Concept for Universal Access and Connectivity in Mobile Radio Networks Dietrich Hunold*, André Noll Barreto*, Gerhard P. Fettweis*, and Michael Mecking# *Dresden University of Technology, Mannesmann Mobilfunk Chair for Mobile Communications Systems, Germany E-Mail: {hunold | noll | fettweis}@ifn.et.tu-dresden.de #Munich University of Technology, Institute for Communications Engineering (LNT), Germany E-Mail:
[email protected] ABSTRACT 1Mobile
communications is characterized by a fast growing number of users and an increasingly heterogeneous data traffic due to emerging multimedia packet services. Likewise, users desire more mobility support, thus demanding permanent availability at any place and any speed. The conventional answer to this problem is building new wireless standards. But these standards require totally new hardware, users cannot use their old terminals, and no wireless communication among networks is possible which considerably constrains the availability demand. Instead, we propose a unified way for basic network access and connectivity maintenance. It is embodied by a perpetually available signalling channel – the Network Access and Connectivity Channel (NACCH) and a common functionality trunk, shared by several networks. With this concept, networks can communicate and, hence, form a mega-network where users have universal access and roaming support.
proposal is then given in Section III, while Section IV will show some performance results for this approach. Section V presents an overview of ideas that are related to the NACCH concept. As a summary, Section VI gives an outlook on the potential evolution of the proposed concept. II. NACCH CONCEPT The suggested solution can be motivated from a user-centralized view. •
•
•
A user wants to run complex multimedia applications on the move with just the same convenience as he is used to in a fixed network. A user will not care about the types of surrounding wireless networks; he just desires permanent access to any arbitrary network that matches his needs. Moving around, a user demands connectivity maintenance according to the QoS requirements of his application. This includes handover and even seamless roaming between networks.
I. INTRODUCTION Progress in wireless communications has ever been driven by enhanced user needs. This situation led to the 2nd generation mobile networks providing digital low rate (speech) services and is currently accelerating the development of the 3GPP UMTS networks enabling medium rate (e.g. video) applications. In future mobile communications, users will ask for multimedia applications with increasingly heterogeneous nature resulting in diverse Quality of Service (QoS) requirements. Moreover, number and variety of mobile networks will increase due to the development of new advanced wireless communications technologies.
These requirements lead to a concept of universal network access and connectivity support. It could be thought of as a Wireless System BIOS (W-BIOS) in analogy to a computer operating system. It comprises a signalling channel (the Network Access and Connectivity Channel – NACCH) with a minimum specification air interface [1] and a base functionality trunk, embodied by the NACCH Core Protocol (NCP), that controls the NACCH. The channel can even be shared among different networks for interoperability. Fig. 1 shows a schematic network architecture that clarifies the fundamental role of the NACCH.
It is our vision of wireless mobile communications [1] to enable the required flexibility by two things: the deployment of the emerging software radio technology and a common signalling channel shared by several networks. This channel is used for concerted spectrum allocation, network access, and connectivity maintenance. This concept is presented in Section II. A specific implementation
A. Basic Principles One inference of the user requirements is a separation between the functionality set and the signalling channel. This separation assures interoperability (same functionality) between networks with different NACCH specifications (different air interfaces). Second, the NACCH itself occurs in two different views: On one hand, it is a logical channel that is characterized by the transferred functionality; on the other hand, it is a physical channel that transfers the corresponding signalling messages.
1. This work has been supported by a grant from BMBF (German Ministry for Education and Research) within the Priority Program ATMmobil [2].
Additionally, several physical requirements can be formulated for the NACCH in order to fulfil the proposed role
Fixed Network Part
W-BIOS
Network 1
Network 2
NACCH Core Protocol
NACCH Core Protocol
Network n
...
NACCH Core Protocol
NACCH Radio Channel
Access & Connectivity
Access & Connectivity
Access & Connectivity
Mobile Users
Fig. 1: Schematic network environment with the NACCH for network operation. It shall: • •
be permanently and everywhere available. transmit messages with highest possible security and robustness. comprise a common base functionality ensuring network interoperability. be easily extensible for system specific features. comprise preferably several modes (e.g. broadcast, random access, etc.). realize small delays and short messages.
