ece442 communications lecture 2. system design principles...

Elements of OFDMA System Design Principles Fading Inter-cell User Multiplexing Inter-cell Interference Scheduling Handoff and IP Network

ECE442 CommunicationsLecture 2. System Design Principles and

Instances in 4G Cellular Systems

Husheng Li

Dept. of Electrical Engineering and Computer Science

Spring, 2014


Cellular Systems

1 In a cellular system, the service area is divided into anumber of cells.

2 The interference could be inter-cell or intra-cell.


Cellular Systems: Development

1 Cellular systems have experienced four generations ofdevelopment.

2 The 4G system mainly includes LTE and WiMAX.


New Challenges in Mobile Broadband

1 There are many new challenges and features for themobile broadband cellular systems, in contrast withtraditional cellular telephony.


Why OFDMA in 4G?

1 There is a performance gain over 3G CDMA systems:

COFDMA =1 + f

fCCDMA, (1)

where f ≈ 0.5 ∼ 1 is the relative inter-cell interferencefactor, which equals the ratio of mean inter-cell andintra-cell interference powers.

2 The orthogonality enables OFDMA to multiple users withlarge signal dynamic range.

3 The fine granularity of bandwidth resource enablesOFDMA to allocate and utilize bandwidth resource moreflexibly.

4 OFDMA decomposes the wireless channel into a numberof parallel flat sub channels in frequency.


Elements of OFDMA

1 OFDM is a multi carrier communication scheme where theentire system bandwidth is divided into Nc equally spacedtones.

2 The time-domain transmitted signal is given by

x(t) =Nc−1∑k=0

X [k ] exp(

j2πkt

Ts

), (2)

where Ts is the OFDM symbol duration and Nc is thenumber of subcarriers.


Illustration of OFDM Modulation

1 Different symbols on different subcarriers are modulatedwith different frequencies.


Wireless Channel for OFDMA

1 After two orthogonal tone signals travel over the wirelesschannel, the received signals are also sinusoids at exactlythe same tone frequencies, thus still keeping theorthogonality.

2 The channel response at any tone frequency is just acomplex coefficient, which may vary with tone in afrequency selective channel.


Cyclic Prefix

1 The transmitted signal needs to be at least as long asTs + Tmax (Tmax is the maximum channel response time).

2 We set a cyclic prefix for each OFDM symbol; i.e, theOFDM symbol in [0,Ts] should be extended to [−Tcp,Ts]such that

x(t) = x(t + Ts), ∈ [−Tcp,0]. (3)


Orthogonality

1 The above figure shows another explanation of OFDMorthogonality.


Time-frequency Resource

1 Each small square represents the basic unit of bandwidthresource, referred to as tone-symbol, which is a tone in anOFDM symbol.


Modulation and Demodulation

1 The modulation and demodulation can be carried out byIDFT and DFT.


OFDMA: Basic Concepts

1 Cell; sector; uplink; downlink


OFDMA: Frequency Usage

1 Zero intra-cell interference; the inter-cell is averaged.2 Tone hopping: the tone-symbols allocated to a given user

hop across different times; which causes interferencediversity or interference averaging.


OFDMA: Comparison with CDMA and FDMA


Tone Hopping: Averaging vs. Peaking

Peaking improves instantaneous SINR, while averagingimproves the reliability. A tradeoff should be achieved.


Example: LTE

In LTE, most of the time-frequency resources (PRBs) aredynamically allocated to different UEs.


Type of Resource Allocation in LTE: Types 0 and 1

There are three types of resource allocation in PDSCH of LTE.A resource is expressed as a bitmap in types 0 and 1.


Type of Resource Allocation in LTE: Type 2

In type 2, the resource allocation is not expressed as a bitmap.


Frequency Hopping in LTE

The frequent hopping in LTE is based on the uplink slot.


Block Signal Processing of OFDMA

The block diagram of signal processing of OFDMA issimilar to that of OFDM, except for multiplexing.


PAPR

HIgh peak-to-average power ratio (PAPR) is a well knowndrawback of OFDM.


SC-FDMA

SC-FDMA is used to reduce the peak-average power ratio(PAPR).SC-FDMA is similar to OFDMA in the sense that both arethe sum of tone signals at the tones allocated to the userand emily the cyclic prefix.Two SC-FDMA signals in an OFDM symbol do not interferewith each other, as long as the allocated tones do notoverlap.


