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Impact of Highly Active Primary Users on IEEE 802.22 Network: A Single Cell Case
2010 Bi-Weekly Cores Lab Meeting
Jihoon Park, Pål Grønsund, Przemysław Pawełczak
Motivation
IEEE 802.22 is a Wireless Regional Access Network standard developed by IEEE since early 2006 Standard is still in the draft phase (latest version is 2.0, July 2009)
This is one of three currently available standards that focus on TV white spaces The other two are IEEE 1900.4 (no networking, just information sharing) and ECMA-392 MAC and TV operation in the White Spaces IEEE 802.11af group has submitted its Project Authorization Request for new standard: IEEE 802.11 in the TV white spaces
There seem to be no papers that would analyze this network in detail Whenever IEEE 802.22 name appears the system analyzed has actually nothing to do with the real standard Example of papers: Zhao et al. (Tridentcom’09), Liu et al. (EMC’07), Hu et al. (Comm Mag’07), Song et al. (Chinacom’08),
2010 Bi-Weekly Cores Lab Meeting
Question
Given1. Realistic activity of Primary Users (channel bandwidths, signal levels, activity
patterns)2. In the area with densely populated Primary Users3. Detailed implementation of IEEE 802.22 (traffic types, admission strategies, frame
structure, modulation and coding types, bandwidth, subcarrier allocation and channel sizes and channel numbers)
2010 Bi-Weekly Cores Lab Meeting
What is the average throughput and delay of IEEE 802.22 network user
experience?
Approach
Investigation is composed of two parts1.Analysis of steady state system behavior (throughput only) for a simplified network (more in system model) for tractability reasons2.Further investigation (throughput and delay) via extensive NS-2 simulations with presumably first in the world implementation of IEEE 802.22 stack
NS-2 simulations will be also used to see how severe the simplifications of analytical model were and how well the analysis follow NS-2 traces
2010 Bi-Weekly Cores Lab Meeting
2010 Bi-Weekly Cores Lab Meeting
Illustration
Preliminaries
IEEE 802.22 has tons of similarities with existing IEEE 802.16e
IEEE 802.22 IEEE 802.16e
OFDMA channel profile (MHz)
6,7,8 20,28,17.5,14,10,8.75,7,3.5,1.25
Air interface OFDMA OFDMA, OFDM, Single Carrier
Burst allocation Linear Two dimensional
Subcarrier permutation Distributed (with enhanced interleaver)
Adjacent/distributed
MIMO No STC, beamforming
Frame size 10 ms, superframe: 16 frames
No superframe, multipme frame sizes: 2, 5, 10, 20 ms
2010 Bi-Weekly Cores Lab Meeting
Preliminaries: superframe structure
2010 Bi-Weekly Cores Lab Meeting
frame 0
Superframe n-1 Superframe n Superframe n+1 . . .Time
. . .
SuperframePreamble
SCH
. . .
FramePreamble
FramePreamble
160 ms
frame 1
10 ms
frame 15
FramePreamble
Preliminaries: frame structure
2010 Bi-Weekly Cores Lab Meeting
frame n-1 frame n frame n+1 ... Time...
DS PHY PDU
FramePreamble FCH DS burst 1 DS burst 2 DS burst x...
UDC
DCD
MAC PDU 1 ... MAC PDU y Pad
MAC Header MAC Payload CRC
DS subframeRanging
slots
BW
slots
US PHY PDU(CPE m)
US PHY PDU(CPE p)
...
US subframe
US burst
MAC PDU 1 ... MAC PDU k Pad
MAC Header MAC Payload CRC
Self-coexistence
window
UCSNotification
slots
US-MAP
DS-MAP
OptionalbroadcastMAC PDU
TTGRTG
request
Preliminaries: frame structure
2010 Bi-Weekly Cores Lab Meeting
frame n-1 frame n frame n+1
Adaptive
N time slots
DownstreamSubframe
UpstreamSubframe
...Time
...
