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7 Traffic Considerations in Comparing Access Techniques for WLL Stefan Mangold, Ingo Forkel, Roger Easo and Bernhard Walke 7.1 Introduction The focus of interest is the multiple access technology that will be employed in Wireless Local Loop (WLL) systems, here referred to as Fixed Wireless Access (FWA) networks. The discussion of whether to employ Code Division Multiple Access (CDMA) or Time Division Multiple Access (TDMA) has gone on for a long time with no result to be expected in the near future. In this chapter, a capacity comparison for FWA based on two access technologies is performed. The TDMA system is analysed with the help of 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 capacity utilization of the system. The CDMA system is investigated analytically in this chapter. It is clear that in performing an analytical calculation certain simplifications and assump- tions will need to be made. The structure of this chapter is as follows. In the next section the FWA network is explained in more detail. Apart from the technological description of this access scheme, the section will also examine the economic viability. The impact of the FWA network in developed and developing countries is investigated 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 following section. 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 and finally 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 of this chapter. It is shown how the expected capacity for both technologies can be esti- mated. 141 Wireless Local Loops: Theory and Applications, Peter Stavroulakis 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 taken for granted in the developed countries [6,11]. It is therefore surprising to note that the world 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. The demand for communication is driven not only by business alliances and exchanges but also through personal relations like friends and relatives that live around the globe. This revolution in communication requirement is abetted by three major forces. Computing power increases while the costs of providing this power are reduced through economies of scale. 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 comput- ing have pushed the merging of segmented industries into a large information industry. The world information technology market which includes products such as personal computers, mobile phones, and communication has grown by 12.2 % between 1985 and 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 delivering knowledge. For developing countries delivering knowledge can mean fighting illiteracy and poverty. Therefore, especially these countries need to increase their teledensity. The International Telecommunications Union (ITU) recommends that the teledensity of a nation should be at least 20 % so that economic growth is not hampered by the lack of telecommunications. Wireless access systems provide a suitable method of providing this access to telecom- munication services. Currently wireless telephony is experiencing a tremendous growth for the last 10 years with the number of subscribers globally estimated at 55 million people until mid 1995 [7]. Most of this usage is for mobile communications. The prediction of which access technique will have the greatest impact must be based on the 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 is proportional to the distance of the subscriber to the distribution point. This would mean that 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. These facts 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. Allowing Internet users to perform international calls at the local call rate. This application is a true 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 will make its impact. The result will be a restructuring of the tariff system. This so-called voice over Internet Protocol (IP) is not expected to last very long as the Internet will be flooded with voice and data movement. The economic impact of FWA networks will largely depend upon the success of Digital Subscriber Line (xDSL) technologies. These so-called `killer' technologies have the possib- ility of rendering all other access techniques obsolete. However, the penetration of xDSL is questionable as some figures state that only 30 % of all telephone lines can be utilized for 142 Traffic Considerations in Comparing Access Techniques for WLL xDSL. Another problem of xDSL is that the lines are owned by the PTO. There is a certain amount of control which a private operator must relinquish when renting a line from the PTO. 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 predictions were 170 million subscribers by the year 2000, the impact of this technology has been slow. 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 cost advantage over all the other technologies [17]. However, the introduction of FWA is very expensive 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 bandwidth must be allocated. In some cases this allocation has been too low. Another deciding factor apart from cost will be the subscriber's demand for high- bandwidth services. Test carried out with Video on Demand (VoD) and home shopping do not reflect heavy user interest. This is different, however, for teleworkers and business users 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 for developing countries and Eastern Europe. The higher risk is clearly bound with the deployment in the developed countries. 