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Resource Management in FemtoCells Malik Muhammad Gul
Abstract—Femtocells are an alternative for traditional
data traffic has increased considerably in the past
indoor cellular communications. These are home base-
few years. The demands of the reliable data traffic
stations which have a DSL based IP backhaul and cel-
are much higher as the data rates are in megabits per
lular air interface through which reliable and high speed
second (Mbps as compared to kilobits per second communications is offered to the home user. The plug-nplay nature of the femtocells along with their co-existence
(kbps) in voice traffic.
with the macrocell calls for new techniques for spectrum
The efforts to meet these ever increasing trends
management and power control in order to reduce macro-
and to increase the reliability and capacity of cellu-
femto and femto-femto interference. This project report
lar networks have given rise to different solutions.
aims to study in detail, the architecture and key challenges of femtocells, along with the available solutions in the
The trend has always been to decrease the cell size and interference in order to accommodate more
literature.
users and higher data rates. Techniques like cell Index Terms—femtocells, resource management, hand-
sectoring, use of directional antennas, distributed off, power control, interference,
antenna systems [1], micro and picocells [2] have I. I NTRODUCTION
T
HE The need for high data rate and reliable communication has always been on the in-
creasing trend. In addition to voice traffic in cellular networks which requires low transmission rates, the
been used towards these goals. Cost of the initial setup and maintenance of these solutions is also a key parameter in deciding their success. Femtocells are a recent advancement in these efforts. These are small portable, user deployed devices which act like base stations and provide
Malik Muhammad Gul is a Graduate Student in the Department of Electrical and Computer Engineering, Georgia Institute of Tech-
reliable and high data rate service to the home
nology, Atlanta, GA, 30332 USA. e-mail:
[email protected].
users. The basic difference between femtocells and
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microcells lies in the fact that microcells act like the outdoor coverage area. This allows the network repeaters; which are used to increase the cellular provider to accommodate more users with the existcoverage in buildings or high density user areas. On ing infrastructure. High setup and running costs for the other hand, femtocells are small base stations the macrocell base stations are thus avoided along which use the high speed Wireline Internet Access with the increase in overall capacity. Network like DSL/cable to connect to the cellular network. Thus, the backbone of a femtocell is this Wireline Access Network and the performance of a femtocell is highly dependent on it.
For users, femtocells offers better indoor coverage and prolonged mobile battery life. As the transmitreceive distance is very small (10 - 50 meters) in a femtocell as compared to macrocell (300 - 2000
Apart from better coverage for the home users, meters), smaller transmit powers are required and femtocells bring many attractive and cost effective thus battery life is increased. Femtocells are thus a benefits to the cellular network provider, and this is very attractive solution for both the consumer and the main reason for the success of femtocells. These cellular network provider. benefits include: 1. Lower setup and maintenance cost:
However, the deployment of femtocells also brings many implementation challenges. As stated,
Femtocells use subscribers’ money for their op- the femtocell backhaul consists of the high speed eration. Even if the femtocell device is provided internet connection. The reliability and availability by the cellular network provider, the operating cost of this connection plays an important role in the is nonexistent. This is because femtocell uses sub- successful operation of femtocells. In addition to scribers’ electric power and internet connection for this, the deployment of femtocells requires effective its operation. Also it requires no site rental as is the resource management in cellular networks. These case with macrocell base station.
resource management issues comprise handoff al-
2. Better outdoor coverage and increase in gorithms, power control, interference avoidance, recapacity: The fact that home users are no more connected to the Macrocell base station, frees up channels in
source allocation and definition of access methods in femtocell networks. The purpose of this course project is to gain an
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and interference management.
A. Backhaul of a Femtocell Network A femtocell uses the subscriber’s internet connection on its backhaul while on the air interface it uses some cellular technology like WiMAX, GSM, cdma 2000, OFDMA etc. Femtocells are connected Fig. 1.