Time slot 0 Time slot N-1Time slot
Preliminaries: frame structure
2010 Bi-Weekly Cores Lab Meeting
DS sub-frame
TTG
RTG
US sub-frame(smallest US burst portion on a given subchannel= 7 symbols)
26 to 42 symbols corresponding to bandwidths from 6 MHz to 8 MHz and cyclic prefixes from 1/ 4 to 1/ 32
Fra
me P
ream
ble
FCH
DS-
MAP
Burs
t 1DCD
Burs
t 2 ti
me
buff
er
tim
e bu
ffer
Self-c
oexi
stence
win
dow
(4 o
r 5 s
ymbol
s w
hen
sch
edu
led)
Burst 1
60 s
ubch
anne
ls
Burst 2
Burst 3more than 7 OFDMA symbols
Burst
Burst n
Burst
Burs
t m
Ranging/ BW request/ UCS notification
Burst
Burst
Bursts
Burs
ts
frame n-1 frame n frame n+1... Time...
10 msUS-
MAP
US-
MAP
UCD
Analytical Model: Assumptions
2010 Bi-Weekly Cores Lab Meeting
A bandwidth B of one TV channel is fully available to IEEE 802.22 network, provided that no Primary User is actively transmitting
Bandwidth is divided into multiple (logical) sub-channels
Two types of Primary Users are considered Wireless Microphones (high variation in channel occupancy), occupies Z (currently Z=1) channel; it can appear on any sub channel Other auxiliary device (low variation in channel occupancy) – tries to resemble a TV transmission, occupies Y (currently Y=2) channels; it can also appear on any sub-channel
Activity of two types of PUS follow a Poisson process Parameterized by individual arrival and departure rate
Transmission is slotted (a parameter of our model – one slot is one frame size)
One base station only (no spectrum sharing among multiple IEEE 802.22 base stations)
Spectrum sensing process is assumed to be non-perfect False alarm probability affects the throughput of IEEE 802.22 Note that mis-detection does not, as given in the standard (detection is done per frame basis)
Analytical Model: Assumptions
2010 Bi-Weekly Cores Lab Meeting
Admission control strategy Whenever a VBR call occupies a channel and CBR call arrives, VBR must free space for VBR call by “squeezing” the number of occupied channels to allow CBR to access When any of PU occupies a channel both CBR and VBR must vacate its corresponding sub-channels CBR: switches to idle channel, if nothing available then connections is being buffered; no requirement on continuous channel availability, Y channels can appear anywhere in the bandwidth B VBR: tries to squeeze the connection, if no space available then it is being buffered; just like in CBR no requirement on continuous channel availability
IEEE 802.22 users generate two types of traffic Elastic traffic (Variable Bit Rate - VBR), occupies X (X is a real number) logical channels Non-Elastic Traffic (Constant Bit Rate - CBR), occupies Y (at the moment Y=1) logical channels Both streams are Poisson, described by individual values of arrival and departure
Analytical Model: Limitations
2010 Bi-Weekly Cores Lab Meeting
No other connection strategies considered (in relation to our previous work), like just buffering or switching only Obviously only one cell considered The opposite requires designing of channel sharing strategies among many IEEE 802.22 base stations Example: what to do when in one location only two full TV channels are present with three base stations?
No adaptive modulation features considered (yet) No two-stage spectrum sensing considered (yet) No co-channel interference (should we?) Infinite number of users
Analysis
2010 Bi-Weekly Cores Lab Meeting
P(Ut ,Uw ,Uc ,Uv → Ut + kt,Uw + kw,Uc + kc,Uv + kv) =Pt(Ut → Ut + kt)Pw(Uw → Uw + kw |Ut)×Psu(Uc → Uc + kc,Uv → Uv + kv |Ut,Um)
0 ≤Ux ≤Ux,max,0 ≤Ux + kx ≤Ux,max,−Ux ≤kx ≤Ux,max −Ux
Pt (Ut → Ut + kt) = ft(n|Ut, μt,Ts)gt(kt + n|λt,Ts)n=0
Ut
∑ ,0 ≤kt <Ux,max −Ux
Pt(Ut → Ut + kt) = ft(n|Ut, μt,Ts)n=|kt |
Ut
∑ gt(n−|kt ||λt,Ts),−Ut ≤kt < 0
Pt(Ut → Ut + kt) = ft(n|Ut, μt,Ts) gt(kt + m|λt,Ts),Ut,max −Utm=n
∞
∑n=0
Ut
∑
π (nt ,nw ,nc ,nv )
Transition probability for PU class 1
#PU class 1, #PU class 2, #CBR flows, #VBR flows
Analysis, cont.
