ELEC5508-LECTURE5 无线通信, 无线工程

Wireless Engineering
Chapter 5
Capacity of Wireless Systems
Chapter 3 pp. 77-96
Wireless Engineering
Traffic and Capacity
Trunking and Grade of Service
Cellular radio systems rely on trunking to accommodate a large number of random users in a limited radio spectrum (fixed number of channels or circuits).
Trunking theory exploits the statistical behavior of users. It was first used by telephone companies to determine the number of telephone circuits that need to be allocated for buildings.
The fundamentals of trunking theory were developed by Erlang.
The measure of traffic intensity has the unit of Erlang.One Erlang represents the amount of traffic intensity carried by a channel that is completely occupied.
Wireless Engineering
Trunking and Grade of Service
The grade of service (GOS) is a measure of the ability of a user to access a trunked system during the busiest hour.
The GOS is an important benchmark used to define the desired performance of
a particular trunked system.
It is the wireless designer’s job to estimate the maximum required capacity and to allocate the proper number of channels to meet the GOS.
GOS is typically given as the likelihood that a called is blocked, or the likelihood of a call experiencing a delay greater than a certain queuing time.
Wireless Engineering
Trunking and Grade of Service
Wireless Engineering
Wireless Engineering
Trunking and Grade of Service
The traffic intensity offered by each user in Erlangs
The total offered traffic for a system containing U users
Traffic intensity per channel of a trunked system with C channels (assuming traffic is equally distributed among the channels)
The maximum possible carried traffic is the total number of channels, C ,in Erlangs.
H
A u λ=H
U UA A u λ==C
UA A u c =
Wireless Engineering
L
,2,1,0!}Pr{==−k k e k k λ
λ A discrete distribution that expresses the probability of a number of events occurring in a fixed period of time if these events occur with a known average rate, and are independent of the time since the last event.
Poisson Distribution
Mean value:
Variance: λσ=2λ
µ
=Poisson Distribution, 10=λ
k : the number of occurrence of an event the expected number of occurrences that occur
during the given interval.
:λ
Wireless Engineering The Blocked Calls Cleared System
The blocked calls cleared system (Erlang-B system)
--No queuing for call requests (no setup time, and immediate access to channel if available.
--Arrived calls follow a Poisson distribution.
--An infinite number of users, a finite number of channels
--Arrivals of requests are memoryless
--The probability of a user occupying follows an exponential distribution The probability that a call is blocked (Erlang-B formula , Erlang loss function )
GOS k A C A A E C k k C
C ===∑
=0!
!)(]blocked is call Pr[C : the number of trunked channels A : the total offered traffic
Wireless Engineering
The Blocked Calls Cleared System
The Erlang loss function is best calculated iteratively by:
with
Erlang B formula provides a conservative estimate of the GOS, as the finite number of users always predict a smaller likelihood of blocking.
)
(11)(11A E A C A E C C −+=1
)(0=A E
Wireless Engineering
The Blocked Calls Cleared System
Large groups are more efficient than small ones.
Wireless Engineering
The Blocked Calls Cleared System
Wireless Engineering
The Blocked Calls Cleared System
Trunking efficiency is a measure of the number of users which can be offered a particular GOS with a particular configuration of fixed channels.
The way in which channels are grouped can substantially alter the number of users handled by a trunked system.
Example
How many users could be supported for 0.5% blocking probability for 5 trunked channels? Assume that each user generates 0.1 Erlangs of traffic.
Solution: C =5. From Fig. 3.6, we have traffic intensity A =1.13.3
.111.0/13.1/===u A A U Therefore, total number of users that can be supported is 11.
Improving Coverage and Capacity in
Cellular Systems
Cellular design techniques are needed to provide more channels per unit coverage area if GOS degrades beyond a threshold.
Major techniques to expand the capacity:
---Cell Splitting: Allows an orderly growth of the cellular system.