Two-Tier Femtocell Network taken from [3]
insight into these implementation issues of femtocells and study the available solutions in the literature. This project report consists of 7 sections. After this section I of introduction, we discuss the infrastructure of femtocells along with a brief discussion of the key challenges, in section II. In the subsequent sections, we discuss each key issue separately along with the available solutions. In the end, there are conclusion, comments and future work recommendations in section VII.
to a femto gateway through a home router as shown in figure 2. The purpose of femto gateway is to hide the details of the femtocells from the cellular network. As femtocells service both the voice and data traffic of home user, their successful operation is heavily dependent on the availability of bandwidth from the wireline internet access network. Three different situations can arise in this scenario [5]. If the cellular operator also owns the wireline access network, then it can provide the required bandwidth to a femtocell based on the static or dynamic bandwidth allocation. Same is the case,
II. A RCHITECTURE OF F EMTOCELLS
if there exists a Service Level Agreement (SLA)
Femtocells are plug-n-play devices which auto- between the cellular operator and the wireline access matically configure and integrate themselves into network provider. However the femtocells can exist the existing cellular network. Thus, the introduc- even without any SLA or privately owned wireline tion of femtocells in the cellular networks gives network. In that case, the femtocell is provided rise to a two-tier networks as shown in Figure 1. with the best effort service. This best effort serThese two-tier networks pose several deployment vice may be sufficient for the voice traffic but for and operational challenges like spectrum allocation high bandwidth demanding application (like video
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Fig. 2.
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Architecture of Femtocells taken from [4]
applications) or during congestion of the wireline structure. The orthogonal subchannels in OFDMA network, the femtocell performance will be severely help in designing resource allocation algorithms degraded. In this case, a femtocell can either reject for femtocell deployment. Resource allocation techthe data traffic to service the voice traffic only niques will be discussed in section IV. or may handoff to the macrocell. An alternative solution is to place the femtocell in between the home router and the DSL modem, so that it can The successful deployment of femtocells requires prioritize its traffic over the traffic by other devices solution to a number of key challenges. The most in home connected to the wireline network. important challenge in this respect is the interB. Air interface of femtocells
ference avoidance or minimization. Interference in
The air interface of a femtocell uses different cel- macro/femto networks arises due to spectrum reuse. lular technologies. Organizations like 3GPP2, 3GPP, This co-channel interference can be macro-femto WiMAX forum, UMTS have started modifying their interference or femto-femto interference. Both of standards for the inclusion of femtocells in their these are dependent on a number of factors which networks. Specifications are being developed for include access methods, resource/spectrum allocacdma2000, WiMAX, GSM and LTE femtocells. tion and power control. These factors and their OFDMA being used in LTE and WiMAX seems implications on interference management have been to be more appropriate for femtocells because if its discussed in the following sections.
ECE 6604- PERSONAL AND MOBILE COMMUNICATIONS - PROJECT REPORT
III. ACCESS M ETHODS IN F EMTOCELLS
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OFDMA can be used to mitigate this interfer-
Access methods in femtocells have a significant ence through the use of orthogonal sub-channels in impact on the type of interference experienced in close by femtocells. In this case, a femtocell self femtocells. Three broadly defined access methods configures itself on startup and uses air interface in femtocells are Closed Access, Open Access and sensing to allocate orthogonal sub-channels to itself. Hybrid Access.
Use of sector antennas has been proposed in [10] to reduce the co-layer and cross-layer interference in
A. Closed Access in femtocells
femtocells networks. Power control can also be used
Closed Access refers to the access method in to lower the leakage of power from the femtocell which only a limited number of subscribers are network to reduce the interference. allowed to connect to the femtocell. The list of allowed subscribers is managed by the femtocell B. Open Access in femtocells owner.
