Gerhard P. Fettweis, Kay Iversen*, Marcus Bronzel, Holger Schubert**, Volker Aue, Detlef Mämpel**,. Jens Voigt, Adam Wolisz***, Godehard Walf**, Jean-Pierre ...
A Closed Solution For An Integrated Broadband Mobile System (IBMS)+ Gerhard P. Fettweis, Kay Iversen*, Marcus Bronzel, Holger Schubert**, Volker Aue, Detlef Mämpel**, Jens Voigt, Adam Wolisz***, Godehard Walf**, Jean-Pierre Ebert*** Dresden University of Technology *Ilmenau University of Technology **Heinrich-Hertz-Institute Berlin ***Technical University of Berlin Abstract Most current development strategies for future mobile systems are concerned with partial solutions only regarding the mobility-bit rate problem. In this paper the basic ideas of the Integrated Broadband Mobile System (IBMS), supporting in a unified way a variety of communication classes ranging from high mobility with low data rates towards quasi-stationarity at high data rates, are presented. The key feature of this proposal is a closed system approach for both outdoor and indoor environment. Essential is the use of a single common, low bit-rate, generally available signalling channel for negotiation of the required service class or supporting the fallback to a lower service class if necessary. In the outdoor area, the use of smart antennas with strongly sectorized coverage will support high bit rates on behalf of reduced mobility speed, while simultaneously assuring efficient use of frequency spectrum. The same signalling channel will also be used to coordinate possible change of the mobile terminal to and forth between the outdoor provision and space limited indoor domains, operating on the picocell principle. Mobile terminals can be designed in a modular way to support all, or only some lower, bit rates. IBMS allows for an evolutionary extension starting from the infrastructure of today’s digital cellular systems towards a multimedia and multiservice support of future universal personal communications using an ATM based communication backbone. IBMS is a joint research effort of Dresden University of Technology, Ilmenau University of Technology, Technical University of Berlin, and the Heinrich-Hertz-Institute Berlin sponsored by the German federal ministry of education, science, research and technology (BMBF) within the project line ATMmobil.
Introduction In modern wireless communication systems, a trade-off between mobility and maximum obtainable bit rate has to be made. Second generation mobile digital cellular systems and personal communications systems (PCS), e.g. GSM, IS-54, or IS-95 offer a high mobility (at velocities up to 250 km/h) for users restricted to transmitting voice, fax, or other services requiring low data rates only. To provide for wide coverage in these systems, cell sizes vary from a few hundred meters up to several kilometers in diameter. Different wireless digital systems such as wireless LAN or digital cordless phone systems have emerged in recent years for inhouse environments. A wireless LAN for instance offers high transmission rates, but limits the mobility at high data rates to portability due to the costs involved for interference rejection and channel equalization. A digital cordless phone system provides inhouse mobility for users at medium data rates which are sufficient for high voice quality, medium rate file transfer, and fax. Generally, each system is designed to force trade-offs between bit +
Sponsored by BMBF, under the ATMmobil project line
rate and mobility. Third generation systems currently under development, such as UMTS/FPLMTS will provide only limited support for higher data rates. On the other hand recent efforts within ATMForum towards merging ATM and wireless are not designed to support high mobility (see. e.g. ATMForum). Fig. 1 shows graphically, how existing and proposed future systems cover only confined regions in a mobility / bit rate plane. A single consistent solution covering the entire range of low rate services with high mobility and high data rate movable services has not been developed so far. We consider this paper as a contribution towards such a consistent solution.
