Class Test-1Mobile Computing (ECS-087)

Solution

Section-A

Q1. What are the pros and cons of having different size cells for wireless networking?Advantages of cellular systems with small cells are the following:

1. Higher capacity: Implementing SDM allows frequency reuse. If one transmitter is far away from another, i.e., outside the interference range, it can reuse the same frequencies. As most mobile phone systems assign frequencies to certain users (or certain hopping patterns), this frequency is blocked for other users. But frequencies are a scarce resource and, the number of concurrent users per cell is very limited. Huge cells do not allow for more users. On the contrary, they are limited to less possible users per km. This is also the reason for using very small cells in cities where many more people use mobile phones.

2. Less transmission power: While power aspects are not a big problem for base stations, they are indeed problematic for mobile stations. A receiver far away from a base station would need much more transmit power than the current few Watts. But energy is a serious problem for mobile handheld devices.

3. Local interference only: Having long distances between sender and receiver results in even more interference problems. With small cells, mobile stations and base stations only have to deal with ‘local’ interference.

4. Robustness: Cellular systems are decentralized and so, more robust against the failure of single components. If one antenna fails, this only influences communication within a small area.

Small cells also have some disadvantages:1. Infrastructure needed: Cellular systems need a complex infrastructure to connect all base

stations. This includes many antennas, switches for call forwarding, location registers to find a mobile station etc, which makes the whole system quite expensive.

2. Handover needed: The mobile station has to perform a handover when changing from one cell to another. Depending on the cell size and the speed of movement, this can happen quite often.

3. Frequency planning: To avoid interference between transmitters using the same frequencies, frequencies have to be distributed carefully. On the one hand, interference should be avoided, on the other, only a limited number of frequencies is available.

Q2. How GPRS is made possible over GSM?The GPRS architecture introduces two new network elements, which are called GPRS support nodes (GSN) and are in fact routers. All GSNs are integrated into the standard GSM architecture. The gateway GPRS support node (GGSN) is the interworking unit between the GPRS network and external packet data networks (PDN). This node contains routing information for GPRS users, performs address conversion, and tunnels data to a user via encapsulation. The GGSN is connected to external networks (e.g., IP or X.25) via the Gi interface and transfers packets to the SGSN via an IP-based GPRS

backbone network (Gn interface). The other new element is the serving GPRS support node (SGSN) which supports the MS via the Gb interface. The SGSN, for example, requests user addresses from the GPRS register (GR), keeps track of the individual MS’s location, is responsible for collecting billing information (e.g., counting bytes), and performs several security functions such as access control. The SGSN is connected to a BSC via frame relay and is basically on the same hierarchy level as an MSC. The GR, which is typically a part of the HLR, stores all GPRS-relevant data.

Q3. Define co-channel reuse ratio.For a hexagonal geometry

Q = D/R = √3 N By increasing Q, the spatial separation between co-channel cells relative to the

coverage distance of a cell is increased. Therefore, a large value of Q improves the transmission quality, due to a smaller level of co-channel interference.

A small value of Q provides larger capacity since the cluster size N is small. A trade-off must be made between these two objectives in actual cellular design.

Q4. What is the function of VLR?This represents a temporary data store, and generally there is one VLR per MSC. It contains information about the mobile subscribers who are currently in the service area covered by the MSC/VLR. VLR also contains information like call forwarding on busy. The temporary subscriber information resident in a VLR includes:

Temporary Mobile Subscriber Identity (TMSI). Features currently activated and available to the subscriber. Current location information about the MS (like, location area and cell identities).

Q5. What is the need of pilot channel?The pilot channel is intended to provide a reference signal for all MSS within a cell provides the phase reference for coherent demodulation.