In Open Access femtocells, any user can con-
Closed access methods give rise to strong macro- nect to a femtocell. This strategy greatly reduces femto interference. For example a non subscriber the cross-layer interference as the user is always connected to a macrocell receives strong interfer- connected to the strongest signal. It also increase the ence signal from a close-by femtocell to which it overall through put of the network. But this access is not allowed to connect. Also, due to adaptive method has two disadvantages. First, the femtocell power control in macrocell, a non subscriber far off owners are generally reluctant to allow other users from the macrocell and close to a femtocell , raises to use their resources especially when one pays for its power levels and presents a strong interfering the femtocell service. Also, open access results in an signal to the nearby femtocell subscriber. Also, the increase in hand offs when for example a user moves user defined random deployment of femtocells in through a residential area with dense deployment dense deployment areas gives rise to femto-femto of femtocells. The neighboring cell list of the user interference as well. This interferences results in also has to be updated continuously in order to coverage holes [16] and higher outage probability make successful handoffs. In dense deployment of in closed access femtocells.
femtocells, reuse of the neighboring cell list can
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cause collisions/confusions and thus increase the affect the throughput of the user if the subscriber outage probability. However, the outage probability and non subscriber are allocated orthogonal subof open access is still less than the closed access channels in a static resource allocation. The outage [6] and its high through put makes open access a probability is also significantly reduced even if a feasible solution for femtocells deployed in small small portion of the total resources are allocated to offices. However efficient handoff and neighboring the non subscribers. cell list updating algorithms are still need to be researched [6].
IV. S PECTRUM M ANAGEMENT The spectrum allocation for femtocells can ei-
C. Hybrid Access in femtocells
ther be disjoint/orthogonal from the macrocell or
As the name suggests, hybrid access is a mixture joint/co-channel. A disjoint spectrum for femtocells of open and closed access in which a portion eliminates macro-femto interference but it lowers of the resources of femtocells is allocated to the the frequency reuse and thus spectral efficiency non subscriber of the femtocell. It also becomes a of the overall system. As femtocells operate in natural solution if some the resources are forced licensed spectrum, the scarcity and price for a to be shared in order to enable emergency calls separate spectrum calls for the existence of cofor the non subscribers. This allocation to the non channel femtocells in the cellular networks. This cosubscribers’ can be static or dynamic. The static existence results in cross layer interference between allocation lowers the spectral efficiency but is easier the macro and femtocells and produces coverage to implement and offers constant throughput for the holes in the network. subscriber. The dynamic allocation results in higher
Different approaches have been used in the lit-
spectral efficiency and is more responsive to traffic erature for interference suppression and avoidance. changes.
Interference suppression or cancelation techniques
In CDMA, hybrid access lowers the throughput are usually avoided due to errors in the cancelation for the subscriber [7] as the interference is increased process. The interference avoidance approaches can when the non subscriber is allowed access to the be broadly classified into two categories. Hardware femtocell. On the other hand, OFDMA does not based approaches include use of switches antenna
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elements [9], sectored antennas in TD-CDMA [10] nels for femtocell. F RSx denotes the random cobut these approaches tend to increase the price of channel assignment in which the whole spectrum is the end user device, so software based approaches divided into x fragments. Macrocell uses the whole like power control and intelligent channel assign- spectrum while the femtocell is allowed only one ment techniques are used to minimize interference. randomly chosen fragment out of x fragments. Dchannel assignment techniques will be discussed in DFP denotes the Distributed-Dynamic Frequency this section while power control algorithms will be Planning while C-DFP denotes the Centralized DFP. the discussion of the next section. The optimal solution for intelligent channel assignment requires a centralized frequency planning in which all the femtocells send their information (e.g. geographical location, received signal strength) to the network which assigns the channels in a way that interference or collision is minimized. However, as the nature of femtocells is plug-n-play, their number and position is random which makes the optimization problem too complex to solve in a short period of time. Therefore, distributed frequency planning is preferred over centralized frequency planning.
As shown, DFP has better performance (through put and outage) than Orthogonal and F RSx cochannel assignments. This is because co-channel interference is minimized by distributed or centralized DFP. C-DFP performs better than D-DFP because it performs optimization over the whole cell while DDFP only performs local area channel assignment optimization. But, note that C-DFP has the maximum signaling overhead as discussed before.