Uplink Data and Control Channels in LTE

LTE supports the localized SC-FDMA.PUSCH is used to carry traffic data and control signals forhigher layers.


PUCCH

PUCCH is used to transfer physical layer controlinformation.


How to send both data and control?

The data and control messages can be multiplexed inPUSCH.


SRS

SRS is used to estimate the uplink channel quality.


Realworld Impairments

Carrier frequency offset and Doppler effectArrival time beyond the cyclic prefixSampling rate mismatchPhase noiseI/Q imbalancePower amplifier nonlinear distortion


Determination of Parameters

The main design parameters are ∆f and Tcp.The overhead due to the cyclic prefix is

Tcp

Ts + Tcp=

11 + 1

∆f Tcp

.

Large Tcp helps to reduce the inter symbol interference;large ∆f reduces the relative carrier frequency offset.Hence, a tradeoff should be achieved.


Orthogonality

Orthogonality is achievable within each cell and sector.In multiple-cell case, synchronization may not be possiblefor all transmitter-receiver pairs.


Orthogonality: continued

When the receiver is not synchronized to a tone signal,there could be signal power spillage.


System Level Benefits

No intra-cell interferenceUser multiplexing with large signal dynamic rangeUniversal and fractional frequency reuseFlexible schedulingMIMO enhancements


Fading

We can use diversity to mitigate fading channels.We can also exploit fading channels for betterperformance.


Intra-cell Multiplexing

No intra-cell interference; in contrast, CDMA is notorthogonal.The near-far problem can be well solved.


Inter-cell Interference: CDMA and FDMA

In narrow band FDMA, frequency reuse is used to mitigatethe inter-cell interference.In CDMA, direct sequence is used to mitigate the inter-cellinterference. But there is a tradeoff between the SINR andbandwidth.


Homework 1

Problem 1. Consider an OFDM system with time varyingchannel gains; i.e., the received signals are given by

y(t) = h(t)x(t),

where h(t) is the time varying channel gain. Prove that thesubcarriers are no longer orthogonal, unless h(t) keepsconstant.Problem 2. Write Matlab codes to simulate OFDMtransmitter and receiver. Generate Fig. 2.34 from your ownsimulation code.


Inter-cell Interference: Uplink and Downlink

Interference averaging is mainly in the uplink, since thelocations of interferers in the downlink are fixed.It is desirable to actively control the inter-cell interferencethan simply averaging the interference.


Fractional Frequency Reuse

Fractional frequency reuse can better utilize the spectrumthan universal frequency reuse.


MIMO

MIMO can be used for both single user and multiple usercases.OFDMA has two advantages for MIMO.


Scheduling

In mobile broadband, a key idea is to apply statisticalmultiplexing principles to share system resources (time,bandwidth, and power) instead of partitioning them in afixed manner among users.The resource demand of users varies rapidly.The following issues are important for efficient schedulingsystem: (a) IP aware base stations; (b) Low systemoverhead and agile operation; (c) MAC state scheduling.


IP based Network

The mobile broadband system should be IP-basedbecause such a network architecture greatly simplifiesnetwork installation, operation and maintenance.


Two More Issues

The deployment of base station could be unplanned.Mobile IP-based handoff should be considered.


Diversity

1 Rayleigh fading and log normal shadowing induce a verylarge power penalty.

2 One approach to combat fading is diversity-combining ofindependently fading signal paths. Diversity-combininguses the fact that independent signal paths have a lowprobability of experiencing deep fades simultaneously.

3 Diversity to mitigate the effect of multipath fading(shadowing fading) is called microdiversity(macrodiversity).


Realization of Independent Fading Paths

1 Space diversity: we can use multiple receive antennas torealize independent fading paths.

2 Polarization: We can use either two transmit antennas ortwo receive antennas with different polarization.

3 Smart antennas: an array of antennas can be steered tothe incoming angle of the strongest multipath component.

4 Frequency diversity: We can transmit the samenarrowband signal at different carrier frequencies.


Receiver Diversity

In receiver diversity, the independent fading paths associatedwith multiple receive antennas are combined to obtain aresultant signal that is then passed through a standarddemodulator.