mobility cellular/PCS
fast
WLAN slow
WLAN/ATM movable Integrated Broadband
Mobile System 16 kbit/s
2.048 Mbit/s
155 Mbit/s
bit rate
Figure 1: Trade-off between bit rate and mobility. Future Personal Communication Systems Recently it has become obvious [1,5] that the next generation of personal mobile communication systems should integrate current and future services, ranging from voice, paging and other low rate cellular services to E-mail, general computer data, still image and even high quality video, including multimedia facilities (see Fig.2). In fact it is expected [5], that shares of traffic volume will shift from uniformity of telephony towards the diversity of data, text, image, video, often referred to as multimedia. These services exhibit a great amount of heterogeneity [5] in terms of traditional quality of service (QoS) which is determined mainly by the permissible bit rate, bit error rate (BER), delay and delay jitter. In addition, other design parameters namely: mobility, coverage, and service continuity have to be considered. The requirements for typical services expected in next generation PCN are summarized in [5]. The need for bit rates up to 2 Mbit/s (compressed) has been identified there. A tendency for avoiding complex compression because of both: desire to power economy
?...? Data, Graphics, Fax
Messaging PDA
Speech Video
Figure 2: Mobile communications for multimedia. (and thus avoiding extensive signal processing within the portable end systems) and vulnerability of compressed data against transmission errors justifies consideration of higher bit rates as well. Objective For supporting this service spectrum [1] a universal, modular, layered, standardized concept - mobile multimedia supporting infrastructure is needed, as well as new end systems. In fact there is a recognized need to permit an interface on the air for more than one bit rate - possibly in cells of different radius and with different levels of mobility. The emerging broadband communication infrastructure, based on the ATM technology with its support for flexible bit rates seems to create a proper framework for the desired wide spectrum of different services. Thus, the solution for future PCS should assure at least simple interconnection to the ATM infrastructure, and perhaps even follow, as far as possible, the ATM paradigm. Our goal is to find a solution for an integrated broadband mobile system that meets the heterogeneous or conflicting needs for each service by combining different existing narrowband and broadband digital communication systems for inhouse and outdoor use. We will achieve that by • Defining a hierarchy of network service classes with respect to bit rate and mobility. • Using as a basis a single cell size which is independent of the provided service class. (A separate overlaid micro-cell size for indoor use is considered.) • Defining a common signalling channel for service class negotiation/setup, itself using the lowest service class • Supporting an evolutionary introduction of the system starting from the existing cellular standards • Defining a modular communication subsystem for the future universal communicator with compulsory support for the lowest (and just cheapest in support) network service class and
optionally extendible for higher classes. We will present a cost effective design strategy for a common wireless broadband communication system which meets the aforementioned objectives. While high mobility will be available to users of low rate services only, higher rates up to 155 Mbit/s wireless ATM will be supported for applications which are restricted to movable users. This represents a tolerable trade-off between bit rate and mobility. The proposed IBMS approach is based on an evolutionary and integrative concept supporting mobile and movable services for low bit rates required for voice services up to 155 Mbit/s ATM based applications for inhouse and outdoor use. Hierarchy of Network Service Classes In order to support the different requirements of the wide spectrum of future Personal Communication System services we introduce several network services classes (NSC) with respect to the bit rate and mobility. The system presented in this paper supports four NSC summarized in Table 1, where hints on the cost and power consumption are also included. Services of NSC A, with low bit rate, are provided with mobility similar to existing digital cellular systems. Current modulation schemes permit usage of simple, low cost and low power consuming receiver implementations. NSC B services with medium bit rates will be restricted in their moving speed, presumably to a few kilometers per hour. Receiver implementations are allowed to be more complex for this NSC. For both NSC A and NSC B services a disruption -free provision is aimed at. The highest bit rate NSC of 34/155 Mbit/s (technology dependent) will be provided only in the case the mobile terminal is almost stationary - in case of moving the terminal a disruption of NSC C service is permitted. A complex receiver implementation with high performance signal processing is required for this NSC. Finally in the NSC D slow mobility is allowed, however disruption free service will be guaranteed only within a pico-cell (crossing the picocell border might cause short NSC D service
NSC D
e.g. 2 or 5GHz
e.g. 2 or 5GHz
BS
BS NSC C
NSC A 20 kb/s
38 GHz 155 Mb/s
2 Mb/s
200 Mb/s
NSC B e.g. 6GHz ‘MSC’
ATM Backbone
BS e.g. 2 or 5GHz
Figure 3: Integrated Broadband Mobile System (IBMS) for inhouse and outdoor communications. disruption).