• • • •
B. Views of the NACCH The logical view describes the functional range of the WBIOS. It is shown in Tables 1 and 2. The bold items have immediate networking importance. Hence, the corresponding NCP protocol units must be standardized to ensure network interoperability. The remaining items bear a great importance, too, but they are mainly system specific. The physical view describes the radio interface of the NACCH which, in general, can be arbitrarily implemented. In order to allow for network interoperability, however, networks must share a common minimum specification air interface. Table 1: NACCH Functionality (1) Network Accessing • • • • • • •
registration authentication paging location update network selection beacon for channel measurement / synchronization measurement reports III. EXAMPLE SYSTEM
To make a proof of concept, a particular implementation
Table 2: NACCH Functionality (2) Connectivity Maintenance • • • • • • • •
radio connection establishment handover mobility support QoS negotiation radio resource management adaptive transmission modem configuration power control / power saving
proposal in a 4th generation multimedia mobile communications system approach is presented (Integrated Broadband Mobile System – IBMS [2], [3]). In the following, a short description of this novel system approach is given. A. IBMS Overview The IBMS system is a TDD-based broadband spread spectrum system operating on the 5 GHz frequency range, with a bandwidth of 25 MHz per carrier. In general, it is a CDMA/SDMA system which uses smart antennas for user separation and achievement of higher data rates. In order to realize this, a hierarchical system of three Traffic Channel Classes (TCC) is defined, establishing a trade-off between mobility and data rate (see Fig. 2). Hence, the hierarchical system of the TCCs is the means to handle the adaptation of the air interface to the user needs. Characteristics of a Traffic Channel Class are modulation scheme, error coding, interleaving, data rate, antenna configuration, multiple access method, frequency range, etc. A unified modem concept [4] allows for seamless transitions between the TCCs during a connection. The NACCH is depicted as a low-rate channel in Fig. 2, available at any speed. Moreover, IBMS provides a corresponding set of three Network Service Classes (NSC) to efficiently handle the different QoS requirements of the applications. The NSCs each are assigned QoS frame parameters and a functional
Data Rate
TCC-C 34 Mb/s
BS
•
TCC-B 2 Mb/s
TCC-A
64 kb/s
ling channel as given in Table 3, but here several shorter codes are used for spreading. These uplink codes must be unambiguously derivable from the broadcast code. For instance, they could be selected from a given set of codes by some broadcast flags. After registration a mobile terminal switches to dedicated mode. If no data connection has been established yet, separate signalling channels as broadcast and random access channels are used (outband signalling). As soon as an application is started, inband signalling should be striven for.
NACCH moveable
portable
high
Mobility
Fig. 2: Trade-off between mobility and data rate, realized in a hierarchical set of Traffic Channel Classes set to configure the corresponding TCC. They inherit a common basic functionality from the NCP. B. NACCH Implementation The NACCH must comply with the restrictions that apply to the data transmission. The TDD frames have a length of 1.5 ms, equally divided between uplink and downlink periods, which are separated by 10 µs-long guard intervals. The system will operate with a symbol rate of 18.162 Mbauds, and in each 0.74 ms uplink/downlink phase 12.440 symbols can be used for data transmission or signalling, the rest being reserved for a training sequence (about 1000 symbols). All TCCs must operate frame-synchronized in order to avoid interference between uplink and downlink which also applies for the NACCH. In general, two types of signalling are proposed: inband and outband signalling. When a data connection is established the signalling should be switched to inband. That is, the NACCH signalling messages are transmitted piggybacked with the data. This realization assures that there is only one physical channel active at the same time. This keeps the modem complexity down and allows for the use of nonlinear power amplifiers. Vice versa, in higher TCCs where smart antennas are employed the inband signalling can be disrupted if the direction of the main beam is suddenly obstructed. In this case there must be an automatic fall-back to the separate signalling channel in order to guarantee seamless NACCH signalling. Despite the two mentioned kinds of signalling, the NACCH will be realized in different modes: •
•
In broadcast mode, the NACCH works as a separate downlink signalling channel with the specifications given in Table 3. The base stations send broadcast messages on this channel periodically which include basic information like the spreading codes used etc. It is realized with a single long spreading code in all cells where the distinct base stations each use a different phase of the code. The random access mode is needed in uplink for network contact. It is also realized as a separate signal-
Table 3: Separate signalling channel specification Parameter
Value
multiple access spreading gain bit rate packet size outer code
CDMA ≈ 200 40 kbits/s 60 bits Reed-Solomon Code over F32
block length correctable errors probability of undetected error
24 3
inner code block length (bits) codeword interleaving segmentation delay (ms)
≤ 5.2·10-11 if e > 8 convolutional code R = 1/6, ν = 8 768 32 x 24 1 x 768 0.75
IV. PERFORMANCE TRIALS The interference induced by the NACCH and the resulting data rate on the NACCH was investigated by dynamic discrete-event simulations of a complex radio network model [5]. Table 4 shows simple assumptions on the packet sizes of some NACCH signalling procedures that were used. Notice that the broadcast channel is modelled to be permanently active in downlink with the same constant power as a normal traffic channel. This is a pessimistic assumption in order to get an upper bound for the interference. The dynamic simulation approach [6] allows for a realistic modelling of transient processes as registration, connection establishment, handover, adaptation of air interface (TCC transitions), etc. A larger cell cluster with 27 cells (diameter 300 m) was used as a network model. The assumed constant user speed was 20 m/s. Only TCC-A (lowest data rate; see Fig. 2) was investigated. This TCC has the worst BER performance because the relatively short block length does not permit turbo coding [4]. The circuit-switched calls were generated according to a Poisson process. The channel was modelled with a path loss exponent of 3 and overlaid log-normal shadowing with a standard deviation of 6 dB.
