Interference evaluation and MS controlled Handoff Technique for FemtoCell

Ahmad Ali Habeeb Software Engineer,

Accenture Services Pvt. Ltd., Bangalore 560037, India

ahmed.ali.habeeb@accenture.com

Mohammed Abdul Qadeer Department of Computer Engineering,

Aligarh Muslim University, Aligarh 202002, India maqadeer@ieee.org

Abstract- Femtocells (aka Home Node B) are low-power wireless access points that operate in licensed spectrum to connect standard mobile devices to a mobile operator’s network using residential DSL or cable broadband connections [1]. This device has evolved out of the research that despite of large deployment of Macrocell (BTS/BSC) in high density apartment areas and densely populated urban population, the signal strength and voice quality is a major challenge to achieve. But on the other side of the coin there are potential handoff challenges in the implementation of femtocells because of the limitations in the number of adjacent cell sites and unavailability of adequate neighboring cell information during mobility.

This paper focuses on the overview of femtocell technology, its potential implementation challenges, Interference evaluation, traditional handoff technique and the suggestion to adopt Mobile Station (MS) controlled handoff technique that should be adopted in order to successfully handover the control from the Mobile operator cell (Macrocell) to the femtocell and vice versa.

Keywords: Femtocell, Home Node B, Handoff, 3GPP,

Femto Forum, GSM, UMTS, WiMAX, DSL, Hand over. 1. Introduction to Femtocell

Femtocells are low-power access points, providing wireless services in licensed spectrum to customers primarily in the home. The femtocell communicates back to the wireless operator’s core network over the end-user’s home broadband connection.

Femtocells can be described as a scaled down model of a typical base station (BTS/BSC) extended to allow easier deployment as an autonomous Customer Premises Equipment (CPE). Although much attention is being focused on UMTS, but the concept is applicable to all previous technologies such as GSM, CDMA-2K, TDSCDMA and WiFi.

The typical architecture of Femtocell is depicted in the Fig. 1 below.

Fig. 1: Femtocell Architecture The Femtocell promotion body “Femto Forum”

defines the following key attributes, all of which are essential for a device to qualify as a femtocell [2]:

• Mature mobile technology: Femtocells use fully standard wireless protocols over the air to communicate with standard mobile devices, including mobile phones and a wide range of other mobile-enabled devices. Qualifying standard protocols include GSM, UMTS, Mobile WiMAX, CDMA and other current and future protocols standardized by 3GPP, 3GPP2 and the WiMAX Forum. The use of such protocols allows femtocells to provide services to over 3 billion existing devices worldwide and to provide services which users can access from almost any location as part of a wide-area network.

• Operating in licensed spectrum: By operating in licensed spectrum, operators are able to provide assured quality of service to customers over the air, free from interference but making efficient use of their spectrum.

978-1-4244-4570-7/09/$25.00 ©2009 IEEE

• Generating coverage and capacity: As well as improving coverage within the home, femtocells also create extra network capacity, serving a greater number of users with high data-rate services. They differ in this from simple repeaters or ‘boosters’ which may only enhance the coverage.

• Using internet-grade backhaul: As shown in Fig 1, femtocells backhaul their data over standard residential broadband connections, including DSL and cable, using standard internet protocols. This may be over a specific internet-service provider’s network, over the internet itself or over a dedicated link.

• At low prices: The large volumes envisaged for femtocells will allow substantial economies of scale, accessing consumer electronics manufacturing approaches and allowing a very low price point which will be acceptable to most residential customers and will be comparable with access points for other wireless technologies.

• Fully managed by licensed operators: Femtocells only operate within parameters set by the licensed operator. While they have a high degree of intelligence to automatically ensure that they operate at power levels and frequencies which are unlikely to create interference, the limits on these parameters are always set by operators, not the end user. The operator is always able to create or deny service to individual femtocells or users. This control is maintained whether the femtocell itself is owned by the operator or the end user.