Q6. Why did we choose hexagonal shape for cells in cellular phone system? Adjacent circles cannot be overlaid upon a map without leaving gaps or creating

overlapping regions. When considering geometric shapes which cover an entire region without overlap and with

equal area, there are three sensible choices: a square, an equilateral triangle, and a hexagon. A cell must be designed to serve the weakest mobiles within the footprint, and these are

typically located at the edge of the cell.- For a given distance between the centre of a polygon and its farthest

perimeter points, the hexagon has the largest area of the three.- By using the hexagon geometry, the fewest number of cells can cover a

geographic region Closely approximate a circular radiation pattern which would occur for an omni-directional

base station antenna and free space propagation. Permit easy and manageable analysis of a cellular system.

Section-B

Q1. How do the signaling sequences address the following features in GSM? (i) Location Updating (ii) Mobile call OriginationLocation Updating:

1. The MS sends a Location Update request to the VLR (new) via the BSC and MSC.2. The VLR (new) sends a Location Update message to the HLR serving the MS which includes

the address of the VLR (new) and IMSI of MS. This updating of the HLR is not required if the new LA is served by the same VLR as the old LA.

3. The service and security related data for the MS is downloaded to the new VLR.4. The MS is sent an acknowledgement of successful location update.5. The HLR requests the old VLR to delete data relating to the relocated MS.The signaling sequence is as follows:

Mobile Originated Call(MOC):

1. The MS transmits a request for a new connection.2. The BSS forwards this request to the MSC.

3,4. The MSC then checks if this user is allowed to set up a call with the requested service

and checks the availability of resources through the GSM network and into the PSTN. If all resources are available, the MSC sets up a connection between the MS and the fixed network.

In addition to the steps mentioned above, other messages are exchanged between an MS and BTS during connection setup (in either direction). These messages can be quite often heard in radios or badly shielded loudspeakers as crackling noise before the phone rings.

The signaling sequence is as follows:

Q2. Write short notes on the following: (i)Radio subsystem in GSM (ii) Security in GSM

Radio subsystem

As the name implies, the radio subsystem (RSS) comprises all radio specific entities, i.e., the mobile stations (MS) and the base station subsystem (BSS). Figure shows the connection between the RSS and the NSS via the A interface (solid lines) and the connection to the OSS via the O interface (dashed lines). The A interface is typically based on circuit-switched PCM-30 systems (2.048 Mbit/s), carrying up to 30 64 kbit/s connections, whereas the O interface uses the Signalling System No. 7 (SS7) based on X.25 carrying management data to/from the RSS.

Base Station Subsystem (BSS): A GSM network comprises many BSSs, each controlled by a base station controller (BSC). The BSS performs all functions necessary to maintain radio connections to an MS, coding/decoding of voice, and rate adaptation to/from the wireless network part. Besides a BSC,the BSS contains several BTSs.

Base Transceiver Station (BTS): A BTS comprises all radio equipment, i.e. antennas, signal processing, amplifiers necessary for radio transmission. A BTS can form a radio cell or, using sectorized antennas, several cells, and is connected to MS via the Um interface (ISDN U interface for mobile use), and to the BSC via the Abis interface. The Um interface contains all the mechanisms necessary for wireless transmission (TDMA, FDMA etc.) and will be discussed in more detail below. The Abis interface consists of 16 or 64 kbit/s connections. A GSM cell can measure between some 100 m and 35 km depending on the environment (buildings, open space, mountains.etc.) but also expected traffic.

Base Station Controller (BSC): The BSC basically manages the BTSs. It reserves radio frequencies, handles the handover from one BTS to another within the BSS, and performs paging of the MS. The BSC also multiplexes the radio channels onto the fixed network connections at the A interface.

Mobile Station (MS): The MS comprises all user equipment and software needed for communication with a GSM network. An MS consists of user independent hard- and software and of the subscriber identity module (SIM), which stores all user-specific data that is relevant to GSM. While an MS can be identified via the international mobile equipment identity (IMEI), a user can personalize any MS using his or her SIM, i.e., user-specific mechanisms like charging and authentication are based on the SIM, not on the device itself. Device-specific mechanisms, e.g., theft protection, use the device specific IMEI. Without the SIM, only emergency calls are possible. The SIM card contains many identifiers and tables, such as card-type, serial number, a list of subscribed services, a personal identity number (PIN), a PIN unblocking key (PUK), an authentication key Ki, and the international mobile subscriber identity (IMSI) (ETSI, 1991c).