Distributed dynamic frequency planning implies that auto-configuration and self-optimization should be included in femtocells so that they can configure their channel assignment and power levels on start
[8] shows the performance of different channel up and then optimize them by communicating with allocation strategies for OFDMA. Figure 3 shows other femtocells in close proximity. Thus, the opera comparison of performance for orthogonal and ation of a femtocell consists of a sensing phase in co-channel assignments. For orthogonal channel as- which it allocates channels to itself after sensing signment, 8 subchannels of OFDMA are divided the air interface, and a tuning phase in which as SM subchannels for macrocell and SF chan- the femtocells adapts itself to varying traffic and
ECE 6604- PERSONAL AND MOBILE COMMUNICATIONS - PROJECT REPORT
Fig. 3.
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Performance of Spectrum Allocation schemes for OFDMA taken from [8]
channel conditions. A. Auto Configuration
computes a badness factor [17] badnessj (k) =
X
(k).pusage (k) pinterf i i
(1)
i∈Nj interf (k) [17] presents three appraoches for distributed where Nj is the set of neighbors of Fj and pi
channel assignment during the sensing phase of an denotes the possible interference coming from Fi to OFDMA femtocell.
Fj . With each update, the femtocell allocates the
First approach is to use a network listening mode subchannel with minimum badness value to itself. (k) is calculated using a probabilistic model periodically, in which the femtocell senses the air pusage i interface and allocates those channels to itself which which models the entry or exit of a user from a have the minimum received signal strength.
femtocell by a poisson process and exponential pro-
Second approach is to use information ex- cess respectively. pinterf (k) is calculated through the i change between femtocells through broadcast mes- interference, a user can tolerate and the transmission sages. These broadcast messages can be exchanged methodology (modulation and code rate) [17]. through femto gateways, or relaying through mobile
Third approach is to use measurement reports in
terminals over the air interface. These message con- which users send information to femto base stations tains the information of power of each subchannel periodically, about received signal strength in each and pilot signal, and the estimated probability of us- subchannel. These measurements reports are then age pusage (k) of each subchannel k in femtocell Fi . i
used to calculate a Zi × K interference matrix Wi
Based on these broadcast messages, each femtocells at each femtocell Fi , where K is the total number
ECE 6604- PERSONAL AND MOBILE COMMUNICATIONS - PROJECT REPORT
TABLE I
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price paid off for this improved through put is the
T HROUGH PUT PERFORMANCE OF CHANNEL ASSIGNMENT STRATEGIES TAKEN FROM
signaling overhead in the transmission of measure-
[17]
ment reports, but as there are typically, only a small Channel Assignment Strategy
Average Throughput
measurement reports
641.34
number of users per femtocell, this transmission
broadcast messages
600.11
takes only a small part of the total bandwidth.
network listening
585.9
random assignment
477.33
B. Other Approaches for Spectrum Management of OFDMA subchannels and Zi is the number
[18] also discusses the spectrum allocation for
of active users in Fi . The channel assignment is OFDMA and compares orthogonal channel assignthen performed based on the following optimization ment with co-channel assignment. It shows that orthogonal channel assignment results in higher data
procedure min
Z i −1 K−1 X X
wz,k .δz,k
(2)
rate coverage as compared to co-channel assign-
z=0 k=0
ment. The results are for a frequency allocation ratio
subject to Z i −1 X
δz,k ≤ 1
of ρ = 0.2 where ρ is defined as
z=0 K−1 X
ρ=
f emtocellBW T otalsystemBW
(3)
δz,k = 1
k=0
δz,k ∈ 0, 1
It is similar to F RS5 in [8]. The reason for better performance of orthogonal scheme is that no
The first constraint assumes that a subchannel is as- centralized or distributed frequency planning in cosigned to only one user, while the second constraint channel assignment is performed and therefore, the assumes that a user has only one subchannel. The resulting co-channel interference causes worse perupdate of channel assignment is performed after a formance for co-channel assignment. randomly chosen time between [0,Tupdate ].
