Significant Technical Differences In Mobile Communication Computer Science Essay

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The Fourth Generation (4G) technology candidates is foreseen most likely to be between Long Term Evolution Advanced (LTE-Advanced) proposed by the Third Generation Partnership Project (3GPP) and Mobile WiMAX using 802.16m standardised by the Institute of Electrical and Electronics Engineers (IEEE). Distinguish the technical differences between these two technologies and give your opinion on the scenario and deployment of 4G in the near future.

Significant Technical Differences

1. LTE and WiMaX employ the orthogonal frequency division multiple access (OFDMA) in the downlink. But WiMax optimizes for highest channel handling by developing all the information in a wide channel. LTE, in contrast, arranges the available spectrum into smaller portions. WiMax pays a charge for high channel utilization, however, because developing that much information might involve a 1,000-point fast Fourier transform. LTE can obtain by with a 16-point FFT. This interprets into higher power consumption, because it's complicated to design fixed-function WiMax hardware that is also resourceful in LTE designs.

2. LTE applies single-carrier frequency division multiple access (SC-FDMA)[11] for uplink signaling, whilst WiMax sticks on OFDMA.

3. Even though both the IEEE 802.13e standard and the evolving LTE standard hold up frequency division duplexing(FDD) and time division duplexing(TDD), WiMax applications are principally TDD. LTE seems to be going in the FDD direction because it is right full-duplex operation: Adjacent channels are applied for uplink and downlink. LTE can therefore estimate a better spec for downlink data rates, though at a cost of placing very severe latency conditions for forward error correction. The main thing is that the WiMax radio is much uncomplicated.

At the end of 2007, the overall mobile users were 3 billion, with GSM based users to raise over 2 billion. Some research reports are expecting that the WiMAX will viably be implemented by 2009 and the LTE (Long Term Evolution) by 2015. Nevertheless, the standards conflict towards the 4G launch is a major concern. ITU and IEEE are attempting to secure a smooth conversion into the new technology. (Figure 1)

Figure 1: Smooth conversion into the new technology

There are various methods that can be used to increase user and system capacity such as by using new multiplexing scheme with enhanced modulation schemes. Tabulate and summarize some important features related to the multiplexing and modulation techniques for analog system (e.g. ETACS, AMPS) and digital (e.g. GSM, GPRS, EDGE, HSPA, LTE, WiMAX) system. Among them are FDMA, TDMA, CDMA, SDMA, etc. for multiplexing and FM, GMSK, 8-PSK, QPSK, OFDM, MIMO, Adaptive Modulation and Adaptive Modulation and Coding (AMC).

Multiplexing

Modulation

Analog

ETACS

Frequency Division Multiple Access (FDMA)

Frequency Modulation(FM)

AMPS

Frequency Division Multiple Access(FDMA)

Frequency Moduation(FM)

Digital

GSM

Time Division Multiple Access(TDMA)

Gaussian Minimum Shift Keying (GMSK)

GPRS

Time Division Multiple Access(TDMA)

Gaussian minimum-shift keying (GMSK)

EDGE

Time Division Multiple Access(TDMA)

8 Phase Shift Keying (8PSK)

HSPA

Code Division Mutiple Access(CDMA)

16QAM

LTE

Multiple Input Multiple Output (MIMO)

Orthogonal Frequency Division Multiple Access(OFDMA)

WIMAX

SDMA

Orthogonal Frequency Division Multiple Access(OFDMA)

Table 1: Multiplexing and modulation technique for analog and digital system

The public cellular service operator in Malaysia are subjected to mandatory standards for Quality of Service (QoS) or Grade of Service (GOS) by Malaysian Communication and Multimedia Commission (MCMC). List various parameters and schemes used for providing QoS/GOS in cellular network and discuss their advantages/disadvantages to the subscribers and operators. How can QoS provisioning be managed in the future 4G cellular network?

There are difference schemes of QoS in cellular netwoks:

1.0 Fault Tolerant Dynamic Channel Allocation Scheme [1]

In this scheme the channels are assigned dynamically based on requirement and therefore increase the channel utilization as well as the quality of service (QoS). The channel allocation schemes are unified and distributed. In a unified approach, the central controller is in charge for channel allocation and the it will send request to the controller while in distributed approach, a Mobile Service Station (MSS) is exist in each cell which handles the channel allocation for that particular cell. Distributed approach is more scalable and trustworthy [1]. Therefore, algorithm is used in this approach.

