Wireless địa phương vòng - lý thuyết và ứng dụng P7 pps

An analysis is performed for a single cell scenario offering only one service class.The approach is then adopted for a single radio cell with multiple service classes andfinally for a mu

a simulator in chapter by S V Krishnamurthy et al [8] With this approach it is possible

to adjust different scenarios and system parameters in order to find the best capacityutilization of the system The CDMA system is investigated analytically in this chapter It

is clear that in performing an analytical calculation certain simplifications and tions will need to be made

assump-The structure of this chapter is as follows

In the next section the FWA network is explained in more detail Apart from thetechnological description of this access scheme, the section will also examine the economicviability The impact of the FWA network in developed and developing countries isinvestigated and a prognosis is made on the future market possibilities of FWA networks.Following is a brief overview of multiple access schemes

A derivation for capacity equations of a CDMA system is contained in the followingsection An analysis is performed for a single cell scenario offering only one service class.The approach is then adopted for a single radio cell with multiple service classes andfinally for a multiple radio cell environment with multiple service classes

A comparison of the capacity result for TDMA and CDMA is presented at the end ofthis chapter It is shown how the expected capacity for both technologies can be esti-mated

141

Copyright # 2001 John Wiley & Sons Ltd ISBNs: 0±471±49846±7 (Hardback); 0±470±84187±7 (Electronic)

7.2 Fixed Wireless Access Networks

The accessing of telecommunication services such as telephone, fax and Internet is takenfor granted in the developed countries [6,11] It is therefore surprising to note that theworld average teledensity (number of telephone lines per hundred people) is less than

10 % In fact, almost half of the world's population has never made a phone call Thedemand for communication is driven not only by business alliances and exchanges butalso through personal relations like friends and relatives that live around the globe Thisrevolution in communication requirement is abetted by three major forces Computingpower increases while the costs of providing this power are reduced through economies ofscale Secondly the cost of providing transmission of information has fallen by a factor of

10 000 over the last 20 years Finally the convergence of telecommunications and ing have pushed the merging of segmented industries into a large information industry.The world information technology market which includes products such as personalcomputers, mobile phones, and communication has grown by 12.2 % between 1985 and

comput-1995 This is a growth five times faster than the average world Gross Domestic Product(GDP) [19]

It is without a doubt established that delivering telecommunication is akin to deliveringknowledge For developing countries delivering knowledge can mean fighting illiteracyand poverty Therefore, especially these countries need to increase their teledensity TheInternational Telecommunications Union (ITU) recommends that the teledensity of anation should be at least 20 % so that economic growth is not hampered by the lack oftelecommunications

Wireless access systems provide a suitable method of providing this access to munication services Currently wireless telephony is experiencing a tremendous growth forthe last 10 years with the number of subscribers globally estimated at 55 million peopleuntil mid 1995 [7] Most of this usage is for mobile communications

telecom-The prediction of which access technique will have the greatest impact must be based onthe current tariff ideology The tariff structure of telephone calls does not provide re-semblance to the real cost involved The highest costs are incurred in the local loop and isproportional to the distance of the subscriber to the distribution point This would meanthat a call coming from a rural area should be more expensive than an urban call Further,

an international call should be only nominally more expensive than a local call Thesefacts are not reflected in the current tariffs This is partly due to the monopolistic history

of most Public Telephony Operators (PTO) Being regulated by the national governments,the PTO had the obligation of offering each citizen a connection at a universal price.The advent of Internet telephony will break the current tariff structure AllowingInternet users to perform international calls at the local call rate This application is atrue reflection of the actual situation Since operators inflate the cost of international calls

to reduce the loss made on subsidising the local loop, popular Internet telephony willmake its impact The result will be a restructuring of the tariff system This so-called voiceover Internet Protocol (IP) is not expected to last very long as the Internet will be floodedwith voice and data movement

The economic impact of FWA networks will largely depend upon the success of DigitalSubscriber Line (xDSL) technologies These so-called `killer' technologies have the possib-ility of rendering all other access techniques obsolete However, the penetration of xDSL isquestionable as some figures state that only 30 % of all telephone lines can be utilized for

xDSL Another problem of xDSL is that the lines are owned by the PTO There is a certainamount of control which a private operator must relinquish when renting a line from thePTO It might be that xDSL will need another five years for a breakthrough in the local loop.However, the success of xDSL will be crucial for the existence of other access technologies.Despite the generous forecasts that were made for FWA networks, some predictionswere 170 million subscribers by the year 2000, the impact of this technology has beenslow For 1998, the subscriber count is at best a few million (some say just 1 million).Companies offering FWA networks in the market have even seen considerable drop-ping of share value This is a surprising since wireless access does have a considerable costadvantage over all the other technologies [17] However, the introduction of FWA is veryexpensive if a wired solution is present Further, the operation of FWA networks gen-erally require the acquirement of two licenses, one enabling the offer of telecommunica-tion services and the other the use of the radio spectrum The allocation of radio spectrum

is also a problem To be able to offer high transmission bit rates, sufficient bandwidthmust be allocated In some cases this allocation has been too low