[11] proposes a strategy for orthogonal channel
The resulting throughput achieved in each chan- assignment in closed access OFDMA femtocells. K nel assignment approach is shown in Table 1 [17].
subchannels are divided into two mutually exclusive
As shown, measurement report based channel sets of size Kmacro and Kf emto for macro and assignment achieves the highest throughput. The femtocells respectively. macrocell uses the whole al-
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located spectrum while femtocells uses a randomly for each user in macrocell is
η (1−η)
with respect to
chosen subset (k) out of the Kf emto subchannels. the user in the femtocell. So, decreasing η provides So, the net portion of spectrum occupied by each more throughput and thus more spectrum is allofemtocell is ρf emto = k/Ff emto . This spectrum cated for femtocells. So, the optimization problem allocation scheme is termed as Frequency ALOHA is formulated as (F-ALOHA). Another difference between the work of these authors like [12], [11], [10] is that they
ρ = arg max wTmacro (w, Umacro )+ 0≤w≤1
(7)
use the stochastic geometry framework and the distribution of femtocells is modeled by a Spatial
(1 − w)Nf emto ρf emto Tf emto (ρf emto Nf emto )
Poisson Point Process. The optimal distribution (ρ) subject to the QOS constraint. The optimum ρ is of the bandwidth between the macrocell and femtofound to be cell is found by maximizing the spatial reuse under ρ ∈ {x, 1 − x}
the QOS constraint η. The area spectral efficiency
(8)
(ASE) is defined as ASE = ρASEmacro + (1 − ρ)ASEf emto
(4)
where
x = [1 +
1 − η Tmacro Uf emto ]−1 η Umacro ρf emto Tf emto
The simulation results are shown in Figure 4, ASEmacro =
Tmacro (ρ, Nmacro ) |H|
(5)
which shows the spectrum allocation for varying η in two scenarios. HA refers to High Attenuation
where ρ = Kmacro /K , Tmacro is the average macrocell throughput with Nmacro users and |H| is the area of the cell. and ASEf emto = Nf emto ρf emto
with path loss exponent of 4, while LA refers to High Attenuation with path loss exponent of 3.5. Note that η = 0.5 means that macrocell and fem-
Tf emto (ρf emto , Nf emto ) |H| (6)
where Nf emto is the number of femtocells.
tocell users have the same average throughput and therefore, almost 90% of the resources are allocated to the macrocell in order to increase its throughput.
The QOS is specified through the factor η (0< As η decreases, femtocells are allocated more power η ≤0.5) which means that the expected throughput
in order to increase their throughput.
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be achieved for the femtocell user with a minimal affect on the macrocell performance. [13], [12] and [14] discuss different power control algorithms for closed access and open access scenarios. [14] proposes a power control algorithm for both the downlink and uplink of a femtocell using open access and considering the macro-femto interference only. The downlink and uplink powers denoted by Pf emto and PF U E (FUE ⇒ Femto User Equipment) Fig. 4. Optimal Spectrum Allocation for varying η taken from [12]
have a maximum limit of 125 mW. This upper bound is chosen to keep the macrocell interference
V. P OWER C ONTROL FOR I NTERFERENCE levels below a certain threshold and minimize the M ANAGEMENT handoffs for the users near the femtocell coverage. Power control is another technique to mini- The main idea is to keep P f emto in a femtocell of mize the femto-femto and femto-macro interfer- radius r (typically around 10m) equal to the average ence. Power control is imperative in co-channel power received in that femtocell. assignment, because in that case femtocell appears
max
Pf emto = min(Pmacro + G(θ) − Lmacro (d) + Lf emto(r) Pf emto )
(9)
as a strong interference for the near-by macrocell where Pmacro is the macrocell power, Lmacro is the user and a coverage hole is formed. The general avarage macrocell line of sight path loss at the disstrategy adopted in the literature for minimizing tance d of femtocell, G(θ) is the gain of macrocell the interference is to minimize the downlink power antenna in femtocell direction and Lf emto(r) is the from the femtocell so that the resulting macro-femto femtocell line of sight path loss at the radius r. interference remains below a specified threshold and
On the uplink, FUE’s transmit power is controlled
the femtocell user still gets an improved coverage. such that the total interference offered from all the Results from the literature [14] show that despite the users in all the femtocells to the macrocell is below max low transmitted power from the femto base station, a defined threshold i.e. Pinterf . This is to guarantee
high throughputs (in the range of several Mb/s ) can that the macrocell performance is not degraded.