NSC
basic bit rate
mobility
hardware cost
Power consumptio n
A
16 kbit/s
high
low
low-
B
2 Mbit/s
medium
medium
low
C
34/155 Mbit/s
low
high
low+
D
34/155 Mbit/s
movable pico-cell
high
medium
Table 1: Network Service Classification Scheme. The above listed network service classes correspond to requirements of typical mobile services. In addition, other bit rates can be supported either by using multiples of these basic rates (channel bundling), by time-sharing a basic rate using timedivision multiple access (TDMA) or by varying channel coding. Modulation techniques, channel coding and media access schemes are chosen to meet the requirements for each NSC with respect to QoS and burstiness. While different techniques have been developed and are still under investigation, they are beyond the scope of this paper and will be published separately (e. g.[2 4]). Finally a fallback from classes of higher bit rates to classes of lower bit rates will be allowed. This could be desirable, if a high bit rate cannot be supported, e.g. the signal-to-noise ratio is not sufficient or the user favors a higher mobility. The fallback to a lower NSC does not necessarily impose the
rejection of more bandwidth consuming services - like motion video. Using hierarchical, adaptive source coding schemes (see e.g. [8]) such services might be provided, although with reduced quality, even over relatively low bit-rate channels. An Integrated Broadband Mobile System for Inhouse and Outdoor Wireless Personal Communications The proposed system uses similar cell sizes and transmission powers for the outdoor environment as in today's cellular communication systems, thus allowing the integration of existing standards. The inhouse environment is similar to existing WLANs with an overlaying micro-cellular structure to grant higher mobility to low rate users. The overall concept is shown in Fig. 3. This figure depicts how users with different demands on data rate and mobility are supported in outdoor and inhouse environments as well. The system consists of a set of base stations connected through an ATM based backbone broadband infrastructure. The ATM backbone provides connectivity between base stations, to management facilities (Mobile Switching Center - MSC) and to endsystems which are (in/direct) connected to the fixed ATM network. There is also a way to connect base stations through a ATM-WLL link (e.g. NSC C) to the fixed ATM network to reduce installation costs. Each of the base stations supports at least NSC A communication, more precisely one or more NSC A channels. This is done to assure universal coverage with this service class - both for the outdoor and indoor use. In addition it is expected that the positioning of the base stations as well as the number of provided NSC A channels will cover the demand for NSC A services. It is expected, that this requirement will lead to a density of base sta-
tions comparable with the density of the contemporary cellular networks. High mobility is enhanced for NSC A through the fewer number of hand-offs which occur compared to systems with smaller cell sizes. Within the set of NSC A channels, some will be distinguished in order to act as common signalling channels supporting the assignment/release of the rest of the channels called further on utility channels. In addition to the basic set of NSC A channels, each base station might provide also a group of NSC B and NSC C channels. It is assumed that for the outdoor scenario the coverage of NSC B and NSC C channels will be identical as the coverage of the NSC A channels. This will be achieved by the use of smart antennas supporting sectorized communication (see next section for the details). The assignment of the users to NSC B and NSC C channels, as well as release of these channels will take place using the same common signalling channels mentioned previously. So a unique signalling scheme, provided with the lowest, NSC A, communication bit rate, will be used for all communication requests. This assumption has several advantages. Roaming between the outdoor and inhouse environment, as described below is assured. Signalling for the sake of fallback to lower NSC is always supported. Finally, any kind of standby/alert mode, needs to be supported only for the NSC A, contributing to a lower energy consumption. Last but not least, as can also be seen from this discussion, not all base stations need to be installed simultaneously so that they can provide all three data rates immediately. Instead, an evolutionary gradual installation of cells that also provide higher data rate transmissions can be carried out. Outdoor Environment Most broadband systems currently under investigation (see e.g. [5,6,7]), use very small cells (picocells) in order to support high bit-rates. The reason is straightforward. Usually, because transmit power is proportionally