7.3 Multiple Access Technologies Presented in this section is a brief description of the two major access technologies for wireless networks. The communication medium for a radio system is a commonly shared radio channel. Considering the uplink, the link from the Radio Network Terminals (RNT) to the 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 signals over the radio channel which are then ultimately detected at a corresponding receiver. For the sake of completeness the Frequency Division Multiple Access (FDMA) method should be mentioned but is not explained in more detail. For a general overview it can be referred 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 slots are assigned to users either in a cyclic fashion or upon demand. Within this time slot an exclusive user is able to transmit across the medium. To avoid collisions the system must be synchronized 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 a software 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 transmission and guard periods. Multiple Access Technologies 143 Frequency Time Guard time Guard time Guard time Slot 1 Slot 2 Slot 3 Slot 4 Figure 7.1 TDMA (Walke, 1999) TDMA is a common multiple access technique employed in digital cellular systems. Its standards 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 tech- nique. Instead of using frequencies or time slots, as do traditional technologies, it uses mathematical 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 the information contained in a signal of interest over a much greater bandwidth. However, because the conversations taking place are distinguished by digital codes, many users can share the same bandwidth simultaneously, as seen in Figure 7.2. Although not shown, it is possible for a user to use more than one code, as is foreseen for third-generation mobile systems. The advanced methods used in commercial CDMA technology improve capacity, coverage and voice quality, leading to a new generation of wireless networks. 7.3.3 Interference in Multiple Access Systems A multiple access scheme must warrant that a user can access the radio channel without causing interference to the other users. If interference is caused, it is then known as Multiple Access 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. 144 Traffic Considerations in Comparing Access Techniques for WLL User N User 1 User 2 Frequency Time Code Figure 7.2 CDMA (Rappaport, 1996) If there is multipath propagation on the channel, then the symbols in the signal of a single 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 MAI and ISI can be grouped together and classified as intra-cell interference, the interference present in a radio cell. A radio cell in a multicellular environment additionally experi- ences interference caused by the transmitting stations in neighbouring radio cells. This interference is known as inter-cell interference. 7.4 CDMA Capacity Analysis Presented here is an analytical method to determine the capacity of a multiclass multi- cellular spread sequence (CDMA) systems based on an approach by S. J. Lee et al. [9]. The basis of the method assumes an a-priori E b =I 0 level which must be maintained to assure a satisfactory performance with respect to the Bit Error Ratio (BER) for a desired service class. Capacity is defined here as the number of simultaneous connections that can be admitted into the system for a particular service class so that the quality constraint can still be guaranteed. The capacity analysis is carried out for the reverse link (RNT to RBS uplink) since this link is considered to be critical for a CDMA system [8]. 7.4.1 CDMA Traffic Model The aim of a broadband FWA network is to carry different types of service classes, each requiring a different service bit rate. A survey conducted for integrated services on CDMA Capacity Analysis 145 wireless multiple access networks has come up with a possible service performance for these networks [12]. Bit-Energy to Interference Spectral Power The bit-energy to interference spectral power denoted here as g  E b =I 0 is the constraining factor for a CDMA system when allocating capacity to a new connection. The term is mainly dependent on the maximum BER the service can sustain and the modulation type selected for the transmission. Spreading Gain The spreading gain G (equalling the spreading factor in a CDMA system) depends on the service bit rate, the transmission bandwidth and the multirate transmission 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 all codes used, but a number of codes can be multiplexed in order to offer the required transmission bit rate. Considering the service bit rates from Table 7.1 and the transmission bandwidth W  112 Mbit/s, a certain spreading gain G for the services could be assigned as proposed in Table 7.2. The base transmission bit rate R b for MC-CDMA was chosen to be the lowest service bit rate of the system, the bit rate for the voice calls. The spreading gain is the quotient 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 (DS- CDMA) system. Generally the service class under scrutiny are voice calls with a service bit rate of 32 kbit/s. The resulting capacity equation derived here is of little importance for a FWA network desired to work on a broadband system. Table 7.1 Service classes for FWA networks Service Maximum BER Delay Bit rate required g Class 1 (voice) 10 À3 Sensitive 32 kbit=s 6.8 dB Class 2 (Packet Data) 10 À4 Insensitive 64 kbit=s 7.0 dB Class 3 (video) 10 À5 Sensitive 128 kbit=s 9.5 dB Table 7.2 Spreading gains for different service classes Service SC-CDMA MC-CDMA Bit rate Class 1 (voice) G  3500 G  3500; 1 Code 32 kbit=s Class 2 (Packet Data) G  1750 G  3500; 2 Code 64 kbit=s Class 3 (video) G  875 G  3500; 4 Code 128 kbit=s 146 Traffic Considerations in Comparing Access Techniques for WLL However, the approach and the trail of thought will be the same one for the pursuit of capacity 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 I intra  S N 7:1 where S portrays the sending power reception level of a user signal at a receiver and I intra is the total interference power experienced within a single cell (intra-cell inter- ference). Assume it is possible to construct a source where the sending signal spectrum is constant between ÀW=2 f W=2 and disappears outside this interval. Let E b be the energy per bit of this signal and the bit rate be R  1=T. The sending power is now equal to E b R. The term N 0 in Equation (7.2) is the power density of the noise power N resulting from the effects of thermal noise and spurious interference in the bandwidth. It can be written that E b  S R and N 0  N W 7:2 Now consider a system with n sources, each possessing the before described character- istic. The ith receiver correlates the received signal with all the other n À 1 signals. Assuming that the sending signal of all the other sources are uncorrelated, then the ith receiver regards the other signals as uncorrelated white noise sources. Further, it is assumed that the received power level of the different sources are all equal at the site of the receiver (perfectly power controlled). This yields E b I intra  S R n À 1 S W  W R n À 1 7:3 This equation can be modified