Security in GSM:

GSM offers several security services using confidential information stored in the AuC and in the individual SIM (which is plugged into an arbitrary MS). The SIM stores personal, secret data and is protected with a PIN against unauthorized use.

The security services offered by GSM are explained below:

Access control and authentication: The first step includes the authentication of a valid user for the SIM. The user needs a secret PIN to access the SIM. The next step is the subscriber authentication.Confidentiality: All user-related data is encrypted. After authentication, BTS and MS apply encryption to voice, data, and signaling but it exists only between MS and BTS, and not end-to-end or within the whole fixed GSM/telephone network.Anonymity: To provide user anonymity, all data is encrypted before transmission, and user identifiers (which would reveal an identity) are not used over the air. Instead, GSM transmits a temporary identifier (TMSI), which is newly assigned by the VLR after each location update. Additionally, the VLR can change the TMSI at any time.

Three algorithms have been specified to provide security services in GSM.

Algorithm A3 is used for authentication, A5 for encryption, and A8 for the generation of a cipher key. In the GSM standard only algorithm A5 was publicly available, whereas A3 and A8 were secret, but standardized with open interfaces. Algorithms A3 and A8 (or their replacements) are located on the SIM and in the AuC and can be proprietary. Only A5 which is implemented in the devices has to be identical for all providers.

Authentication:

Before a subscriber can use any service from the GSM network, he or she must be authenticated. Authentication is based on the SIM, which stores the individual authentication key K i, the user identification IMSI, and the algorithm used for authentication A3. Authentication uses a challenge-response method:

The access control AC generates a random number RAND as challenge, and the SIM within the MS answers with SRES (signed response) as response (see Figure).

The AuC performs the basic generation of random values RAND, signed responses SRES, and cipher keys Kc for each IMSI, and then forwards this information to the HLR.

The current VLR requests the appropriate values for RAND, SRES, and Kc from the HLR. For authentication, the VLR sends the random value RAND to the SIM. Both sides, network and subscriber module, perform the same operation with RAND and the

key Ki, called A3. The MS sends back the SRES generated by the SIM. The VLR can now compare both values. If they are the same, the VLR accepts the

subscriber, otherwise the subscriber is rejected.

Encryption:

To ensure privacy, all messages containing user-related information are encrypted in GSM over the air interface. After authentication, MS and BSS can start using encryption by applying the cipher key Kc (the precise location of security functions for encryption, BTS and/or BSC are vendor dependent). Kc is generated using the individual key Ki and a random value by applying the algorithm A8. Note that the SIM in the MS and the network both calculate the same K c based on the random value RAND. The key Kc itself is not transmitted over the air interface.MS and BTS can now encrypt and decrypt data using the algorithm A5 and the cipher key K c. As Figure shows, Kc should be a 64 bit key – which is not very strong, but is at least a good protection against simple eavesdropping.

Section-C

Q1. In context to cellular network, discuss the following: (i) Cluster (ii) Frequency Reuse (iii) Cell Splitting (iv) Sectorization Cluster:

The total coverage area is divided into clusters. There can be no co-channel interference within a cluster. The number of cells in a cluster is called the cluster size. This number is denoted

by N. The N cells collectively use the complete set of available frequencies.

There are two possible models to create minimal interference: three cell cluster and seven cell cluster.

