control

DL PC: UE master, nodeB “slave”UL PC: nodeB master, UE slaveTPC commands 1500 times per second, up/down


Inner and Outer loop power control

Not standardized, but most use the inner/outer loop controlstructure


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.


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.


power control in wcdma–background

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