Overview of Vertical Handover

Handover is a fundamental feature in any wireless networks. When a mobile terminal moves away from a base station, the signal level will decrease and there is a need to switch to other base stations. The handover control is the procedure to handle the base station migration for the mobile. There are a lot of studies related to handover [14] and can be divided into two categories: handover architecture and handover algorithms (see Figure 2-2). Architecture includes the methodologies, controls, and software/hardware elements involving in the re-routing of a connection. In handover methodologies, hard handover is limited by only one

connected BS and the handover procedure is “break-before-make”. Due to the technology of CDMA, soft handover is proposed to allow multiple BS connections and the handover procedure becomes “make-before-break”. The soft handover will take the benefit of the diversity technique and will enhance the transmission performance.

Handover

Figure 2-2. Important studies involved in the handover mechanism

For handover controls, the trigger decision in network-controlled handover is decided by base station (BS) only, on the contrary, a mobile station (MS) dominates the decision in mobile-controlled handover. However, the common solution in handover control is mobile-assisted handover. This is because to make a precise decision, both the BS and MS should be responsible for the handover decision. Issues related to the handover algorithms are the triggering metrics used by the algorithm. The common metrics include the received signal strength (RSS), signal-to-noise ratio (SIR), power budget, and etc. Different measurements will affect the handover decision and cause different handover performance. The handover performance usually includes the handover frequency, handover blocking rate, and handover delay. Different services might have different requirements in handover performance, so a service-based handover control algorithm is important in handover designs.

The conventional handover used to switch between homogeneous networks is called the horizontal handover. In the heterogeneous networks, the handover for switching from one system to another system is called the vertical handover. Due to different systems have different characteristics; the vertical handover could be asymmetrical. In our scenario, as shown in Figure 2-3, the handover from UMTS to WLAN systems is called the downlink handover and the handover from WLAN to UMTS systems is called the uplink handover. The objectives of the downlink and uplink vertical handovers are different. The downlink handover is focusing on improving transmission rates while the uplink handover is focusing on the connectivity of the call. Besides of the above, like the conventional handover problem, the vertical handover also needs to prevent the traditional ping-pong effect between two

networks.

Figure 2-3. Two-way handover in heterogeneous networks

2.3 Relative Features for the Vertical Handover

In order to have proper handover decisions and make the integrated system efficient, an accurate reference measurement is required for designing a proper vertical handover control.

However, when UMTS operates in a frequency division duplex (FDD) mode, the transmission is continuous and there is no available gap time for measuring WLAN systems to get the sufficient information. Besides, even if the measurement of WLAN systems is possible, how to obtain the reliable information in time is also a challenge. Therefore as depicted in Figure 2-4, a compressed mode in UMTS system and a reliable active scanning (RAS) scheme in WLAN system provide the first level RF measurements for the vertical handover trigger.

Figure 2-4 Extra feature of vertical handover 2.3.1 Compressed mode

The compressed mode [15, 16] is employed to create a transmission gap in FDD mode for measuring the RF from other systems. By compressing the data stream during a few slots, a gap is created for measuring without having the disconnection of the UMTS call. The policies of the compressed mode and related power controls have been defined in UMTS specification [17-20]. During the compressed frame, more power is required to guarantee the quality of increasing transmission rate. Therefore, there exists a trade-off between handover

performance (sufficient information) and power consumptions. Several investigations are proposed [21-23]. In this vertical handover design, reducing the spread factor is used to simplify the operating environment and to provide a measured period for WLAN system.

2.3.2 Scanning

After getting the idle period in UMTS system, how to use this period to scan and to get sufficient information in WLAN systems is a challenge. According to the IEEE 802.11 standard, the scanning can be accomplished by using either a passive or active mode [24]. In the passive scanning mode, the mobile station (MS) moves to each channel on the channel list and waits for the beacon signal. In this mode, it could save the battery power but will take longer time to finish the measurements. In the active mode, a MS actively broadcasts additional probing request frames on the channel and expects to receive the response from the access point (AP), as a result, the active scanning mode is a fast way to collect the information and to reduce the time delay for handover. However, the efficiency of active scanning strongly depends on the whether the AP can successfully receive the request or not. A reliable active scanning (RAS) [25] which has a response detection and retransmission scheme is proposed.

Therefore, the RAS scheme will be assumed in this vertical handover designs.

2.4 Prior Works in Vertical Handover

Several aspects of vertical handover algorithms have been investigated. First, Stemm and Katz [26] introduced the vertical handover between different networks. A combination of mobility management and a virtual connectivity manager was used to maintain the connection

in the integrated system [27]. A policy-based handover algorithm was proposed by considering the tradeoff between network performance and individual requirements [28].