3-cell cluster 7-cell cluster

Frequency Reuse/ Reuse Distance: Cellular radio systems rely on an intelligent allocation and reuse

of channels throughout a coverage region. Each cellular base station is allocated a group of radio channels to be used within a small geographic area called a cell. Base stations in adjacent cells are assigned channel groups which contain completely different channels than neighboring cells. The base station antennas are designed to achieve the desired coverage within the particular cell. By limiting the coverage area within the boundaries of a cell, the same group of channel may be used to cover different cells that are separated from one another by distances large enough to keep interference levels within tolerable limits. The design process of selecting and allocating channel groups for all of the cellular base stations within a system is called frequency reuse or frequency planning.(see Figure-1)

Figure-1

In hexagonal geometry, there are six neighbors of each cell and the line joining the centers of any cell and each of its neighbors are separated by 60 degrees. This restricts the number of usable cluster sizes and their layouts. In order to tessellate-to connect cells without gap-the number of cells per cluster, N, can only have values, which satisfy the following equation:

The frequency reuse factor is given by 1/ N.To locate the nearest co-channel neighbors of a particular cell, one must do the following:

move i cells along any chain of hexagons and then turn 60 degrees counter-clockwise and move j cells. (see Figure-2)

For example, the GSM uses 3 or 4.

N=i2 + i j + j2

where i and j are non-negative integers and N=1,3,4,7,9,12,13,16,19,21.

Figure-2 a

Figure-2 b

Cell Splitting:

Cell splitting is the process of subdividing a congested cell into smaller cells (called microcells), each with its own base station and a corresponding reduction in antenna height and transmitter power. Splitting the cell reduces the cell size and thus more number of cells have to be used For the new cells to be smaller in size the transmit power of these cells must be reduced. More number of cells more number of clustersmore channels high capacity The new cell radius = old cell radius/2. It can be permanent and dynamic.

Center-to-center distance between closest co-channels (interfering cells)

Also called reuse distance.

Reuse factor is q = D/R = √3 N

Power Issues:

Suppose the cell radius (R) of cells is reduced by half then what is the required transmit power for these new cells? Pr[at old cell boundary] α Pt1R-n Pr[at new cell boundary] α Pt2(R/2)-n where Pt1and Pt2 are the transmit powers of the larger and smaller cell base stations respectively, and n is the path loss exponent. So, Pt2= Pt1/2n

In practice not all the cells are split at the same time. This means that different size cells will exist simultaneously.

In such situations, special care needs to be taken to keep the distance between co-channel cells at the required minimum, and hence channel assignments become more complicated.

When there are two cell sizes in the same region, one cannot use original transmit power for all new cells or the new transmit power for all original cells

Larger transmit power for allsome channels used by smaller cells would not be sufficiently separated from co-channel cells

Smaller transmit power for allsome parts of larger cells left un-served Channels in the old cell must be broken down into two channel groups, one for smaller cell and

other for larger cell The larger cell is usually dedicated to high speed traffic so that handoffs occur less frequently

Two channel group sizes depend on the stage of splitting process: At the beginning of splitting process, there will be fewer channels in small power groups With increasing demand, smaller groups will require more groups Splitting continues until all channels in area are used in lower power group Entire system by that time is rescaled to have smaller radius per cell Antenna down tilting is used to focus energy from BS toward ground, to limit radio coverage of

newly formed microcells

Large Cell is also called Macro cell, and similarly, Small as Micro and Smaller as Pico cell.

Sectoring: As opposed to cell splitting, where D/R is kept constant while decreasing R, sectoring keeps R

untouched and reduces the D/R Capacity improvement is achieved by reducing the number of cells per cluster, thus increasing

frequency reuse In this approach first SIR is improved using directional antennas The CCI may be decreased by replacing the single omni-directional antenna by several

directional antennas, each radiating within a specified sector The factor by which the co-channel interference is reduced depends on the amount of sectoring

used. A cell is normally partitioned into three 120º sectors or six 60º sectors as shown in Figure-3.

Figure-3 120 degree sectoring 60 degree sectoring

Problems with Sectoring: Increases the number of antennas at each BS Decrease in trunking efficiency due to sectoring(dividing the bigger pool of channels into

smaller groups) Increase number of handoffs(sector-to sector) Trunking Efficiency:

It is a measure of the number of users which can be offered a particular Grade of Service (GOS) using fixed number of channels

Q2. What are the different handoff detection strategies? Discuss different types of handoff with reference to network. Handoff or Handover refers to a process of transferring an ongoing call or data session from one channel connected to the core network to another. In other words, it is the process of transferring a MS from one base station to another. However, a handover should not cause a cut-off, also called call drop. GSM aims at maximum handover duration of 60 ms.Reasons for a Handoff to be conducted:

To avoid call termination: call drops When the capacity for connecting new calls of a given cell is used up(load Balancing) Interference in the channels. When the user behaviors change (Speed and mobility).