2. Implementation Challenges

As Femtocell is dependent on DSL or WiMAX for its implementation, the stakeholders includes Internet Service Providers (ISP’s), telephone companies, cable operators and wireless service providers to work together for it’s successful deployment.

Some of the potential implementation challenges are discussed below

1) Interference – Because of the small size of Femtocell, it can attain spectrum efficiency much greater than what can be achieved by a Macrocell and thus can be easily deployed as a Customer Premises Equipment (CPE). This can further reduce the operating expense of the Network Providers.

But with these apparent benefits, there are some issues with the interactions between the femtocell technology and the host macro-cellular radio network into which they are deployed.

If femtocells can only achieve their potential by disrupting the macro network, then they will be relegated to niche deployments, of little overall relevance to next generation networks. On the other

hand, if the interactions between macro and femto radio layers can be managed to the benefit of all, then their properties (in terms of lowered cost, improved spectrum efficiency and link budget and general performance) can be fully realized, and femtocells will find themselves an essential component of all future radio access network designs.

In a nutshell if a single frequency system is being operated, where the macro and femtocell network utilize the same frequency band, then the power control algorithms of the macrocell and femtocell can create interference, where for example a mobile increases its transmit power to the femtocell as part of the 'near-far' power control inherent in some of the telecommunication technology like CDMA, whilst it is within the coverage area of a macro unit. The resultant high power transmitter in the macro field acts as an interferer since the frequency is shared. Finally, there is the issue of coverage area, where in apartments; femtocell users on different floors can create interference to other users. There are several partial solutions to this problem, but primarily the only way to prevent interference is to use a different frequency for the femtocell coverage, particularly for CDMA deployments. The partial solutions include utilizing the mode-2 fixed power option available in the 3G configuration parameters, which would prevent the mobile unit power from increasing and causing interference, though there is an obvious performance trade-off if this approach is used.

Let’s talk about another scenario of Macrocell downlink interference to the femtocell UE receiver.

A Femtocell is located on a table next to the apartment window that is in the direct bore sight of a rooftop macrocell (approx 30m distance). The macrocell is fully loaded, while a UE is connected to the femtocell (i.e. FUE) at the edge of its range. In this scenario the victim link is the downlink from the femtocell to the FUE whilst the aggressor transmitter is the downlink from the macrocell. This interpretation of Scenario A is summarized in the below Figure 2

Fig. 2: Macro & Femtocell Interference

In order for the Femto User Equipment to detect the femtocell and camp onto it, the P‐CPICH Ec/No must be sufficient. It is assumed that a level of ‐20dB will be adequate in this respect. To find the range of the femtocell we need to find the distance below which the PCPICH power is less than 20dB below the power from the macrocell. The lines crossing the P‐CPICH power curves denote the range of the femtocell. By observing in Figure 3 where the P‐CPICH power exceeds the bounds on the macro interference power minus 20 dB, it is concluded that the 21dBm femtocell must be located less than about 4m from the UE to satisfy the assumption that the “edge of range” is at the window. Lower power femtocells must be even closer and the requirement cannot always be met if the path loss from the macro is near the lower bound. Graph in Figure 3 shows the received signal strength at UE, from macrocell and femtocell.

Fig. 3: Received signal strength at UE, from Macrocell and Femtocell

Sparse/Dense deployment data for Indoor-to-Outdoor Interference and Co-Channel Interference in different simulation scenario is as below [3]

Fig 4: Indoor-to-Outdoor Interference

Figure 5: Co-Channel Interference

2) Quality of Service (QoS) - A voice call over a femtocell typically (depending on the codec) requires as an example 40Kbits/sec (in both the uplink and downlink). The impact on a broadband backhaul is minimal for voice services, but there are other variables that can affect user experiences. With multimedia applications, the femtocell shares the cable or DSL connection with the home’s other broadband devices. When some family members are downloading or uploading high bandwidth applications through the femtocells, for instance, priority for femtocell voice traffic can quickly become an important consideration.