Another deciding factor apart from cost will be the subscriber's demand for bandwidth services Test carried out with Video on Demand (VoD) and home shopping

high-do not reflect heavy user interest This is different, however, for teleworkers and businessusers who need to work with the company network at comparable Local Area Network(LAN) schemes

In summary, it can be said that FWA networks will be a very viable technology fordeveloping countries and Eastern Europe The higher risk is clearly bound with thedeployment in the developed countries

7.3 Multiple Access Technologies

Presented in this section is a brief description of the two major access technologies forwireless networks The communication medium for a radio system is a commonly sharedradio channel Considering the uplink, the link from the Radio Network Terminals (RNT) tothe Radio Base Station (RBS) the system can be classified as a MultiPoint-to-Point (MPP)system With multiple access technology it is possible for several users to send their signalsover the radio channel which are then ultimately detected at a corresponding receiver.For the sake of completeness the Frequency Division Multiple Access (FDMA) methodshould be mentioned but is not explained in more detail For a general overview it can bereferred to B Walke [16]

7.3.1 Time Division Multiple Access

With TDMA the radio resource is divided in the time domain into time slots The time slotsare assigned to users either in a cyclic fashion or upon demand Within this time slot anexclusive user is able to transmit across the medium To avoid collisions the system must besynchronized and additionally a guard time is inserted between slots No other conversa-tions can access an occupied TDMA channel until the channel is vacated TDMA is asoftware intensive protocol so the gathering of results is possible by means of simulations.Figure 7.1 illustrates the basic principle of TDMA with the alternating transmissionand guard periods

Time

Figure 7.1 TDMA (Walke, 1999)

TDMA is a common multiple access technique employed in digital cellular systems Itsstandards include North American Digital Cellular, Global System for Mobile Commu-nications (GSM), and Personal Digital Cellular (PDC)

7.3.2 Code Division Multiple Access

CDMA is a form of spread-spectrum, an advanced digital wireless transmission nique Instead of using frequencies or time slots, as do traditional technologies, it usesmathematical codes to transmit and distinguish between multiple wireless conversations[10] Its bandwidth is much wider than that required for simple point-to-point commu-nications at the same data rate because it uses noise-like carrier waves to spread theinformation contained in a signal of interest over a much greater bandwidth However,because the conversations taking place are distinguished by digital codes, many users canshare the same bandwidth simultaneously, as seen in Figure 7.2

tech-Although not shown, it is possible for a user to use more than one code, as is foreseenfor third-generation mobile systems The advanced methods used in commercial CDMAtechnology improve capacity, coverage and voice quality, leading to a new generation ofwireless networks

7.3.3 Interference in Multiple Access Systems

A multiple access scheme must warrant that a user can access the radio channel withoutcausing interference to the other users If interference is caused, it is then known as MultipleAccess Interference (MAI), interference caused by the multiple accession to the radio chan-nel In the presence of MAI the data symbols of the different users interfere with each other

User N

User 1 User 2

Frequency

Time

Figure 7.2 CDMA (Rappaport, 1996)

If there is multipath propagation on the channel, then the symbols in the signal of asingle user cause interference upon each other, leading to Inter-Symbol Interference (ISI).ISI takes place if the symbol duration is less than the time dispersion on the channel,

a phenomenon which can take place if the transmission bit rate is very high Both MAIand ISI can be grouped together and classified as intra-cell interference, the interferencepresent in a radio cell A radio cell in a multicellular environment additionally experi-ences interference caused by the transmitting stations in neighbouring radio cells Thisinterference is known as inter-cell interference

7.4 CDMA Capacity Analysis

Presented here is an analytical method to determine the capacity of a multiclass cellular spread sequence (CDMA) systems based on an approach by S J Lee et al [9] Thebasis of the method assumes an a-priori Eb=I0level which must be maintained to assure asatisfactory performance with respect to the Bit Error Ratio (BER) for a desired serviceclass Capacity is defined here as the number of simultaneous connections that can beadmitted into the system for a particular service class so that the quality constraint canstill be guaranteed The capacity analysis is carried out for the reverse link (RNT to RBSuplink) since this link is considered to be critical for a CDMA system [8]

multi-7.4.1 CDMA Traffic Model

The aim of a broadband FWA network is to carry different types of service classes, eachrequiring a different service bit rate A survey conducted for integrated services on

wireless multiple access networks has come up with a possible service performance forthese networks [12].