Another handover control mechanism which adopts the concept of a dwell timer was implemented to ensure the stability of the handover and the improvement of the mean throughput [29]. From network layers, based on Mobile IP [30], new protocols like Session Initiation Protocol (SIP) and Stream Control Transmission Protocol (SCTP) have been proposed to improve the delay and throughput performance between any heterogeneous wireless networks [31]. In this thesis, to address different service needs for both real-time and non-real-time services, a novel vertical handover control algorithm based on effective signal-to-noise (SNR) values and quality of service (QoS) requirements will be proposed.

Chapter 3

Dynamic Vertical Handover Control Algorithm

In the heterogeneous network, there are three design challenges: (1) no common pilot between networks (2) various QoS requirements and (3) ping-pong effects. To effectively support the vertical handover, three corresponding control elements are designed in the proposed vertical handover control algorithm: throughput-based mapping function, QoS-based dynamic handover threshold, and performance-based trigger timer. In this chapter, all challenges will be discussed in detail and the corresponding solutions will be proposed and analyzed.

3.1 Challenges

3.1.1 No common pilot between networks

In UMTS, a pilot channel is used as an indication for triggering a handover and power control [32]. More advanced handover controls based on aggregate pilot Ec/Io, average pilot Ec/Io, and etc for handover decisions are investigated [33-35]. However, in the integrated

system, the handover trigger in the WLAN will be different than the trigger in the UMTS. As compared to the pilot channel in UMTS, a beacon signal is used in the WLAN. The signal-to-noise ratio (SNR) has different indication of the achievable transmission rate and the coverage. Therefore, the decision of the vertical handover trigger becomes difficult.

3.1.2 QoS requirements

Quality of Services (QoS) has become an important requirement in wireless systems. The ultimate goal of QoS is to support “sufficient quality” for all services. Different services might have different definitions of “sufficient quality”. For example, in the non-real-time services, the QoS could depend on the transmission rate. On the other hand, in the real-time services, the QoS means the delay bound and a smooth video playout. In the wireless system, due to the time varying channel and competing resources, it could be more challenge to maintain QoS among users. Due to different system characteristics, the effort of maintaining the QoS in various systems becomes even harder when the integrated wireless system is considered.

3.1.3 Ping-pong effect

In a conventional handover, the ping-pong effect happens when the connection channel is not stable. In the unstable channel, any trigger of handover does not guarantee the improvement of the connection quality. In the integrated system, the ping-pong effect also exists in the vertical handover. This is due to the extended delay in handing over between two different systems as compared to two base stations within the same system. Therefore how to avoid the occurrence of ping-pong effect is also an important challenge in the integrated systems.

3.2 Throughput-Based Mapping Function

In the UMTS-WLAN integrated system, there is no common pilot to be used as an indication for the vertical handover trigger. Thus, a common reference should be defined to

differentiate the achievable performance. The performance in WLAN system could be observed from two perspectives: one is from user perspective and the other is from system perspective. In this section, the throughput-based mapping function from user perspective is first proposed, and then the general form of the mapping function is discussed by observing the performance from the system perspective.

3.2.1 Mapping function from user perspective

In different networks, the same signal-to-noise ratio (SNR) might have different throughput performances. Therefore a direct comparison of the SNR values will cause a misinterpretation of the resulting performance. To resolve the problem, an effective SNR, based on the same throughput reference, could be used. For example, as shown in Figure 3-1, based on a WLAN performance curve, a mapping between the achievable rates from UMTS and WLAN, RUMTS and RWLAN, and the corresponding effective SNR values, SUMTS and SWLAN

can be identified. In short, the original SNR is first used to estimate the achievable throughput which depends on the RF condition only. Through the mapping module, the corresponding effective SNR, SWLAN and SUMTS, are calculated based on a WLAN throughput performance curve. A dynamic vertical handover control algorithm can then be derived based on the effective SNR. Basically, the vertical handover is triggered when the differences between the S_WLAN and S_UMTS values exceed or drop below a dynamic threshold, H, for a period of time. As stated in Eq. (3-1), the downlink handover from UMTS to WLAN is triggered when the effective SNR from WLAN is greater than that of the UMTS by H for ∆T_downlink seconds.