Whether a wireless operator can leverage those devices depends largely on whether the cable or DSL owner exercises the power to determine which service get priority. An ISP may throttle the traffic on their network when huge amounts of data are downloaded or uploaded on the network, to avoid impacting service to their other customers. And packet switched network such as broadband backhaul has jitter/recover clock accuracy. One of the solutions to address this issue is the IEEE 1588 time synchronization standard.

In broader perspective handoff is also one of the key parameters of QoS as discussed below.

3) Handoff - Mobility is the most important feature of a wireless cellular communication system. Usually, continuous service is achieved by supporting handoff (or handover) from one cell to another. Handoff is the process of changing the channel (frequency, time slot, spreading code, or combination of them) associated with the current connection while a call is in progress. It is often initiated either by crossing a cell boundary or by deterioration in quality of the signal in the current channel. Handoff is divided into two broad categories - hard and soft handoffs. They are also characterized by “break before make” and “make before break.” In hard handoffs, current resources are released before new resources are used; in soft handoffs, both existing and new resources are used during the handoff process. Poorly designed handoff schemes tend to generate very heavy signaling traffic and, thereby, a dramatic decrease in quality of service (QoS). (In this chapter, a handoff is assumed to occur only at the cell boundary.) The reason why handoffs are critical in cellular communication systems is that neighboring cells are always using a disjoint subset of frequency bands, so negotiations must take place between the mobile station (MS), the current serving base station (BS), and the next potential BS. Other related issues, such as decision making and priority strategies during overloading, might influence the overall performance.

• Types of Handoff Handoffs are broadly classified into two

categories - hard and soft handoffs. Usually, the hard handoff can be further divided into two different types – intra and intercell handoffs. The soft handoff can also be divided into two different types - Multiway soft handoffs and softer handoffs. A typical example of hard handoff is depicted in Fig 6.

Fig 6: Hard handoff

A hard handoff is essentially a “break before make” connection. Under the control of the MSC, the BS hands off the MS’s call to another cell and then drops the call. In a hard handoff, the link to the prior BS is terminated before or as the user is transferred to the new cell’s BS; the MS is linked to no more than one BS at any given time. Hard handoff is primarily used in FDMA (frequency division multiple access) and TDMA (time division

multiple access), where different frequency ranges are used in adjacent channels in order to minimize channel interference. So when the MS moves from one BS to another BS, it becomes impossible for it to communicate with both BSs (since different frequencies are used). Figure 6 illustrates hard handoff between the MS and the BSs

2.3.1 Handoff Initiation A hard handoff occurs when the old connection is

broken before a new connection is activated. The performance evaluation of a hard handoff is based on various initiation criteria. It is assumed that the signal is averaged over time, so that rapid fluctuations due to the multipath nature of the radio environment can be eliminated. Numerous studies have been done to determine the shape as well as the length of the averaging window and the older measurements may be unreliable. Figure 1.2 shows a MS moving from one BS (BS1) to another (BS2). The mean signal strength of BS1 decreases as the MS moves away from it. Similarly, the mean signal strength of BS2 increases as the MS approaches it. This figure is used to explain various approaches described in the following subsection.

2.3.1.1 Relative Signal Strength: This method

selects the strongest received BS at all times. The decision is based on a mean measurement of the received signal. In Figure 1.2, the handoff would occur at position A. This method is observed to provoke too many unnecessary handoffs, even when the signal of the current BS is still at an acceptable level.

2.3.1.2 Relative Signal Strength with

Threshold: This method allows a MS to hand off only if the current signal is sufficiently weak (less than threshold) and the other is the stronger of the two. The effect of the threshold depends on its relative value as compared to the signal strengths of the two BSs at the point at which they are equal. If the threshold is higher than this value, say T1 in Fig 7, this scheme performs exactly like the relative signal strength scheme, so the handoff occurs at position A. If the threshold is lower than this value, say T2 in Fig 7, the MS would delay handoff until the current signal level crosses the threshold at position B. In the case of T3, the delay may be so long that the MS drifts too far into the new cell. This reduces the quality of the communication link from BS1 and may result in a dropped call. In addition, this results in additional interference to co-channel users. Thus, this scheme may create overlapping cell coverage areas. A threshold is not used alone in actual practice because its effectiveness depends on prior knowledge of the crossover signal strength between the current and candidate BSs.