Figure 1: A Model of Wireless Communication Networks [1]

Figure 1 shows a wireless communication model. Many cells consist Mobile Hosts (MH) and a Mobile Service Station (MSS). Each cell is connected to a fixed network. Each MH when it wants to communicate with another mobile host has to request a channel from MSS for communication [3].In communication, there are communication channel and control channel [3]. A communication between a MH and a MSS is supported by communication channel while control channel is employed for sending control messages like MH requesting the MSS [3] in order to set a communication channel. The channels are assigned by MSS to the MH to see if there is co-channel interference occurs. A channel for communication is allocated to the MH if the co-channel interference does not exist.

There two different approaches for requiring channels. In Centralized approach, Mobile Switching Center (MSC) will be used to provide channels for each MH if MH asked for a channel from the central controller. Hence, the MSC will prepare for channel allocation in condition that no co-channel interference exists. In this approach, the MSC might become a bottleneck in the network [3] and the whole network will be affected if the MSC experiences break down. This is the drawbacks of the Centralized approach. Hence this approach is not scalable and robust [3].

There is no central controller and each cell has its own MSS in the Distributed approach [3] .The neighbors of a cell will receive an update about channel utilization by the cell and notifies them when to quit the channel so that others can use it. A cell can use a channel right after getting a reply from its entire neighbors informing the cell that the channel is ready to be used.

1.1 System Model [3]

Channels are assigned dynamically and not assigned in advance to any cells. A cluster model is employed where three cells are used with each cell has six neighbors. If a cell uses a channel, its neighbor cannot use the same channel in order to avoid co-channel interference. If the cell does not have any channel, it can borrow a channel with a condition that the cell's neighbors satisfied its request and also ensure that co-channel interference does not exist. There are two types of operation mode which are update mode and search mode[3]. A notification of a cell about its channel utilization information to all its neighbors is known as update mode. A cell is able to send an update about its status to its entire neighbors by using this mode. In addition, a cell can send a request message to its neighbor if it wants to borrow a channel and this type of operation is called search mode.

1.2 Distributed Channel Allocation Algorithm [3]

In this model, three cell cluster model and each cell has six neighbors and it has a unique neighbor id ordered from one to six[3]. Channels are not pre-assigned to any cell. The channel with lowest frequency has the least order and the channel with highest frequency has the highest order[3]. A cell Ci selects a channel with the highest order to support a call if it is available[3].

Figure 2: Cell Cluster Model to Illustrate Distributed Channel Allocation[3]

In figure 2 each cell is numbered from 1 to 6 and we are going to use cell Ci to indicate a particular cell. When Ci ask for a channel to use, it is in search mode and it's called a borrower[3]. It sends a broadcast message to all its neighbors for the channel and sets a timer and remains till the timer times out. When a cell is in borrower mode it ignores to any query from any other cell asking for channels. This is because the borrower itself is in seek for channels and so it's not going to be of any help to other channels which are requesting. This scheme is good in the way that it avoids unnecessary congestion in the network[3]. Now after the timer times out, the cell Ci will borrow a channel based on the replies which it got from its neighbors[3]..

To verify that this algorithm works lets assume that the cell Ci has got replies only from 2 neighbors, 1 and 4 and there is a channel r which is assigned to both the cells 1 and 4. In this case the cell Ci can borrow the channel r even though other cells (2,3,5,6) have not responded other than cells 1 and 4[3].. The reason is that since channel r is assigned to cells 1 and 4, it cannot be assigned to the neighbors of cells 1 and 4 (cells 2, 6, 3 and 5) due to co - channel interference[3].. Thus channel Ci can borrow the channel with the responds it got from just two channels. This confirms that this algorithm is fault tolerant[3].

As a result the above algorithm which we saw was fault tolerant as it need not wait for[3] responds from all the channels. This also indicates that the algorithm is more scalable. It employs distributed channel allocation scheme in which channels are allocated dynamically based on request. This algorithm also lets reuse of channels. This implies that a lender can lend a channel to two or more cells simultaneously offered that no co-channel interference occurs[3].

2.0 Call Admission Control (CAC) Scheme

In the CAC algorithm new call arrival rates are estimated continuously and if they are higher than a predetermined level some calls are blocked irrespective of whether a channel is available or not[3].. The goal of this scheme is to retain the new call arrival rate lesser than a predetermined level. In this scheme a comparison is done with the existing two schemes namely prerequest scheme and the guard channel scheme and various benefits and drawbacks are given for the two schemes and then a CAC algorithm is developed which offers a better QoS than the existing two schemes. The two metrics used for QoS in this algorithm are Forced Termination Probability (FTP) which is named as the ratio of the number of calls which are forced to terminate because of failed handoff to the number of calls that successfully penetrated the network. Another metric is the Successful Call Completion Rate (SCCR) which is named as the number of calls which are finished successfully in a unit time by each cell [3]. So lower FTP and higher SCCR is what ideal algorithms will try to get and this algorithm achieves that.