Performance Metrics

Array gain: it results from coherent combining of multiplereceive signals. Even in the absence of fading, this can stilllead to an increase in average received SNR.Diversity gain: it can mitigate the negative effect of fadingand result in the error probability as

P̄s = cγ̄−M ,

where M is called the diversity order.


Selection Combining

In selection combining (SC), the combiner outputs thesignal on the branch with the highest SNR.For Rayleigh fading, we can prove that the average SNR ofthe combiner output isγ̄Σ = γ̄

∑Mi=1

1i .


Threshold Combining

In threshold combining, the receiver scans each of thebranches in sequential order and outputs the first signalwith SNR above a certain threshold.Similarly to SC, only one branch output is used at a time.Different from SC, TC monitors the SNR at only onebranch.


Maximal Ratio Combining

In MRC, the output is a weighted sum of all branches bysetting

αi = aie−jθj ,

and the SNR is given by

γΣ =1

N0

(∑Mi=1 ai ri

)2

∑Mi=1 a2

i

When the SNR is high, the diversity order of MRC is M.


Equal Gain Combining

In EGC, the weights are the same and the SNR is given by

γΣ =1

N0M

(M∑

i=1

ri

)2

The performance of EGC is quite close to that of MRC,which typically exhibits less than 1dB of power penalty.


Transmitter Diversity

In transmit diversity, we use multiple transmit antennaswith the power divided among them.Transmit diversity is desirable in systems such as cellularsystems where more space, power and processingcapability is available on the transmit side.Transmit diversity is highly dependent on whether thecomplex channel gain is known or not.


Transmitter Diversity: Known Channel Gain

The transmit signal is given by

r(t) =M∑

i=1

ai ris(t).

When the channel gain is known, MRC can be used andthus achieve the diversity order M.


Alamouti Scheme

Alamouti scheme was found by Alamout in Bell Lab andpublished in JSAC (1998). It achieves full diversity for 2× 2MIMO system even when the channel gains are unknownat the transmitter.


Alamouti Scheme

Let h1 and h2 be the channel gains of two transmit antennas,respectively. Received signal y = (y1, y∗

2 )T is given by (withoutnoise)

y =

(h1 h2h∗

2 −h∗1

)(s1s2

)= HAs.

Multiplying HHA HA, we have

zi = (|h1|2 + |h2|2)si , i = 1,2.


Time and Frequency Diversity

In OFDMA, we can achieve diversities of both time andfrequency.


Multiuser Diversity: General Principle

When there are multiple users, we can choose the userwith the best channel condition to transmit.


Mitigating Fading Using Channel State Feedback

The receiver can feed back the channel state to thetransmitter.The transmitter can control the power.The transmitter can control the rate.The transmitter can control both power and rate.


Diversity or Feedback?

The two approaches contradict with each other and dealwith fading with fundamentally different ways. Hence, atradeoff should be achieved between them.At low SNR, the feedback can improve the performance. Inthe high SNR regime, the feedback does not improvemuch.Hence, the SNR regime is the key consideration indetermining a diversity or feedback strategy.Other concerns such as delay, data rate variation andchannel prediction.


Multiuser Diversity: EV-DO

In the downlink, EV-DO uses rate control for multiple users.


Multiuser Diversity: Flash-OFDM and LTE

Users can measure the downlink channel gain and use it toapproximate the uplink channel gain. It works in TDDsystem; but not in FDD system.Only the base station can measure the receive power ofthe uplink signal from the user and derive the channel gainif the transmit power is known.In Flash-OFDM, different policies are used for boundaryand interior users.


Utilizing Frequency Diversity or Selectivity

We can either use frequency diversity scheduling orfrequency selective scheduling.


Utilizing Multiple Transmit Antennas

Transmit diversity is opposite to the multiuser diversity.When the channel changes very slowly, it is unfair to keepselecting the best user.The idea of opportunistic beam forming is to artificiallyinduce channel fluctuation using multiple antennas.


Downlink Multiplexing

We can investigate the Pareto-optimal allocation of powerand bandwidth in FDM.


Downlink Multiplexing in LTE

In LTE, users are multiplexing in time and in frequency.Three control channels, PCFICH, PHICH and PDCCH areused.


Uplink Multiplexing

For two users, there are four regimes in the uplinkmultiplexing.


Other Concerns

There could exist cross interference and self-noise.We can carry out non-orthogonal multiplexing.There could exist interference across different sectors in acell.