Figure-4 shows four possible handover scenarios in GSM:

Figure-4 Types of Handoff in GSM Intra-cell handover: Within a cell, narrow-band interference could make transmission at a

certain frequency impossible. The BSC could then decide to change the carrier frequency (scenario 1).

Inter-cell, intra-BSC handover: This is a typical handover scenario. The mobile station moves from one cell to another, but stays within the control of the same BSC. The BSC then performs a handover, assigns a new radio channel in the new cell and releases the old one (scenario 2).

Inter-BSC, intra-MSC handover: As a BSC only controls a limited number of cells; GSM also has to perform handovers between cells controlled by different BSCs. This handover then has to be controlled by the MSC (scenario 3). This situation is also shown in Figure- 5.

Inter MSC handover: A handover could be required between two cells belonging to different MSCs. Now both MSCs perform the handover together (scenario 4).

Figure-5

The MS sends its periodic measurements reports, the BTSold forwards these reports to the BSCold

together with its own measurements.

Based on these values and, e.g., on current traffic conditions, the BSCold may decide to perform a handover and sends the message HO_required to the MSC.

The task of the MSC then comprises the request of the resources needed for the handover from the new BSC, BSCnew.

This BSC checks if enough resources (typically frequencies or time slots) are available and activates a physical channel at the BTSnew to prepare for the arrival of the MS.

The BTSnew acknowledges the successful channel activation, BSCnew acknowledges the handover request.

The MSC then issues a handover command that is forwarded to the MS. The MS now breaks its old radio link and accesses the new BTS.

The next steps include the establishment of the link (this includes layer two link establishment and handover complete messages from the MS).

Basically, the MS has then finished the handover, but it is important to release the resources at the old BSC and BTS and to signal the successful handover using the handover and clear complete messages as shown.

Beside all these four types of hand-off they can also be divided into hard and soft handoffs.Hard Handoff:

It is defines as “break before make” connection (Intra and inter-cell handoffs, see Figure-6).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

A major problem with this approach to handoff decision is that the received signals of both base stations often fluctuate. When the mobile is between the base stations, the effect is to cause the mobile to wildly switch links with either base station. The base stations bounce the link with the mobile back and forth. This phenomenon is called, “Ping Ponging”.

The solution is to allow MS continue maintain a radio link with the current BS, until the signal strength from the new BS exceeds that of current BS by some pre-specified threshold value.

Soft Handoff:It is defined as “make-before-break” connection (Mobile directed handoff, see Figure-7). With soft handoff, a conditional decision is made on whether to hand off. Depending on the changes in pilot signal strength from the two or more base stations involved, a hard decision will

eventually be made to communicate with only one. This normally happens after it is clear that the signal from one base station is considerably stronger than those from the others. In the interim period, the user has simultaneous traffic channel communication with all candidate base stations. It is used in CDMA.The difference between hard and soft handoffs is like the difference between swimming relay events and track-and-field relay events. In swimming relays, the next swimmer starts just as the preceding one touches the wall, analogous to the switch from one base station to another in a hard handoff. In track-and-field relays, the baton is passed from one runner to the next after the second runner starts running, and so for a short time they are both running together, analogous to a soft handoff.

Figure-7

In both soft and hard handoffs, there will normally be some simultaneous control channel communication between the two base stations and the user according to the signaling protocol in use, so we must look at traffic channels to distinguish between hard and soft handoffs.

Handoff can also be between two channels at one base station. If the two channels are in two sectors of a sectorized cell, and the kind of handoff used is soft, this is sometimes known as “softer handoff’’.

The following flow graph represents the overall process/procedure of Handover/hand-off.