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TKK HELSINKI UNIVERSITY OF TECHNOLOGY Euro-NF JRA 2.3 meeting, 2.10.2008

Multi-user scheduling in

HSDPA systems

Samuli Aalto and Pasi Lassila

Department of Communications and Networking

TKK Helsinki University of Technology

Email: {Samuli.Aalto, Pasi.Lassila}@tkk.fi

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TKK HELSINKI UNIVERSITY OF TECHNOLOGY Euro-NF JRA 2.3 meeting, 2.10.2008

HSDPA systems

– Downlink scheduling• BS decides allocation

of radio resources for different users’ traffic

– Radio resource management• In HSDPA, resources = orthogonal codes

• Each user terminal has a ”category”

• Category defines the processing power limitation of the terminal Scheduling:

– Based on user’s channel quality and terminal category a coding scheme and number of codes is used which determines the ”bit rate”

– Scheduler should use all resources (i.e., schedule multiple users)

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TKK HELSINKI UNIVERSITY OF TECHNOLOGY Euro-NF JRA 2.3 meeting, 2.10.2008

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TKK HELSINKI UNIVERSITY OF TECHNOLOGY Euro-NF JRA 2.3 meeting, 2.10.2008

Flow-level model (1)

– Elastic flows with random sizes with a general distribution, Poisson arrivals with rate

• At time t there are N(t) flows, each flow is indexed by n

– We do not consider fast fading• Flows only see average channel behavior

• Flows have different channel’s due to, e.g., distance to the base station

– Codes correspond to servers• Number of servers denoted by K and servers indexed by k

• The service rate of each server k is user dependent, denoted by rnk– e.g., rate attenuates with distance dn, rnk ~ Min{1,(d0/dn)}

• Aggregate rate is linear in number of codes (orthogonality)

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TKK HELSINKI UNIVERSITY OF TECHNOLOGY Euro-NF JRA 2.3 meeting, 2.10.2008

Flow-level model (2)

– Terminal category• Associated with each flow n is terminal category cn telling the number of

codes

– Due to terminal category constraints multiple flows need to be scheduled simultaneously (HSDPA systems)

• Earlier we assumed all codes are given to exactly one user (old CDMA 1xEV-DO systems)

• Multiple servers can serve one flow

– classical multiserver problem assumes one server per queue (flow)

• Servers are heterogeneous (service rate depends on flow)

• Again, size-based information is used to select flows intelligently

– Same problem formulation applies to OFDMA systems• Carriers correspond to codes

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TKK HELSINKI UNIVERSITY OF TECHNOLOGY Euro-NF JRA 2.3 meeting, 2.10.2008

Problem reduction

– Idea: First try to simplify the problem to the simplest possible system amenable to analysis

• Gives insight for analyzing more complex scenarios

– Assumptions• All flows have identical channels (symmetric situation)

• All flows have the same terminal category so that K/2 codes can be allocated per user

– Corresponds to an M/G/2 system (with homogenous servers)• Even for this system the optimal scheduling rule is not known (for

minimizing mean flow delay)

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TKK HELSINKI UNIVERSITY OF TECHNOLOGY Euro-NF JRA 2.3 meeting, 2.10.2008

Collection of useful results (found so far…)

– In a static setting with a fixed number of flows SRPT is optimal1

• Applies even with heterogeneous servers

• Assumes one server / flow

– In the dynamic setting• “long jobs are stuck at the end of the busy period”2

– Gain from (size-based) scheduling• Impact greatest for M/G/1 queue

• For M/G/n, as n increases, scheduling has less and less impact

• In an M/G/∞ queue scheduling does not affect performance 1 Pinedo (1995)2 Wierman (2007)

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TKK HELSINKI UNIVERSITY OF TECHNOLOGY Euro-NF JRA 2.3 meeting, 2.10.2008

Some tests for M/G/2 (relative to PS)

0.5 0.6 0.7 0.8 0.9

0.4

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0.5 0.6 0.7 0.8 0.9

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Exponential flow sizes

Erlang flow sizes Pareto flow sizes

0.5 0.6 0.7 0.8 0.9

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SRPT

0.5 0.6 0.7 0.8 0.9

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SRPT*

SRPTFCFS

SRPT/LRPT

SRPT*SRPT

FCFS

SRPT/LRPT

SRPT*SRPT

SRPT/LRPT

Pareto Exp

Det

Erl

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TKK HELSINKI UNIVERSITY OF TECHNOLOGY Euro-NF JRA 2.3 meeting, 2.10.2008

Two forthcoming ACM MSWiM 2008 papers

– Pasi Lassila and Samuli Aalto”Combining opportunistic and size-based scheduling in wireless systems”

• studying how to optimally combine channel-aware and size-based scheduling of elastic flows in HSPDA/HDR type systems

• channel-awareness exploits variations in the quality of the radio channel• size-based schduling gets rid of flows as soon as possible

– Jarno Nousiainen, Jorma Virtamo and Pasi Lassila”Forwarding capacity of an infinite wireless network”

• studying the maximal forwarding capacity of a massively dense wireless multihop network

• separation of micro (single hop) and macroscopic (end-to-end path) levels• formulation of the forwarding problem and development of simulation

algorithms for obtaing upper bounds