Frequency Reuse Across Cells

It had been widely believed that K > 1 (K is the reusefactor) is necessary until CDMA with K = 1.


Major Design Questions

Is interference averaging always desirable for inter-cellinterference management? Does universal frequencyreuse achieve the best SINR versus bandwidth tradeoff?The tradeoff is different in a data system as opposed to avoice system.Should OFDMA follow the same CDMA power controlprinciple? How should the uplink transmit power bedetermined?


Signal and Interference in Multi-cell

We assume that the users are distributed as a PoissonPoint Process and ignore the thermal noise. The basestations are placed in regular hexagonal grids.


Downlink SIR

The distribution of downlink SIR is given by

F (x) = 1− 1|A|

∫A0

∏n>0

(1 +

x |r − sn|−r

|r − s0|−r

)−1

dr .

An explicit express is very hard to obtain.The uplink can be analyzed similarly.


Uplink Power Control

Power control is necessary in OFDMA, not because ofintra-cell interference, but because of inter-cellinterference.The uplink power control can be formulated as anoptimization problem:

max{αi},{wi}

∑i

wiRi

s.t . Ri = αi log(1 + γi)

ui ≤ Gci ,iPm (4)


Uplink Power Control in LTE

In LTE power control of the PUCCH and PUSCH is quitedifferent.The data rate of PUCCH depends on the PUCCH formatsand power control of PUCCH is used to meet thecorresponding SINR targets.The PUSCH has the flexibility of setting different SINRtargets for different users depending on whether they arenearby or faraway.


Fractional Frequency Reuse

Consider a two-user case shown above.Neither universal reuse or reuse K > 1 always outperformsthe other.


Two-cell Analysis

Consider a two-user case shown above.Neither universal reuse or reuse K > 1 always outperformsthe other.


Rate Region

We extend the two-cell case to multiple cells.


Breathing Cells

We can also carry out FFR in the time domain, thusresulting in breathing cells.


Inter-cell Interference in LTE

In addition to hopping and physical resource allocation,LTE employs a more active mechanism called inter-cellinterference coordination (ICIC).The idea is similar to FFR; i.e., to balance different reusefactors for superior and inferior users, to trade off SINR vs.bandwidth and to coordinate power patterns amongneighboring cells.ICIC defines various signaling messages to be exchangedbetween neighboring base stations over the backhaul,called X2 interface.RNTP and HII messages are used to coordinate thedownline and uplink interferences.


Basic Problems in Scheduling

Scheduling deals with two questions:User Selection: which user to transmit in the uplink or toreceive in the downlink.resource Allocation: what time-frequency bandwidth to beallocated to the selected users and what transmit power tobe used.

A key feature in an OFDMA mobile broadband cellularsystems is scheduling by which a base station dynamicallyselects users and allocate time-frequency-power resourceto them.


Scheduling for Infinitely Backlogged Traffic

Infinitely backlogged traffic is a simplified model where thequeue length of every user is much longer than what canbe served in a scheduling cycle. The QoS is measured bythe fairness among different users.The scheduling can be modeled as an optimizationproblem:

maxR

∑i

U(Ri)

s.t . R = [R1, ...,RM ] ∈ R (5)


Utility Function

There could be various types of utility functions. Inparticular, U(R) could be log(R), which results in theproportional fairness.


Scheduling for Elastic Traffic

In elastic traffic, the traffic sources can adjust the packetarrival rates (e.g., adjusting the video signal quality).Congestion control is used to adjust the packet arrivalrates.


Congestion Control

Denote by Ai the packet arrival rate of source i . Thecongestion control can be formulated as an optimizationproblem:

maxA

∑U(Ai)

s.t . Ai ≤ R̄i , [R̄1, R̄2, ...] ∈ R̄. (6)

The solution is given by

Ai(t) = arg maxAi

(U(Ai)− ηQi(t)Ai), (7)

where η is a tuning parameter (like price) and Qi is thequeue length.


Throughput Maximization

The scheduling policy that stabilizes any arrival rate vectorin the stability region is called the throughput optimal. Theoptimal algorithm has been found by Tassiulous andEphremides in 1992.


Signaling for Scheduling

Control signaling is needed for the scheduling.


