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Hybrid Networks: Cellular-Relay Architecture Harish Viswanathan, Sayandev Mukherjee and Ram Ramjee Bell Labs, Lucent Technologies Murray Hill, NJ Network Architecture for 4G Macro-cell architecture may not have enough capacity and coverage

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Hybrid Networks: Cellular-Relay Architecture

Harish Viswanathan, Sayandev Mukherjee and Ram Ramjee

Bell Labs, Lucent Technologies Murray Hill, NJ

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7/4/2005

2 Lucent Technologies

Network Architecture for 4G

Macro-cell architecture may not have enough capacity

and coverage for ubiquitous high data rates

Alternatives

– Pico-cellular, Hierarchical, Ad Hoc, etc.

– Integrated cellular and WLAN

Focus: Can macro-cellular capacity be significantly

enhanced through deployment of cheap “pseudo base stations” or relays? – Multiple hops to and from base station through the relays – Intermediate step towards multihop through terminals

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Cellular Networks with Relays

Relays can improve coverage and capacity Mobile relays to cover hot spots Adaptation to traffic imbalance Additional delay due to multihop Relays decode received packets, then store and forward them on the same/different frequency to terminals

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Cellular-WLAN Architecture

Intermittent connectivity/low data-rate high data-rate Poor WAN Channel Good WAN Channel 802.11b

Relay Proxy

Base Station

Proxy discovery and maintenance Routing

– Node mobility – 3G channel dynamics

Incentives

–WAN operator –Proxy & relay Opportunistic Relaying using dual-mode terminals

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Cellular-Relay Architecture

Base Station

Wireless Base Stations

Service provider deployed relays

to cover hot spots or enhance coverage (eg. in-building)

Same spectrum is used for base to

relay and relay to user transmission

Low cost base stations with no

backhaul

Routing and Scheduling Signaling Location of Relays

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Ev-DO Based System

Pilot Slot 1 Slot 2 Slot 3 Base TX Preambles Relay Tx/Rx Data Pilot Preamble

Downlink Frame Structure Medium Access Features

Time division multiplex

transmission with spreading

Base and relay transmissions

identified by unique PN scrambling (allows simultaneous transmission)

Rate adaptation based on link

quality

CDMA reverse link
Link quality feedback on the

uplink (as in 1X EV-DO) – helps deal with mobility across relays

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Why do relays provide a gain?

Multiple simultaneous transmissions possible With power control, one transmission for every k relays even for a large number of relays With increasing number of relays the size of each relay region decreases - transmit power can be reduced

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Large System Performance Gain

Number of simultaneous transmissions ~ Number of Hops ~ Throughput Gain ~

k N

N N

Assumes no peak rate limit from base station

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First Ring Bottleneck

In the multihop system, data has to be first transmitted to the first ring of relays before the reuse efficiency kicks in Gain In the limit of large number of hops transmission from the base to the first ring can be the bottleneck since base has to transmit one at a time to each of the receivers in the first ring

peak rate limit

≤ R R

peak avg

Multiple antennas at the base station can alleviate the problem

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First ring upper bound with peak rate limit

12 14 16 18 20 22 24 26 28 30 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 Peak SNR (dB) Gain First Ring Upper Bound 35 dBm 40 dBm 45 dBm

Rate from base to first ring of relays is limited by peak SNR achievable One hop performance is sensitive to the base transmit power while p-hop performance bound is not Cell Radius = 2 Km Higher gains can be achieved with multiple antenna transmission in the first hop

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Routing and Scheduling with relays

Which terminals to schedule at each time step? Constraints:

– Relays cannot transmit and receive in same frame – Queues at base and each relay for every user; packets transmitted by relay must have been queued from base to relay first – Transmission rates dictated by interference power

Optimization: given instantaneous feedback about transmission

rates on all links, determine the optimal set of active links on the next frame so as to achieve throughput-optimality: stable queues for each user for largest possible set of arrival rates

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Throughput-Optimal Scheduling

( ) ,

max

ij j r t i j

D r

∈

∑

R ( ) ( )

( ) ( )

ij iq j ih j

D Q t Q t = − index of user index of the link i j − − ( ) source node of link ( ) destination node of link q j h j − −

R(t) is the feasible set of link rates that depends on power allocation and interference Algorithm determines optimal reuse based on fading conditions

(Derived from Tassiulas & Ephremides 1992)

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Simulation Setup

Cell has three 120º sectors; 20 users/sector and 0, 1, 2, 3 or 4

relays/sector

Relays are at a distance of half the cell radius from the center,

n lines that divide the central angle of the sector equally

In each slot, packets arrive independently for each user with

equal probability. The size of the packets is exponential with a chosen mean

Cell radius is 2 km; we assume full interference from two rings

f cells around the cell in question

Path loss model is COST231-Hata at 1900 MHz, with base height

30m, Relay height 3m, and user terminal height 1.5m

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Impact of number of relays

0.5 1 1.5 2 2.5 3 0.5 1 1.5 2 2.5 3 Mean aggregate load in cell (nats/symbol) Mean aggregate throughput in cell (nats/symbol) Effect of number of relays Base only (40 dBm) Base + 1 Relay (37 dBm) Base + 2 Relays (37 dBm each) Base + 3 Relays (37 dBm each) Base + 4 Relays (37 dBm each)

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Impact of Relay Transmit Power

0.5 1 1.5 2 2.5 0.5 1 1.5 2 2.5 Mean aggregate load in cell (nats/symbol) Mean aggregate throughput in cell (nats/symbol) Effect of relay power Base only (40 dBm) Base + 4 Relays (20 dBm each) Base + 4 Relays (30 dBm each) Base + 4 Relays (37 dBm each)

Gain saturates with increasing relay power

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Results with Constant Total Cell Power

0.5 1 1.5 2 2.5 3 0.5 1 1.5 2 2.5 Mean aggregate load in cell (bits/symbol) Mean aggregate throughput in cell (nats/symbol) Performance of throughput-optimal scheduling policy with constant total cell power Base only (40 dBm) Base (37 dBm) + 1 Relay (37 dBm) Base (37 dBm) + 2 Relays (34 dBm each) Base (37 dBm) + 3 Relays (32 dBm each) Base (37 dBm) + 4 Relays (31 dBm each)

Distributing the power is better

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Summary

Analysis for large number of relays shows gains

increasing linearly with number of hops

– First ring of relays can be a bottleneck

Simulation results show

– About 60% gains for 3 relays in uniform traffic distribution – Increasing gains with with increasing relays when total cell power is held constant – Diversity gains from fading

Potentially large gains for non-uniform traffic with optimal

placement of relays or with multi-hop through WLANs

Coverage enhancement

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Other Results

Cellular-WLAN Architecture

– Proxy assignment and Routing protocols – ~50% throughput improvement

Connectivity

– Significant coverage improvement – Energy savings on the reverse link

Multicasting

– Significant gains from SINR improvement to cell edge users – Use of Network Coding