Table 4: Simple modelling of some NACCH signalling procedures Signalling
Size
broadcast channel registration deregistration connection establishment connection release handover TCC transition
Separate channel occupation
60 bits / timeslot (DL) 300 bits (UL + DL) 300 bits (UL + DL) 180 bits (UL + DL) 180 bits (UL + DL) 300 bits (UL + DL) 300 bits (UL + DL)
In order to assess the impact of the NACCH signalling on the network performance at radio network level, capacity simulations with and without modelling the separate signalling channel were done. The results are shown in Fig. 3 for different traffic loads per cell. Clearly, the SIR in downlink (dashed curves) with NACCH is lower than without NACCH due to the broadcast channel. But in reality, the traffic on the broadcast channel will be bursty. Thus, the downlink will perform better. The additional signalling (due to handover, user registration, etc.) is so low that it has no noticeable impact on the uplink performance as the overlapping solid curves demonstrate. The average required data rate on the NACCH was investigated next. In uplink, the NACCH data rate grows linearly with the offered traffic due to the simple traffic model as apparent from Table 5. However, this mean value is far below the capacity of a separate signalling channel. In downlink, another full separate channel is occupied by the broadcast according to the model. Table 5: Average NACCH data rate per cell (uplink) traffic load (Erlangs/cell)
0.5
1
2
5
10
mean data rate (bit/s)
12.7
26.4
68.7
177.6
397.0
1 NACCH channel in DL permanently 5 timeslots UL + DL 5 timeslots UL + DL 3 timeslot UL + DL 3 timeslot UL + DL – –
V. RELATED APPROACHES Due to the novelty of the NACCH concept, there are hardly comparable approaches. A subset of the NACCH concept was implemented in the CATM demonstrator (Cellular ATM Access Network) [7], a parallel project to IBMS within ATMmobil [2]. Currently, the idea of a common signalling channel is investigated in the COMCAR project [8], where UMTS services shall be realized in new spectrum bands. Software-defined radio architectures are studied in the SDR Forum [9]. Ideas as in the advanced 100
UL, without NACCH DL, without NACCH UL, with NACCH DL, with NACCH
1 channel 2 channels 3 channels
98
percentage of number of signalling channels (%)
40
35
mean SIR (dB)
outband outband outband outband outband inband inband
Due to the high burstiness on the NACCH, it is interesting to ask for the required maximum number of separate signalling channels in the network. This number is decisive for the construction of NACCH code sets. Fig. 4 shows the percentage of the required number of channels. For a traffic load of 10 Erlangs/cell and higher, at least two channels must be available. Below this threshold, one channel is sufficient in conjunction with a congestion resolution mechanism. As a reference, simulations were done with signalling phases 10 times as long as specified in Table 4 (e.g. 3000 bits instead of 300 bits). Then two separate channels are needed for at least 1 Erlang/cell as Fig. 5 shows. When the traffic is higher than 5 Erlangs/ cell, up to three channels are needed in parallel. The occurrence of a higher number of channels is negligible, i.e. possible collisions can be resolved.