Downlink Scheduling

In the downlink, all users send to the base stationscheduler the downlink channel related information.


Uplink Scheduling

In the downlink, all users send the uplink sounding signalsto allow the base station to measure the uplink.


Signaling for Scheduling in LTE

The table summarizes the control signaling that supportsscheduling in LTE.


Semi-persistent Scheduling

Some traffic such as VoIP is characterized by regulararrival patterns. Hence, the dynamic scheduling may notbe efficient.For VoIP in LTE, semi-persistent scheduling is used.


IP Network

TCP-IP is widely used in communication networks. Incontrast to wired communication networks, wirelessnetwork has the features of link reliability and user mobility.


Motivation for IP-based Cellular Network

From the perspective of en users, the IP-based networkarchitecture seamlessly extends the broadband accessexperience that the users have in the wireline world.Existing applications and protocols rum natively across thewired and wireless links without modifications.The PI-based network architecture benefits the operatorsby reducing the capital and operating expenses.


Architecture

The architecture of IP-based cellular network consists of aradio access network (RAN) and a core network.


Protocol Stack

The protocol stack optimizes for IP-based data deliveryand networking.


Requirement for Handoff

Early cellular systems are not based on IP architecturesbecause of circuit-switched voice service:

Low and somewhat constant data rate.Fixed MAC channel resource allocation.Stringent latency requirement.High reliability.


Soft Handoff in CDMA

The CDMA RAN can sully exploit the macro diversity.


Procedure of Soft Handoff in CDMA

Soft handoff can be initiated by the user. During the softhandoff, both base stations send the same signal in thedownlink and receive a single signal from the user in theuplink..


Difference between Voice and Broadband Data

Is soft handoff still good in broadband data service?


Parallel Links to Multiple Base Stations

At both uplink and downlink, the user has parallel chains ofdigital signal processing modules, each of whichprocesses the data to/from a base station.


Make-before-break Handoff in OFDMA

The user in the make-before-break handoff in OFDMAindependently can send/receive different IP packetsto/from the two base stations.


Uplink Macro-diversity

In the MBB handoff mode, the user obtains uplinkmacro-diversity by dynamic link selection.


Stickiness of Link Selection

The constraint of of sticky link selection leads to two implications:

The user may be stuck with an inferior link for some period oftime.

It is possible that the user is scheduled to transmit on both links,thus not having enough transmit power.


Downlink Macro-diversity

Link selection stickiness is more severe in the downlinkthan in the uplink.


Break-before-make Handoff in OFDMA

Sometimes (e.g., drastic deterioration of the wirelesschannel) MBB does not work, the user has to rely onbreak-before-make handoff.


Handoff Initiation

The handoff decision is based on two factors: wirelesschannel quality and network loading condition.In the universal frequency case, the user processes thereceived signal to accomplish the following:

Cell searchSynchronizationSystem information


Wideband Broadcast

The design of wideband broadcast signal is to make it easyto detect the presence of the signal..


Zadoff-Chu based Broadcast

One possible choice is to construct the signal similar to apseudo-random sequence.


Beacon based Broadcast

One possible choice is to use a single tone beacon.


Comparison of Correlation Window

Sample level search is suitable for a sideband broadcastsignal while symbol-level search for a beacon signal.


Beacon Signals in Various Time Durations

In the absence of symbol synchronization, no signal FFTwindow captures a whole OFDM symbol of the beaconsignal.


Flash Signaling in Flash-OFDM

We can combine flash signals andsuperposition-by-position coding in Flash-OFDMassignment channel.


Railway Flash-OFDM Network

Unlike other cellular deployments, here the number of basestations exceeds the number of users (trains).


FFR Case

The beacon signals are sent only in the primary subbandor in every subband.


Mobile or Network Controlled?

In mobile-controlled handoff, the user decides whenhandoff is to take place.In network controlled handoff, the user measures therelative signal strengths from neighboring base stationsand reports the measurement results to the serving basestation.A major advantage of network-controlled handoff is thatthe network can better balance traffic loadings in adjacentcells.


Cell Search in LTE Handoff

There are two handoffs in LTE: namely the idle handoff andactive handoff.


Random Access in LTE Handoff

After UE acquires synchronization and system informationfrom the eNB in cell search, the UE proceeds to the step ofrandom access.

ece442 communications lecture 2. system design principles...

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