2.1 Channel Prioritization Schemes

In the Guard Channel Scheme several channels are exclusively stored for the handoff calls and these channels are called guard channels. A certain number of channels say G out of C channels are exclusively stored for handoff calls of the profiled users. The remaining channels are divided for all types of calls. These calls include new calls, handoff calls of profiled users and handoff calls of non profiled users. This is an illustration of static allocation of channels.

In the channel prerequest scheme the channels are ask for beforehand before the handoff occurs. The information about the mobility patterns of the profiled users is in the Home Location Register (HLR) and employing this information so that the handoff activities of the profiled users can be predicted. The prerequest channels are asking the neighboring cell for a certain amount of time called as the reservation period. By rising the reservation period, the chances of forced termination can be significantly decreased.

2.2 Call Admission Algorithm (CAC)

Figure 3: Flow Chart for CAC Algorithm [3]

In the CAC algorithm the adequate load is calculated based on simulation outcomes and this value is used for evaluation purpose. The estimated load is also calculated and it is verified with the acceptable load. If the estimated load is lower than or equal to the satisfactory load, then attempts are made to assign channels for all the incoming calls. If the estimated load is higher than the satisfactory load then only a fraction of the incoming calls will be assigned channels and the remaining fraction of the calls will be removed even if there are available channels. This is called pre-blocking of channels and this scheme develops the FTP and SCCR of the profiled users.

3.0 QoS based on Mobility Prediction Techniques [4]

Mobile prediction techniques are used to find the path or the trajectory of a mobile node and it is reserved in a database from time to time. This technique helps in storing resources for MH before a hand off occurs so prioritizing resources takes place for a node before its hand off and therefore this reduces the call blocking rate at hand offs. However the new call access rate is diminished as more resources are stored for hand off calls. Forced termination can be decreased by increasing the number of new call blocking probability. But this is not very resourceful utilization of the radio resources. By using mobile prediction methods, we can know in advance when a handoff will take place and so dynamic allocation of resources for the handoff can be done. This makes sure that the new call blocking probability is not raised so much to keep resources for the hand off calls. There are numerous mobile prediction techniques which have been utilized in the past which includes GPS positioning techniques in which every MH is integrated with a GPS receiver so that the path of the MH can be traced.

3.1 Road Topology based Mobile Prediction Techniques [4]

This technique is based on the fact that MH's in vehicles will come across the most frequent hand offs and so studying their characteristics would verify to be more beneficial. Since the vehicles travel on the road, the road topology is considered and is used in the prediction algorithm. In this scheme the base station will do the mobile prediction based on the road topology information it has. Since the base station has more storage capacity and resources than the MH, it is used for better precision.

Figure 4: Road Topology Information for Mobility Prediction [4]

In this technique there are some base stations, which are updated with the MH's most recent position at standard interval of time. Each base station will preserve a database which includes information about the road topology. As shown in figure 5 the road which consists of bends is busted into many piecewise linear line sections and the coordinates for these line sections are stored separately. The database has information such as average time to transit a segment, neighboring segments at each junction and the probability of the MH to do a hand off and go to a next neighboring section. The database is restructured periodically and at every instant the information about the location of the MH is obtained. Segments which have already had hand offs are referred to as handoff feasible segments. Using this model, precise prediction is done, which is practical in prioritizing resources for the handoff of an MH.

4.0 Dynamic Allocation Scheme using Renegotiation [5]

In a Dynamic Allocation scheme using Renegotiation, the unemployed resources of the network are searched and they are allocated to services which got a smaller bandwidth at the time of access when they actually wanted more. In other words, renegotiation of the bandwidth is done for a lesser priority service when the medium is free, thus rising the entire bandwidth of the lower priority services. The scheme on the other hand also keeps the bandwidth of the higher priority sources.

4.1 Different Service Classes

Conversational and streaming classes are correlated with real time traffic and are severely delay sensitive. Examples of these types of services are video telephony, telnet, voice and video. In contrast, Interactive and background procedures are less delay sensitive and they comprise applications like FTP, WWW, email and news. Since they are less sensitive to delay both these classes give better error rates via channel coding techniques. The discrepancy between the interactive and the background services are that interactive services are mostly used by interactive applications like interactive email. Background classes are used for background purposes which include background browsing and background emails.