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The interference allowance is distributed equally between all femtocell users, irrespective of the fact that they are active or not. Thus PF U E
max Pinterf + Lmacro , PFmax = min( UE) Nf emto
(10)
where nf emto is the number of femtocell and 1 user per femtocell is assumed. Lmacro can be measured through measurement reports. The simulation re(A) Downlink sults for N active femtocells and air interface of UMTS are shown in figure 5. These results show that this power allocation strategy has minimal effect on the macrocell throughput (both on the uplink and downlink) while high throughputs are achieved for femtocell users which favors the use of femtocells. The reduction in throughput on the uplink with the increase in (B) Uplink
N is due to the decrease in the PF U E as nf emto is increased. Similar results have been reported for
Fig. 5.
CDF of throughputs taken from [14]
the same power control method used for co-channel femtocell deployment in GSM spectrum [15]. It uses maximum macrocell interference based power allo- trol interference. [13] gives a joint downlink power cation criterion (same as for PF U E discussed above) control (centralized and distributed) and channel for both the downlink and uplink and shows the assignment strategy for closed access co-channel throughput improvements in femtocell’s downlink femtocell deployment in which a different channel and uplink.
is allocated to a femto base station if the transmitted
Power control algorithms are often augmented power falls outside the allowed limit required to with channel assignment strategies in order to con- maintain a certain co-channel interference.
ECE 6604- PERSONAL AND MOBILE COMMUNICATIONS - PROJECT REPORT
VI. H AND OFF T ECHNIQUES
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presented. The key solutions available in the liter-
Designing handoff algorithms for femtocells is ature for interference management through chana challenging issue and there is no good solution nel allocation and power control have also been available in the literature for this. The problem is discussed. The interested reader is encouraged to challenging due to two facts. First is the small consult the references for more details. size of a femtocell. Due to these small sizes, very
As femtocell resource management is a relatively
frequent handoffs may occur in open access fem- new field, concrete solutions are very sparse in tocell networks with dense deployments. Secondly, the literature and there is a quite a lot of room the current standards only allow a limited size for research in this field. Most of the literature of Physical Cell Identities (PCI) and Neighboring on spectrum management in femtocells deals with Cell lists. These two information are critical for OFDMA because of the orthogonal time and freperforming a handoff. Physical Cell identities are quency composition of its subchannels helps in repeated in a macrocell network due to limited size interference avoidance through intelligent channel but they are unique at least in a local area to avoid allocation. confusion. But, in dense deployment of femtocells this confusion cannot be avoided because of small sizes of femtocells. Also, neighboring cell lists are used by a mobile station in order to scan the available received powers on different channels so that a handoff can be performed when needed. Due to plug-n-play nature of femtocell, these neighboring cell lists have to be updated dynamically in contrast
The plug-n-play nature of femtocells calls for distributed solutions for resource management especially for the handoff algorithms. There is no solution available in the literature for handoff algorithms in open access femtocells. Also, the standards for PCI and Neighboring lists used in handoff algorithms need to be updated for open access femtocells.
to static macrocell networks. A major assumptions used in most of the power VII. C ONCLUSIONS AND F UTURE W ORK
control and channel allocation algorithms is the
In this project report, an introduction to the archi- perfect synchronization between the clocks of femtecture and key challenges of femtocells has been tocells and macrocell. Synchronization algorithms
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for femtocell networks are still to be researched and
[10] V. Chanrasekhar and J. G. Andrews, ”Uplink Capacity and
performance evaluation with realistic synchroniza-
Interference Avoidance for Two-Tier Femtocell Networks”, IEEE Transcations on Wireless Communications, Feb. 2008.
tion algorithms can be performed.
[11] V. Chanrasekhar and J. G. Andrews, ”Spectrum Allocation in Tiered Cellular Networks”, IEEE Transcations on Communica-
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