From Eq. (3-2), for the WLAN to UMTS handover, the uplink handover is triggered when the

effective SNR from the UMTS is greater than that from the WLAN by H for ∆T_uplink seconds, where the threshold H and the trigger timer ∆T_{downlink/uplink} could be a function of QoS requirements and would be addressed in later section.

downlink downlink

UMTS

WLAN S H for T

S − > ∆ (3-1)

uplink uplink

UMTS

WLAN S H for T

S − < ∆ (3-2)

Figure 3-1. Throughput vs SNR in WLAN (including link adaptation)

3.2.2 The General form of the mapping function from system perspective

The throughput performance used in the section 3.2.1 is estimated by the individual measurements of signal strength (RF only). However the actual throughput differs a lot, especially in WLAN system. This is because the medium in WLAN is shared by many users;

Effective SNR Throughput

R^UMTS R^WLAN

S^UMTS S^WLAN S^UMTS S^WLAN R^WLAN

R^UMTS

(1)

(2)

collision and simultaneous backoff stages will occupy the medium and reduce the effective throughput. Furthermore, the throughput in WLAN will be saturated when the loading is high.

Thus, call admission control in WLAN is crucial to maintain the throughput and other QoS performances. In this section, the effect throughput, RE, calculating from the system perspective is used as the general form of the mapping function. In calculating effective throughput from the system perspective, it takes the transmission ratio and call admission control into considerations, the relationship between R and RE is:

R medium). Therefore the transmission probability Pt can be calculated in Eq (3-4)

c

Where P_s is the successful transmission probability with associated successful transmission time T_s, P_c is the collision probability with collision time T_c and E[I] is the average idle period.

The expressions of T_s andT_c in Eq. (3-5) and (3-6) are according to the Figure 3-2 diagram which applied the DCF scheme and RTS/CTS mechanism.

Figure 3-2. The expression of successful transmission and collision time

DIFS ACK

SIFS Payload

Header SIFS

CTS SIFS RTS

T_s = + + + + + + + + (3-5)

Timeout CTS

SIFS RTS

T_c = + + _ (3-6)

The remained work is to get the probabilities for all periods. Here a three dimension Markov-chain model [36-38] based on the number of users, the back-off window stages and process time is used to calculate the probabilities. Finally, substituting all parameters into Eq (3-4), the transmission ratio probability Pt can be calculated.

For the allowable incoming probability, Pa, it depends on the call admission control (CAC) in WLAN system. The main objective of CAC is to prevent channel overload and protect existing users. Therefore the allowable incoming probability, P_a, would be a conditional probability which depends on the new collision rate and the limits set for all existing users. If the new incoming user will cause the average collision rate exceed a

threshold, then CAC will block the incoming flow. To trace the new collision rate, a counter rate before admitting the new incoming user and α is the weighted factor which is designed by users’ experience (usually be 0.8). With this counter, the allowable incoming probability Pa

could be expressed as:

mapping flow is shown in Figure 3-3.

Figure 3-3. The mapping flow

3.3 QoS-Based Dynamic Handover Threshold

To reflect the benefit of the vertical handover in various aspects, besides the effective SNR values, the algorithm needs to consider dynamic thresholds and the associated timers to achieve the QoS requirements. In the proposed vertical handover algorithm, the dynamic threshold, H, will depend on service types of either non-real time services or real-time

services. For non-real-time services, the transmission packets come at a burst and are not sensitive to the delay. In this case, the user transmission rate becomes the first priority for the vertical handover. On the other hand, for real-time services, the services have a stringent requirement in the delay bound. Besides, for WLAN-to-UMTS uplink handover, since WLAN has a smaller coverage, the connectivity becomes essential in the uplink handover.

To achieve above design goals, the dynamic threshold is proposed as:

dB respectively. ∆ is the handover latency (the process delay for a vertical handover). The m, n, and k are values of 0 or 1 which will be decided based on following conditions:



Under this design, for non-real-time services, the throughput ratio provides additional weight on the threshold, H, calculations. This is because the effective SNR in some cases can’t accurately represent the absolutely throughput difference between UMTS and WLAN.

See Figure 3-1, the difference of the effective SNR between UMTS and WLAN represented in blue lines (1) is the same as the red lines (2), but the difference of throughput in red lines is much larger than blue lines’. The reason is that WLAN throughput may reach the saturation point no matter how larger SNR is. So the weighted factor for non-real time services is needed.