4.2 Renegotiation Scheme [5]

In this scheme the conversational classes are given the highest priority and they are priority 1 class. The streaming class is also set high priority and it belongs to priority 2 classes. Now these priority 1 and priority 2 classes will be admitted only if there are enough resources (bandwidth) to fulfill their requirements. If not their requests won't be acknowledged. The interactive service is given the lowest priority and it is 3. So the requirements from these classes are admitted even if the networks have a smaller bandwidth than what they had required. The benefit of the Renegotiation scheme when compared to the CAC scheme is that in the CAC scheme when a bandwidth is allocated to priority 1 and 2 they cannot be relocated to a lower priority class even after the higher priority class leaves the network. In renegotiation scheme a priority 3 application can use more bandwidth that what was assigned for it. This is made possible because any unemployed resources by the higher priority classes can be relocated to this class. On the other hand when a high priority class arrives again it will not be blocked if the system has low resources. At that time the bandwidth given to a lower priority class will be taken back and will be allocated to the high priority class. Consequently the higher priority class is not harmed in any way.

4.3 Renegotiation by Flow Termination

The renegotiation by flow termination is a means by which more bandwidth is allocated for a low priority flow when a high priority flow ends or when there are more resources available in the network [5].

Figure 5: Behavior of Two Flows Without (a) and With (b) Renegotiation [5]

In figure 5 behavior of two different flows are shown without and with renegotiation. In case (a) without renegotiation the bandwidths of A and B are shown. Even after the termination of flow A the bandwidth of B remains the same. In case (b) after the termination of flow A the bandwidth of flow B is increased due to the availability of resources [5].

SATELLITE COMMUNICATION

The Very Small Aperture Terminal, VSAT service is becoming more popular in Malaysia. Obtain information on VSAT service operator in Malaysia including examples of application, network topology, user equipments, lease procedure and services offered and their data rate. How can a VSAT system accommodate subscriber that need higher data rate services.

VSAT Applications

VSAT is an ideal satellite network that offers communications support for a wide range of applications [10]:

Point-of-sales transaction

Order-Entry Billing

Inventory Control

Financial Management[10]

Data processing[10]

Reservation System[10]

Telemetry & Data Collection[10]

News Wire Services[10]

Private-Line Voice[10]

Virtual Private Networks[10]

Distance Education[10]

High Speed Internet Access[10]

VSAT Topology

The VSAT system has two types of topologies: Star Topology and Mesh Topology

1.0 Star Topology

The star topology is the conventional VSAT network topology. The remote terminal is connected to the hub for communication. It suitable to used this topology for data collection or broadcasting. The hub station (double hop) is used to communicate between the remote and this makes star topology impractical to offer speech services between the terminals because time delay in the hub station (500 ms) is high. The digital carriers provide connection on the space segment in both directions, which organized with a variety of access schemes. The access techniques used in a star network can be Frequency Division Multiple Access (FDMA)[9], Time Division Multiple Access (TDMA)[9], and Code Division Multiple Access (CDMA), but TDMA is the most common.

2.0 Mesh Topology

In mesh topology, there is direct communication between the remote VSAT terminals. This direct communication reduces the time delay. The signaling processor is placed in the central node, hence this internal signaling is using star topology and this is often referred to as the demand assignment multiple access (DAMA). While in a mesh network, it is typically for Frequency Division Multiple Access (FDMA).

Figure 1: VSAT system topologies: star topology and mesh topology. Mesh topology offers direct connectivity between the remote VSAT terminals [6]

End User Equipment for satellite Internet access[12]

The end user equipment consists of the VSAT dish, BUC LNB and the Modem. Depending on where the equipment is placed, they are classified as indoor or outdoor [12].

1.0 The indoor unit[12]

The IDU is normally a single box satellite Internet modem which is linked to the customer's computer equipment.

1.1 Satellite Modem

A satellite modem or sat modem is used to set up data transfers using a communications satellite as a relay[12]. The VSAT Systems Indoor Unit (IDU) modem has onboard TCP optimization and QoS (Quality of Service) capabilities, router and DNS. The unit is centrally controlled from a Network Operations Center (NOC)[12].

The IDU is compatible with Ku and C band service as well as Ka band service when it becomes available[12].