related to the bit rate for a given SNR using the same type of antennas. Thus a power level in the order of 100 times greater than required for NSC A would be needed for NSC B. This holds again for NSC C rates, where the transmit power needed would be 104 times that of a NSC A rate transmission. Use of very small cells reduces the power needed. In addition, use of the small cells supports efficient frequency reuse. Unfortunately, the necessity to install a large number of base stations creates a serious economical barrier for such approach, especially in the outdoor environment. In contrast to this trend we propose to use for all NSC Classes large cell sizes with diameters comparable to cell sizes used in today's existing cellular systems. Using cell sizes with diameters up to some kilometers, as in PCS, the system can easily provide a wide coverage without a massive deployment of base stations. Supporting NSC B to D will be achieved using smart antennas. Technically speaking, smart antennas will enable higher symbol rates by increasing the Signal-to-Noise-Ratio (SNR) and further enhancing the properties of the mobile communication channel by reducing the delay spread. Therefore, the implementation of equalizers for bandwidth efficient higher modulation
schemes (e.g. COFDM) are made feasible. A higher capacity can be achieved with the same link budget, as the bandwidth efficiency increases. Different bit rate requirements can be made adjustable during transmission by adapting the modulation type, the symbol rate or channel coding. An adaptive beamforming antenna (smart antenna) can provide gains up to 20 dB depending on the size of the adaptive array. Hence, for supporting NSC B rates at the same transmit power as needed for NSC A, the use of one smart antenna is needed. This antenna will be applied to the base station, while the mobile station can still use an omni-directional low cost antenna. The support of NSC C rates up to 155 Mbit/s wireless ATM is enabled by using smart antennas at base and mobile station simultaneously. A possibly achievable overall gain of 40 dB reduces the power needed to a NSC A transmission with no antenna gain. Since adaptive algorithms for smart antennas require time to converge, NSC B rates will only be provided with limited mobility. NSC C rates can only be supported for movable applications. In this case, the NSC C link can be compared to a stationary microwave link. The different scenarios for all classified bit rates are shown in Fig. 4. scenario transmitter omnidirectional
receiver
data rate
degree of mobility
omnidirectional
A
directed
directed
high (mobile)
2.048 Mbit/s
medium (portable)
155 Mbit/s
low (movable)
omnidirectional
B
C
16 kbit/s
directed
Figure 4: Different scenarios for different bit rate classes. The main advantage of the use of smart antennas as proposed here is to ensure nearly equal transmit power and cell size independent of the data rate. In addition, as seen in Figure 4 the sectorization achieved for the NSC B and NSC C contributes to efficient frequency reuse. We have selected carrier frequencies around 2 or 5 GHz for the outdoor environment, and thus as basic frequencies for the whole IBMS. At this frequency, there is still enough advantage which can be taken from diffraction and penetration to provide overall coverage. However, antenna arrays can be built considerably smaller than for lower frequencies around 1 GHz. Inhouse Environment The basic approach described above will assure overall coverage with the NSC A communication service, and optionally NSC B and C. There will however definitely be areas, typically within buildings, where a service equivalent to NSC B and C with respect to the QoS should be offered to relatively numerous cus-
tomers. We shall refer to this variant as the inhouse environment. Here, only a pico-cellular network with cell sizes comparable to the size of offices or even smaller can provide the capacity needed. A dual approach is aimed for the inhouse environment. For the additional NSC B service provision (and perhaps also for NSC A in case of very numerous customers) local coverage will be provided using the same frequency range as for the basic outdoor scenario. This will be achieved using dedicated sub-frequencies, perhaps also with reduced transmission power in order to increase spacial frequency reuse. The use of frequencies in the order of 2-5 GHz in the inhouse environment has been subject to intensive studies and is well understood. This solution supports relatively good mobility within the inhouse area. In addition, the use of a higher carrier frequency might be desirable to support both the high bit rate (up to 155 Mbit/s) as well as the increased cell capacity per volume. This will be done in a picocell approach, and referred to as NSC D service. Fortunately, the attenuation from reflection at walls