The dynamic threshold decreases when the throughput ratio of WLAN and UMTS becomes larger. This will make the UMTS-to-WLAN handover easier. Thus, a mobile with non-real-time services can achieve higher throughput. For real-time services, besides the over-air-error, the packet loss happens when the delay bound expires, the excessive handover delay,∆−D_i, will also cause the loss packets in the real-time services. With the allowable

number of lost packets Bi for application i, the number of lost packets due to the vertical handover, (∆−D )_i T_i (see Figure 3-4) should be less than Bi. If the resulting error

performance exceeds Bi, the effect of the degradation will be considered in the dynamic threshold, H. To emphasize the connection quality, the weighted factor,β , as expressed in Eq _i (3-9) is increased proportionally to the increase of the packet loss.

dB

Figure 3-4. The packet loss for real-time services in handover

Finally, a timer hysteresis,∆T_uplink, is considered to avoid the ping-pong effect in the conventional handover control. However since the coverage in WLAN is small, any excessive delay might result in the discontinuity of the connection. To resolve this potential problem, the dynamic threshold, H, will include (∆+∆T_uplink) ∆ for an earlier uplink handover trigger if the time hysteresis ∆T_uplink is larger. It can be seen that when the time hysteresis is larger, the dynamic handover threshold, H, would be larger. The result forces the early trigger for WLAN-to-UMTS handover, as show in Eq. (3-2).

3.4 Performance-Based Trigger Timer

In conventional handover algorithm, the objective of the trigger timer is to resolve the ping-pong effect [39, 40]. To calculate a proper trigger timer, a performance-based trigger timer, which the length of the trigger timer depends on whether the resulting performance can be improved after the handover, is calculated.

3.4.1 Downlink handover

From Eq. (3-12), the handover from UMTS to WLAN is worthwhile only when the user can transmit more data in WLAN than that in the UMTS after the handover process is finished. To calculate the downlink handover timer,∆T_downlink, it is assumed that the RE WLAN and RE UMTS are stable during this handover period. In this case, the timer∆T_downlinkis calculated from Eq.

The same argument can apply to the uplink handover timer calculation. The uplink handover is worthwhile when Eq. (3-14) is satisfied:

∫

The uplink handover timer,∆T_uplink, is calculated by Eq. (3-15).

UMTS E

WLAN E uplink

R r R

r

T =

−

≥ ∆

∆ ,

1 1

2 (3-15)

The handover timers are not fixed and will be updated based on the value r from time to time.

Figure 3-5. Trigger time functionality for downlink handover

3.5 Summary of Proposed Vertical Handover Control Algorithm

The proposed vertical handover control algorithm is composed by throughput-based mapping function, QoS-based dynamic handover threshold, and performance-based trigger timer. The procedures of the vertical handover control algorithm depicted in Figure 3-6:

1. Measures the signal strength and calculates the effective throughput from system perspective.

Time

∆ ∆

∆T ∆T

Throughput

0 R

^UMTS

R

^WLAN Ping pong effect

(compare throughput performance) handover

No handover

2. Uses the mapping function to get the effective SNR.

3. Applies different services with different QoS requirements to set the dynamic handover threshold and trigger timer for downlink and uplink handover.

4. Finally, trigger equations in Eq (3-1), (3-2) is used to make decision about the handover.

Measurement

Estimation (user perspective)

Calculation (system perspective)

Trigger timer

setting Mapping Threshold setting

Sw - Su > H for ∆Tdownlink Sw–Su < H for ∆Tuplink

services

SNR

R

RE

∆Tuplink,

∆Tdownlink SW, SU H

Yes, Handover

No, stay

Figure 3-6. The procedure of proposed vertical handover algorithm

Chapter 4

Mathematical Model and Numerical Analysis

In this chapter, a mathematical model is created to analyze the handover performance.

Simulation results and the analysis for the proposed vertical handover algorithm are also given

4.1 Mathematical Model

In this section, the performance of the handover frequency and average throughput is analyzed and the impacts from the path loss and shadow fading are considered. The fast fading will be ignored due to the averaging of the measurements. As calculated in Eq. (4-1) and (4-2), the signals (in dB) received at MS from UMTS and WLAN, are U (k) and W (k), respectively.

( ) ( )

dk u dk

K K k

U( )= ₁− ₂log + (4-1)

( )

(

w dk

)

v

(

w

( )

dk

)

K K k

W( )= ₃− ₄log + (4-2)

As shown in Figure 4-1, dk is the distance when the MS is d meters from UMTS at kth sample time and the functionw (dk) calculates the distance from WLAN when the mobile is d meters

from UMTS. K₁, K_2, K_3, and K₄ are parameters for the path loss. The shadow fading, u (d_k) and v (dk) are assumed to be independent and identically distributed stationary Gaussian random processes with zero mean and variance,σ . ²

Figure 4-1. UMTS and WLAN location

The received signal will be averaged by applying an exponential filter, implemented as a low-pass filter:

model with associated probabilities will be used for representing the behavior of the vertical

model with associated probabilities will be used for representing the behavior of the vertical

在文檔中 針對WLAN與UMTS的新穎異質切換控制演算法 (頁 20-0)