2.0 The outdoor unit[12]

2.1 VSAT Satellite dish

The satellite dish which is in parabolic shape is mounted on the roof. It receives the Ku band RF analog signals from the geostationary satellite 23,000 miles in the sky and passes it on to the modem. The dish sizes range between .96m to 2.8m.[12]

VSAT networks in general operate at one of three frequencies: C band, Ku band, or Ka band. C band operation needs a larger satellite dish due to its lesser frequency; however, it suffers less from rain attenuation. Ku band dishes are smaller in size and more vulnerable to rain-fade than C band dishes. On the other hand, Ku band dishes work fine even during rain fade scenarios if the modems are set with power boosting capabilities. Ka-band-based networks are able to use even smaller dish sizes, however, the problem of rain attenuation is exaggerated further at the higher frequency operations such as Ka.[12]

Larger dishes provide for better reception in remote areas and support much higher bandwidth service plans. The dishes can be stationary, mobile or portable.

2.2 Block up Converter[12]

Block up converter (BUC) is employed for the uplink of satellite signals. It works by switching a 'block' (band) of frequencies from a lower frequency to a higher frequency. As the distances to be traveled by the signal for transmission are tremendous, higher frequencies are used as they suffer less attenuation. BUCs convert the L band to a Ku band and a C band to a Ka band[12].

For most applications, BUCs have a rating between 3-6 Watts. For other high powered applications, BUCs with larger ratings are available [6].

2.3 Low Noise Block Converters (LNB)

BUCs are generally used in conjunction with Low Noise Block Converters (LNB). The BUCs make up the 'transmitting' section of the equipment whereas LNBs are down-converters, the receive section of the equipment. The receive signal needs to be down converted because Ku band signals received from the satellite are high frequency signals which suffer greater attenuation while passing through cables used for indoor transmission.[12]

VSAT Services [9]

i. Interactive real time application[9]:

- Point of Sale/retail/Banking (eg. ATM) [9]

- Corporate data[9]

ii. Telephony[9]

- Rural: individual subscribers[9]

- Corporate Telephony[9]

iii. Intranet, Internet and IP infrastructure[9]

- Multimedia delivery (eg. video streaming) [9]

- Interactive distance learning/ training[9]

iv. Direct-to-home[9]

Broadband Internet access for consumers and businesses[9]

VSAT data rates

The list of data rates (in Kbps) is as below;

  1. 1.2 up to 56

    6. 512

  2. 64

    7. 768

  3. 128

    8. 1024

  4. 256

    9. 1536

  5. 384

  10. 2048

How can a VSAT system accommodate subscriber that need higher data rate services.

VSAT networks give a way to set up a switched point to point data network that can have the capability for high data rates of up to 2Mb/s. Connections are set up directly between remote terminals usually on a call by call basis. These networks are generally configured to operate without a large central earth station and carry a mix of data traffic and telephony traffic or only data traffic.

Direct Broadcasting Satellite, DBS service such as ASTRO is becoming more popular. Investigate and research the multiplexing method each TV channels before the channels can be distributed and shown to viewer. How can ASTRO solve the problem of signal impairment and degradation due to heavy rain, and interference from WiMAX operators?

In digital television and radio system, some variable bit-rate data streams are multiplexed together to a fixed bitrate transport stream by means of statistical multiplexing. This makes it possible to transfer several video and audio channels simultaneously over the same frequency channel, together with various services [6].

In the digital television systems, this may involve several standard definition television (SDTV) programmes (particularly on DVB-T, DVB-S2, ISDB and ATSC-C), or one HDTV, probably with a single SDTV companion channel over one 6 to 8 MHz-wide TV channel [6]. The device that achieves this is called a statistical multiplexer. In some of these systems, the multiplexing results in an MPEG transport stream. The newer DVB standards DVB-S2 and DVB-T2 have the ability to carry several HDTV channels in one multiplex. Even the original DVB standards can carry more HDTV channels in a multiplex if the most advanced MPEG-4 compressions hardware is used.

On communications satellites which carry broadcast television and radio network, this is known as multiple channels per carrier or MCPC [6]. Where multiplexing is not realistic (such as where there are different sources using a single transponder), single channel per carrier mode is employed.

Signal multiplexing of satellite TV and radio channels is usually carried out in a central signal playout and uplink centre, such as Astro in Malaysia, which offers playout, digital archiving, encryption, and satellite uplinks, as well as multiplexing, for some of digital TV and radio channels.

How can ASTRO solve the problem of signal impairment and degradation due to heavy rain, and interference from WiMAX operators

The direct-to-home (DTH), is broadcast as high-power Ku-band transmissions using the transponders of the MEASAT satellite system. Reception of the service signals utilizes a fixed 60-cm diameter dish antenna.

Ku-band signals can be influenced by rain attenuation (rain fade), making it vulnerable to frequent outages in heavy rainfall areas such as Malaysia, the operators of Astro have utilized a tailor-made system to increase power of the satellite delivery system to overcome this problem. The service availability of 99.7%, however, is still a issue of much debate by subscribers.

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