Fig. 7: Signal strength between two adjacent BS for potential Handoff

2.3.2 Handoff Decisions: There are numerous

methods for performing handoff, at least as many as the kinds of state information that have been defined for MSs, as well as the kinds of network entities that maintain the state information [4]. The decision-making process of handoff may be centralized or decentralized (i.e., the handoff decision may be made at the MS or network). From the decision process point of view, one can find at least three different kinds of handoff decisions.

2.3.2.1 Network-Controlled Handoff: In a network-controlled handoff protocol, the network makes a handoff decision based on the measurements of the MSs at a number of BSs. In general, the handoff process (including data transmission, channel switching, and network switching) takes 100–200 ms. Information about the signal quality for all users is available at a single point in the network that facilitates appropriate resource allocation. Network-controlled handoff is used in first-generation analog systems such as AMPS (advanced mobile phone system), TACS (total access communication system), and NMT (advanced mobile phone system).

2.3.2.2 MS-Assisted Handoff: In a mobile-assisted handoff process, the MS makes measurements and the network makes the decision. In the circuit-switched GSM, the BSC is in charge of the radio interface management. This mainly means allocation and release of radio channels and handoff management. The handoff time between handoff decision and execution in such a circuit-switched GSM is approximately 1 second.

2.3.2.3 MS Controlled Handoff: In MS-controlled handoff, each MS is completely in control of the handoff process. This type of handoff has a short reaction time (on the order of 0.1 second). MS measures the signal strengths from surrounding BSs and interference levels on all channels. A handoff can be initiated if the signal strength of the serving

BS is lower than that of another BS by a certain threshold. The third technique could not become popular because of so many reasons like abundance of BS resource to control handoff, centralized handoff procedure, interoperability of handsets, priority technique for making handoff decisions etc. But in case of Femtocell the major challenge in handover from non-CSG(Closed Subscriber Group) to an allowed CSG cell is still and open issue as per the latest 3GPP technical specification TS 25.367 V8.1.0 (2009-03) which states “In Cell_DCH state, handover from a non-CSG cell to an allowed CSG cell is not within the scope of Release 8” Adoption of MS controlled handover for Femtocell: In the case of Femtocell, Macrocell cannot contain information about the neighboring femtocells because there might be hundreds of femtocell within a high rise. So when the user is in call mode and moves from macro to femtocell, the BS won’t be in a position to handover all the control to the femtocell. So in this scenario, the only way left is to let MS decide and camp-on to the best signal cell by measuring the signal strength of both the cell. 3. Conclusion: In this paper, the overview of femtocell technology, its implementation challenges and interference were discussed and evaluated and the best handover technique which can be adopted in order to successfully camp on to femtocell from macrocell and vice versa while in the voice mode were also evaluated. As handover is still a potential problem for femtocell, we hereby propose the MS controlled handoff technique for the femtocell. 4. References [1] 3GPP TS 25.367 V8.1.0 (2009-03), “Technical Specification Group Radio Access Network; Mobility Procedures for Home NodeB”, Stage 2 (Release 8), 3GPP, http://www.3gpp.org/ftp/Specs/html-info/25367.htm [2] Femto Forum, http://www.femtoforum.org, as on 05/15/09. [3] Femtocell Networks: A Survey IEEE Communication Magazine Vol. 46, Issue 9, Sep. 2008 [4] N. D. Tripathi, J. H. Reed, and H. F. Vanlandingham, Handoff in Cellular Systems, IEEE Personal Comm., December 1998 [5] R. Inayat, R. Aibara, and K. Nishimura, “A Seamless Handoff for Dual-Interfaced Mobile Devices in Hybrid Wireless Access Networks,” Proc. IEEE Int’l Conf. Advanced Information Networking and Applications (AINA ’04), pp. 373-378, Mar. 2004