Recursive solution is needed to compute departure probability
2010 Bi-Weekly Cores Lab Meeting
ft (n |Ut ,μt,Ts) = P(t1 |Ut, μt,Ts)0
Ts
∫ ft(n−1|Ut −1,μt,Ts −t1)
P(t1 |Utμt) =Utμte−Utμtt1 ,P(0 |Ut −n, μt,Ts −t1 −t2 −K −tn) =e(Ut −n)μt (Ts −t1 −t2 −K −tn )
2010 Bi-Weekly Cores Lab Meeting
Analysis results and model verification
•PU: TV and WM•SU: CBR Only•λt = 5 /s, μt = 3 /s•λw = μw = 3, 30, 300 /s•λc = μc = 100 /s•Pf=0.1•M = 4•Batch–size 10000–num 100–conf 0.9
100
101
102
103
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
0.65
λw
(/s)
Sys
tem
thr
ough
put
(Mbp
s)
2010 Bi-Weekly Cores Lab Meeting
Analysis results and model verification
•PU: TV and WM•SU: CBR Only•λt = 5 /s, μt = 3 /s•λw = μw = 3, 30, 300 /s•λc = μc = 100 /s•Pf=0.9•M = 4•Batch–size 10000–num 100–conf 0.9
100
101
102
103
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
λw
(/s)
Sys
tem
thr
ough
put
(Mbp
s)
2010 Bi-Weekly Cores Lab Meeting
Analysis results and model verification
•PU: TV and WM•SU: CBR and VBR•λt = 5 /s, μt = 3 /s•λw = μw = 3, 10, 30 /s•λc = μc = 10 /s•M = 4 (#channels)•Pf=0.9•Batch–size 10000–num 100–conf 0.9
100
101
102
103
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
λw
(/s)
Sys
tem
thr
ough
put
(Mbp
s)
2010 Bi-Weekly Cores Lab Meeting
•PU: TV and WM•SU: CBR and VBR•λt = 5 /s, μt = 3 /s•λw = μw = 3, 10, 30 /s•λc = μc = 10 /s•M = 4 (#channels)•Pf=0.1•Batch–size 10000–num 100–conf 0.9
100
101
102
103
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
λw
(/s)
Sys
tem
thr
ough
put
(Mbp
s)
Analysis results and model verification
2010 Bi-Weekly Cores Lab Meeting
NS-2 Simulations
Adaptation of existing WiMax forum IEEE 802.16e NS-2 stack More than 20.000 lines of code What is currently implemented Spectrum sensing (single stage) Bandwidths is changed Frame size conforms to IEEE 802.22
2010 Bi-Weekly Cores Lab Meeting
Two stage spectrum sensing
2010 Bi-Weekly Cores Lab Meeting
Two stage spectrum sensing
DLSubframe
ULSubframe
Time
Fre
q
t t+5mst+1ms
RTG = receive transmit gapTTG = transmit transceive gap
This implementations uses some symbols at the end of the UL frame.802.22 uses quiet periods starting from the end of the frame
DLSubframe
ULSubframe
Fast
Sen
sing
2010 Bi-Weekly Cores Lab Meeting
Two stage spectrum sensing
OFDMA Frame Fine
Sensing
Time
Fre
q
25 ms (5 OFDMA frames)
PS: fine sensing can be set to other values, but it must be a multiply of OFDMA frame length
2010 Bi-Weekly Cores Lab Meeting
NS-2 Model
2010 Bi-Weekly Cores Lab Meeting
NS-2 Model
CBR_802.22= 100 * 1/100 = 10 K = 10 * 8 = 80 Kbps
CBR = packetSize_ * pps = packetSize_ * 1/interval_
CBR_WM= 100 * 1/100 = 10 K = 10 * 8 = 80 Kbps
CBR_TV= 1000 * 1/10 = 10 K = 10 * 8 = 80 Kbps
Exponential on/off process:burst (on) = 2 secidle (off) = 3, 4, 5, 6 sec
Exponential on/off process:burst (on) = 60 secidle (off) = 60, 120, 180 sec
Constant
2010 Bi-Weekly Cores Lab Meeting
NS-2 Model
CBR traffic, Wireless microphone only, On time: 2 s, off is a variable