restricts cell sizes to the size of rooms in frequency ranges well above 20 GHz. This will automatically reduce the amount of inter-cell interference. The proposed IBMS concept is currently targeted to use 38 GHz to support NSC B and NSC C rates in this mode (the use of the 60GHz is also under consideration). Furthermore, RF-communications can be replaced by diffuse infrared links, especially where very low limits of electro-magnetic radiation have to be considered [9]. We would like to stress that because of the possible fallback from the service NSC D to the lower service classes it is not necessary to provide the NSC D service support in the whole area, or in a disruptive mode. The inhouse subsystem will share the same signalling and control principle with the outdoor subsystem. Supporting two frequencies does not necessarily have to double the cost for the RF components. The lower frequency band might be used as an intermediate frequency for transmitting at 38 GHz/60 GHz, leading to a modular terminal design, extendable from the basic 5 GHz transmission frequency to the inhouse frequency (see Fig. 5.). Infrared components are cheap anyway. A mirroring of signalling channels from 5 GHz to 38 GHz may be necessary to avoid continuous switching between the two frequencies when using the inhouse frequency for utility channels. So a seamless reception of signalling data is ensured. 5 GHz
38 GHz
baseband modulator
Figure 5: Inhouse terminal. Conclusions An integrated broadband mobile system for inhouse and outdoor wireless communication using a common signalling/paging channel has been presented. Based on a further development of today's PCS strategies for outdoor cellular systems and future
WLANs, the IBMS concept supports a wide range of applications ranging from mobile services with low data rates to movable services at high rates for both, inhouse and outdoor environments. This approach accommodates the heterogeneous demands of future narrowband and wideband services. The proposed system integrates today's cellular service, cordless phone service, wireless LAN and 155 Mbit/s ATM on the local loop. The proposed system therefore allows for: • Mobility versus data rate on demand over a wide range. • One common signalling channel for all signalling in stand-by mode and link/call setup. • The common low-rate signalling channel and the low-rate user channel could be an evolutionary modification of existing cellular standards (e.g. GSM). • The cell size and transmit power is (almost) independent of the data rate. • Transparent access to ATM. There are several interesting technical problems which have to be solved in order to turn the above described concept into reality. One basic issue is definitely development of smart antennas with expected gain, and acceptable tuning time. Another issue is development of the common signalling, supporting in a proper way the assignment of channels in desired network service classes, fallback functions as well as outdoor/indoor roaming. Multiple access protocols and bundling strategies for support of other bit rates have to be designed. These issues, being under investigation, will be reported separately. References [1] H.Armbruester, “The Flexibility of ATM: Supporting Future Multimedia and Mobile communications”, IEEE Communic. Mag. February 1995, pp.76-84 [2] V. Aue, G. P. Fettweis, and H. Nuszkowski, “A multiple-access scheme for multimedia cellular systems,“40th Internationales Wissenschaftliches Kolloqium, vol. 1, (TU Ilmenau), pp. 55-60, Sept. 1995. [3] V. Aue and G. P. Fettweis, “Multi-carrier spread spectrum modulation with reduced dynamic range,“Proceedings of the 46th IEEE Conference on Vehicular Technology, vol. 2, pp. 914-917, Apr. 1996. [4] G. P. Fettweis, A. S. Bahai, and K. Anvari, “On multi-carrier code division multiple access (MC-CDMA) modem design,“Proceedings of the 1994 IEEE Conference on Vehicular Technology, vol. 3, pp. 1670-1674, June 1994. [5] D.Raychaudri et.al. “ATM-Based Transport Architecture for Multiservice Wireless Personal Communication Networks”, JSAC Vol.12, No.8, October 1994, pp.1401 -1414 [6] A.Wolisz, M.Schlaeger, J.Weinmiller, H.Woesner, “Wireless Access to High Speed Networks” in High Speed Networks for Multimedia Applications, W. Effelsberg, O.Spaniol, A. Danthine, D.Ferrari(eds), Kluver Academic Publishing, 1996 [7] ATM Forum Working Document 96-0530 “Charter, Scope and Working Plan for Proposed Wireless ATM”, April 15-19, 1996 [8] J.M. Reason, L.C. Yun, A.Y. Lao, D. Messerschmitt, “Asynchronous Video: Coordinated Video an Transport for Heterogeneous Networks with Wireless Access”, Mobile Computing, H.F. Korth and T. Imielinski, Ed., Kluwer Academic Press, Boston, MA., 1995 [9] M. Wolf, D. Mämpel, and K. Iversen, “Diffuse-Infrared Broadband Communication System Based on Multiple Optical Carriers”, NOC’96, program Nr. 135, 1996