---Sectoring: Uses directional antennas to further control interference and
frequency reuse.
---Zone microcells : Distributes the coverage of a cell and extends the cell boundary to hard-to-reach places.
Cell splitting increase capacity by increasing the number of base stations
Sectoring and zone microcells rely on base station antenna placements to improve capacity by reducing co-channel interference.
Wireless Engineering
Wireless Engineering Cell Basics (Review)
N R
D Q 3==
S duplex channels available for use.
Each cell is allocated a group of k channels
Cluster size N If a cluster is replicated M times within the system, the capacity C=MkN=MS
The co-channel reuse ratio:
The SIR ()003i N i Q I S n
n ==
Cell Splitting
Cell splitting is the process of subdividing a congested cell into smaller cells (microcells), each with its own base station and a corresponding reduction in antenna height and transmitter power.
By decreasing the cell radius R and keeping the co-channel reuse ratio D/R unchanged, cell splitting increases the number of the channels per unit area.
Wireless Engineering
Wireless Engineering
Cell Splitting
Handoff issues must be addressed so that high speed and low speed traffic can
be accommodated simultaneously.
Wireless Engineering
Cell Splitting
Cell splitting will not upset the
channel allocation
scheme required
to maintain the
minimum co-
channel reuse ratio
Q .
Wireless Engineering
Cell Splitting
For the new cells to be smaller in size, the transmit power of these cells must be reduced.
If the radius of each new microcell is half that of the original cell, []n
t r R P P −∝1boundary cell old at []n
t r R P P −∝)2/(boundary cell new at 2 If we take n =4, and set the received powers equal to each other, then
16
12t t P P =
Cell Splitting
In practice, not all cells are split at the same time. Therefore, different cell sizes might exist simultaneously.
Channels in the old cell must be broken down into two channel groups, corresponding to the smaller and larger cell reuse requirements.
Antenna downtilting is often used to limit the radio coverage of newly formed microcells.
Cell splitting achieves capacity improvement by essentially rescaling the system. By decreasing the cell radius R and keeping the co-channel reuse ratio D/R unchanged, cell splitting increases the number of channels per unit area.
Wireless Engineering
Sectoring
Sectoring keeps the cell radius unchanged and seek method to reduce Q.
The channels used in a particular cell are broken down into sectored groups and are used only within a particular sector.
Several directional antennas are used at the base station.
This technique for decreasing co-channel interference and thus increasing system performance is called sectoring .
Wireless Engineering
Wireless Engineering
Sectoring
Wireless Engineering
Sectoring
The signal-to-interference ratio (SIR) for a mobile receiver
Assume n =4 and cluster size N=7.
If there is no sectoring and omnidirectional antennas are used, S/I=18.6 dB.
With sectoring, S/I=25.1 dB (24.2 dB in the textbook).()00
3i N i Q I S n n ==
Sectoring
The reduction in interference offered by sectoring enable planners to reduce the cluster size N, and provides an additional degree of freedom in assigned channels.
Sectoring needs an increased number of antennas at each base station.
Trunking efficiency is decreased due to channel sectoring at the base station.
The number of handovers also increases, since sectoring reduces the coverage area of a particular group of channels.
In urban areas, directional antenna patterns are somewhat ineffective in controlling radio propagation.
Wireless Engineering
A Microcell Zone Concept
The increased number of handoffs required when sectoring is employed results in an increased load on the switching and control link elements..
In microcell zone approach,
when a mobile travels from
one zone to another within
the cell, it retains the same
channel.
Any base station channel
may be assigned to any zone
by the BS.
A handoff is not required at
MSC when the mobile
travels between zones within
a cell.
Wireless Engineering
A Microcell Zone Concept
A given channel is active
only in the particular zone
in which the mobile is
travelling, and the base
station radiation is
localized and interference
is reduced.
Particularly useful along
highways or along urban
traffic corridors
Wireless Engineering。