to include noise effects [4]. These effects are contained in the term N 0 found in Equation (7.2) E b I intra  N 0  S R n À 1 S W  N 0 7:4 Extending Equation (7.4) with the term W/S and using the identity for N 0 from Equation (7.2) E b I intra  N 0  W R n À 1 N S 7:5 CDMA Capacity Analysis 147 Cancelling the denominator, Equation (7.5) can be rewritten as E b I 0  E b I intra  N 0  1 n À 1= 3 2 G  N 0 E b 7:6 where I 0 is referred to as the interference power spectral density and G is the spreading gain defined in the other place. The coefficient 3=2 results from the rectangular chip form of the spreading code [3]. Using the definition G  R chip =R whereby R chip is the chip rate of the spreading sequence and modifying Equation (7.6) to remove the term E b the quality constraint for a service finally becomes E b I 0  S R n À 1S= 3 2 R chip  N 0 7:7 Introducing the term g for the required bit energy to interference power spectral density ratio E b =I 0 , the constraint for an acceptable connection (considering the BER as a connection admission criterion) is S R n À 1S= 3 2 R chip  N 0 ! g 7:8 Hence, the number of simultaneous accepted connections (also called capacity) in a single cell offering only one service is equal to n 3 2 G  g g À 3 2 R chip N 0 S 7:9 7.4.3 MultiClass Services The system is now be extended to include the transmission of different service classes. These services can be voice calls, Internet services or data transmission for example. In the system being analysed there are up to K service classes, each service class having an information bit rate R k . 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 a class k connection is defined by R k  c k R. The term c k denotes the number of codes needed for transmitting a class k connection [2]. Further, it is assumed that there are n k connections in each service class k. The connection 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 power spectral density ratio for the ith connection is modelled as E b I 0  E b I intra  N 0 7:10 The value E b =I 0 is the constraint value for the connection admission and is determined by the modulation technique of the system and the BER which must be guaranteed for the connection. 148 Traffic Considerations in Comparing Access Techniques for WLL In the equations below S i denotes the received level of the signal power of the connection to be accepted. Depending on the transmission scheme, R i is the ith terminal's service bit rate 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. E b I 0     SC i  S i R i P K k1 n k S k À S i  3 2 R chip  N 0 7:11 E b I 0     MC i  S i R P K k1 c k n k S k À c i S i   3 2 R chip  N 0 7:12 It is apparent that c k in Equation (7.12) is the term for the number of codes necessary to service one of the n k connections of service class k for an MC transmission scheme. Similarly c i is the number of codes needed for the ith connection under consideration for the analysis. The I intra term resulting from the existing connections is also known as the Co-Channel Interference (CCI) of the cell. E b I 0  E b CCI  N 0 7:13 This basic interference limited model needs to be modified to include the effects of the channel in which the spreading sequence is propagated. The behaviour of the channel is modelled as a Wide Sense Stationary Uncorrelated Scattered (WSSUS) channel. The multipath propagation induced in this channel leads to interference between the code sequence symbols, hence known as ISI. The I intra is now the sum of both, ISI and CCI E b I 0  E b CCI  ISI  N 0 7:14 Equations (7.11) and (7.12) are extended to consider ISI. This type of interference is contained in the terms F G i  or FG [9]. Additionally, a transmission coefficient u 2 describes the ratio of the primary received signal to the multipath signal of the considered interfering links and acknowledge the effect of multipath propagation for these signal components as well. E b I 0     SC i  S i R i X K k1 n k S k À S i ! Á 1  2u 2 3 2 R chip  S i R i FG i N 0 7:15 E b I 0     MC i  S i R X K k1 c k n k S k À c i S i ! Á 1  2u 2 3 2 R chip  S i R FGN 0 7:16 CDMA Capacity Analysis 149 These equations are the basic frame work for the more complex investigation which will follow. 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 be simultaneously admitted into the transmission system. The basic parameter for the con- nection admission is the E b =I 0 value which is inherently determined by the service quality, e.g. the BER required. Recalling the identity E b =I 0  g and Equation (7.8), the quality constraint condition for a connection i of a particular service class is S i R i X K k1 n k S k À S i ! Á 1  2u 2 3 2 R chip  S i R i FG i N 0 ! g i j SC 7:17 S i R X K k1 c k n k S k À c i S i ! Á 1  2u 2 3 2 R chip  S i R FGN 0 ! g i j MC 7:18 Following simple algebra, these equations can be rewritten as 1  2u 2  3 2 G i 1 g i À F G i   1  2u 2 S i ! X K k1 n k S k  3 2 N 0 R chip 1  2u 2     SC 7:19 1  2u 2 c i  3 2 G 1 g i À FG  1  2u 2 S i ! X K k1 c k n k S k  3 2 N 0 R chip 1  2u 2     MC 7:20 Equations of this form obey the following proposition: a i S i ! X K k1 n k S k  b exists if and only if X K k1 1 1 À " Á n k a k 1 7:21 when "  max iPf1; ;K g b a i S i  7:22 Applying this proposition for the SC calculations, Equation (7.19) yields X K i1 1 1 À " Á 1  2u 2 1  2u 2  3 2 G i 1 g i À FG i   n i 1       SC 7:23 150 Traffic Considerations in Comparing Access Techniques for WLL [...]... transmission scheme Similarly ci is the number of codes needed for the ith connection under consideration for the analysis The Iintra term resulting from the existing connections is also known as the Co-Channel Interference (CCI) of the cell Eb Eb ˆ I0 CCI ‡ N0 …7:13† This basic interference limited model needs to be modified to include the effects of the channel in which the spreading sequence is propagated... N0 R 2R kˆ1 …7:15† …7:16† 150 Traffic Considerations in Comparing Access Techniques for WLL These equations are the basic frame work for the more complex investigation which will follow 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 be simultaneously admitted into the transmission system The basic parameter for . 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. 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 tech- nique 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