45
30
25
20
15
10
In-/Outband
96
94
92
90
88
86
84
82
0
0
1
2
3
4 5 6 7 average traffic load per cell (Erlang)
8
9
10
Fig. 3: Network SIR performance in uplink/downlink for different traffic loads
0.2
0.5 1 2 5 average traffic load per cell (Erlang)
10
Fig. 4: Number of required parallel signalling channels for assumptions from Table 4
100
1 channel 2 channels 3 channels 4 channels 5 channels
percentage of number of signalling channels (%)
98
96
94
92
90
88
86
The deployment of the NACCH concept is closely related to the development of software-defined radio. The NACCH enables communication between networks to realize, e.g., spectrum negotiations. Software-defined radio provides the flexibility for networks to communicate via the NACCH, thus enabling self-configuring and ad-hoc networks. Together they form a fruitful coalition, hence paving the way for a plethora of coexisting networks that will no longer be treated as separate, solitary entities. Instead, they will talk to each other, they will understand each other, and they will probably benefit from each other.
84
REFERENCES 82
0
[1] 0.2
0.5 1 2 5 average traffic load per cell (Erlang)
Fig. 5: Number of required parallel signalling channels for 10-fold signalling compared with Table 4 “cognitive radio” approach [10] could be fabulously merged into the NACCH functionality.
[2]
VI. CONCLUSION AND OUTLOOK The development of future mobile radio systems is challenged by enhanced user requirements and increasingly heterogeneous multimedia applications. Consequently, such systems must become more flexible, requiring a unified and generalized method to maintain wireless access and connectivity. The proposed solution is a common Network Access and Connectivity Channel (NACCH). Along with this channel, a basic functionality trunk is provided with the NACCH Core Protocol. This solution requires a minimum specification air interface and a standardized base set of functionality shared by all networks. The NACCH concept is a fairly new idea providing support for both mobility and wireless aspects. Hence, it turns out that protocols and physical layer have to be jointly developed. Otherwise, such complex systems cannot be designed efficiently in the future. Much of the NACCH functionality is also implemented in existing systems as, e.g., location update, handover, etc. In the NACCH concept, however, all those functions are grouped together in a single network entity, because the focus is on the functionality, not on the physical realization. Moreover, the concept allows for access to arbitrary networks, that implement the NACCH, from the same terminal. A practicable realization of the NACCH concept was implemented and investigated in a comprehensive radio network simulator. Due to the pessimistic assumption of a permanent broadcast channel, the performance in downlink is worse than it will be in reality. However, the remaining signalling for e.g. handover results in no significant SIR degradation. Depending on the length of the signalling packets and the offered traffic load, code sets with two or three NACCH codes should be used.
[3]
[4]
[5]
[6]
[7]
[8]
G. Fettweis, R. Steele, and P. Charas, “Mobile Software Telecommunications,” in Proc. 2nd EPMCC’97/3. ITG-Fachtagung “Mobile Kommunikation”, Sept. 30-Oct. 2, 1997, Bonn (ITGFachbericht 145, VDE-Verlag Berlin, Offenbach, 1997, Germany, pp. 321-325). R. Keller et al., “Wireless ATM for Broadband Multimedia Wireless Access: The ATMmobil Project,” in IEEE Personal Communications Magazine, vol. 6, no. 5, pp. 66-80, October 1999. D. Hunold and G. Fettweis, “A Flexible MobilityAware Transmission Scheme Utilizing Smart Antennas in a Wireless Cellular Network,” 6th Intl. Workshop on Mobile Multimedia Communication MoMuC’99, pp. 262-270, San Diego, CA, USA, November 15-17, 1999. A. Noll Barreto, M. Mecking, and G. Fettweis, “A Flexible Air Interface for Integrated Broadband Mobile Systems,” in VTC Spring 2000, pp. 18991903, Tokyo, May 15-18, 2000. J. Deissner et al., “A Development Platform for the Design and Optimization of Mobile Radio Networks,” in Proc. GI/ITG Special Interest Conference MMB’99, pp. 129-133, University of Trier, Germany, Sept. 1999. J. Fischer et al., “Object-Oriented Modeling of a Generic Mobile Radio System for Dynamic System Simulation,” 1999 Summer Computer Simulation Conference SCSC’99, pp. 240-247, Chicago, Illinois, July 11-15, 1999. B. Stantchev and G. P. Fettweis, “Burst Synchronization for OFDM-based Cellular Systems with Separate Signaling Channel,” in Proc. of VTC’98, pp. 758-762, Ottawa, Canada, May 18-21, 1998. Communication and Mobility by Cellular Advanced Radio (COMCAR) project, available at http://www.comcar.de/
[9] [10]
The Software Defined Radio (SDR) Forum, available at http://www.sdrforum.org/ J. Mitola III, “Cognitive Radio for Flexible Mobile Multimedia Communications,” in IEEE Intl. Workshop on Mobile Multimedia Communications MoMuC’99, pp. 3-10, Nov. 15-17, 1999.