Bit-Energy to Interference Spectral Power The bit-energy to interference spectral powerdenoted here as g ˆ Eb=I0 is the constraining factor for a CDMA system when allocatingcapacity to a new connection The term is mainly dependent on the maximum BER theservice can sustain and the modulation type selected for the transmission

Spreading Gain The spreading gain G (equalling the spreading factor in a CDMAsystem) depends on the service bit rate, the transmission bandwidth and the multiratetransmission technology For the Single-Code (SC) technology there are different values

of G since different bit rates are realized by different spreading of the data sequence.Whereas for MultiCode (MC) technology there is only one spreading gain equal for allcodes used, but a number of codes can be multiplexed in order to offer the requiredtransmission bit rate

Considering the service bit rates from Table 7.1 and the transmission bandwidth W ˆ

112 Mbit/s, a certain spreading gain G for the services could be assigned as proposed inTable 7.2 The base transmission bit rate Rbfor MC-CDMA was chosen to be the lowestservice bit rate of the system, the bit rate for the voice calls The spreading gain is thequotient of transmission bandwidth to service bit rate

7.4.2 Single-Class Services

This is the most common type of capacity analysis for a Direct Sequence CDMA CDMA) system Generally the service class under scrutiny are voice calls with a service bitrate of 32 kbit/s The resulting capacity equation derived here is of little importance for aFWA network desired to work on a broadband system

(DS-Table 7.1 Service classes for FWA networks Service Maximum BER Delay Bit rate required g

Class 2 (Packet Data) 10 4 Insensitive 64 kbit=s 7.0 dB

Table 7.2 Spreading gains for different service classes Service SC-CDMA MC-CDMA Bit rate

Class 2 (Packet Data) G ˆ 1750 G ˆ 3500; 2 Code 64 kbit=s

However, the approach and the trail of thought will be the same one for the pursuit ofcapacity for a multiple service class system.

7.4.2.1 Single-Cell and Single-Class Capacity

The error rate of digital transmission systems only depends on the signal-to-noise ratio.Respectively the Carrier to Interference Ratio (C/I) expressed by

C

Iintra ˆ

S

where S portrays the sending power reception level of a user signal at a receiver and

Iintra is the total interference power experienced within a single cell (intra-cell ference)

inter-Assume it is possible to construct a source where the sending signal spectrum isconstant between W=2  f  ‡W=2 and disappears outside this interval Let Eb bethe energy per bit of this signal and the bit rate be R ˆ 1=T The sending power is nowequal to EbR The term N0 in Equation (7.2) is the power density of the noise power Nresulting from the effects of thermal noise and spurious interference in the bandwidth Itcan be written that

Now consider a system with n sources, each possessing the before described istic The ith receiver correlates the received signal with all the other n 1 signals.Assuming that the sending signal of all the other sources are uncorrelated, then the ithreceiver regards the other signals as uncorrelated white noise sources Further, it isassumed that the received power level of the different sources are all equal at the site ofthe receiver (perfectly power controlled) This yields

character-Eb

Iintraˆ RS

…n 1†S W

Cancelling the denominator, Equation (7.5) can be rewritten as

of the spreading code [3]

Using the definition G ˆ Rchip=R whereby Rchip is the chip rate of the spreadingsequence and modifying Equation (7.6) to remove the term Eb the quality constraint for

a service finally becomes

S R

In the system being analysed there are up to K service classes, each service class having

an information bit rate Rk For single-code transmission this bit rate is an integer multiple

of the line bit rate R In the case of MC transmission, the high information bit rate of aclass k connection is defined by Rkˆ ckR The term ck denotes the number of codesneeded for transmitting a class k connection [2]

Further, it is assumed that there are nk connections in each service class k Theconnection is linked to the RBS with the least path loss

Using a similar line of thought as in Equation (7.7) the bit energy to interference powerspectral density ratio for the ith connection is modelled as

In the equations below Sidenotes the received level of the signal power of the connection

to be accepted Depending on the transmission scheme, Riis the ith terminal's service bitrate for the SC system whereas R is the line rate of the MC system The intra-cell interference

is no longer based on the uncorrelated disturber signals but rather on the interference caused

by the different connections with their corresponding received power levels

Eb

I0

SC

PK kˆ1nkSk Si

PKkˆ1cknkSk ciSi

Eb

I0

SC

S i

R i

XK kˆ1

XK kˆ1

These equations are the basic frame work for the more complex investigation which willfollow.

7.4.3.1 Single-Cell and MultiClass Services Capacity

The aim of the capacity analysis is to determine the number of connections that can besimultaneously admitted into the transmission system The basic parameter for the con-nection admission is the Eb=I0value which is inherently determined by the service quality,e.g the BER required

Recalling the identity Eb=I0ˆ g and Equation (7.8), the quality constraint condition for

a connection i of a particular service class is

S i

R i

XK kˆ1

XK kˆ1

b

aiSi

…7:22†Applying this proposition for the SC calculations, Equation (7.19) yields

XK iˆ1

…7:23†

These equations are the basic frame work for the more complex investigation which willfollow.

7.4.3.1 Single-Cell and MultiClass Services Capacity... that can besimultaneously admitted into the transmission system The basic parameter for the con-nection admission is the Eb=I0value which is inherently determined by the