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Integration of Heterogeneous Wireless Networks using MIRAI Architecture Masugi Inoue, Khaled Mahmud, Homare Murakami, Mikio Hasegawa, and Hiroyuki Morikawa Communications Research Laboratory 3-4 Hikarino-oka, Yokosuka, Kanagawa, 239-0847, Japan, [email protected], www.crl.go.jp/yrc/ Abstract - A key word describing next-generation wireless communications, so called 4G, is “seamless.” As part of the e-Japan Plan promoted by the Japanese Government, the MIRAI (Multimedia Integrated network by Radio Access Innovation) project is aiming at the development of new technologies that will enable seamless integration of various wireless access systems for practical use by 2005. First, this paper describes heterogeneous wireless networks and introduces the concept of MIRAI. The most significant feature of MIRAI is the provision of a set of signaling functions: radio-access-network discovery and selection, heterogeneous paging, and vertical handoff. These functions make it possible to offer seamless services between different wireless systems. A medium access control (MAC) protocol for basic access networks, which are dedicated wireless systems providing the signaling, is then introduced. A proofof-concept demonstration system is also briefly introduced. The delay performances of the signaling procedures are presented. Keywords – 4G, seamless, RAN discovery and selection, IPv6, heterogeneous wireless networks I. INTRODUCTION With the start of 3G wireless services, people in both academia and industry have begun to show more and more interest in the next generation of wireless communications, i.e. 4G (for example, see Refs. [1] and [2]). Figure 1 shows our understanding of the transition from the second and third generations to the new generation in wireless communications. In the new generation, it will be quite hard to meet the various requirements for mobility, data rate, coverage, price, and services with only a single system, like 4G cellular. There will thus be a greater number of wireless systems such as intelligent transportation systems, ultrahighspeed wireless LANs with wireless access, and digital video broadcasting and high-altitude platform systems, as well as 4G cellular systems to meet our insatiable needs. In other words, new-generation wireless networks will be heterogeneous. The Communications Research Laboratory (CRL) has been conducting the MIRAI research project [3], a national project under the e-Japan Plan. MIRAI is Japanese for “future” and an acronym of Multimedia Integrated network by Radio Access Innovation. Its goal is to develop technologies for seamless integration of heterogeneous wireless networks in a new era.

4th Generation Mobility 4G-Cellular




3G-Cellular IMT-2000


DVB Ultrahigh-speed Wireless Access

3rd Generation


High-speed Wireless LAN

2nd Generation

Ultrahigh-speed Wireless LAN


Fixed Networks

N-ISDN 10k



Gigabit-net 10M



Data rate

Fig. 1 Generations of wireless networks.

II. SCOPE OF MIRAI The followings are the main requirements for the provision of seamless services over heterogeneous wireless networks. A. Requirements 1) Radio Access Network (RAN) Discovery and Selection A user at a specific geographic location wanting to start communication first needs to find out which RANs are available in that area. This, however, can be a time- and energy-consuming process because users need to search all the RANs. Generally, three discovery methods are possible: distributed (users search for available RANs), centralized (the network announces which RANs are available), and a combination of both. After finding out which RANs are available, another problem is selecting the most appropriate one from the number of available RANs. The RAN selection can be based on both the user’s preferences on price, data rate, battery life, service grade, etc. and RAN status (e.g., available bandwidth, congestion, etc.). RAN selection makes it possible to deliver each service via a network that is the most efficient for it. 2) Managed Core Network To page a user in a heterogeneous wireless system, the network should know where the user is and which RANs are available to the user. After the user has connected to a RAN, the network should enable QoS guaranteed seamless handover within the same RAN (horizontal) and among different RANs (vertical). One of the main functional

on the network. To answer the question “Which RANs are available to the user?”, the resource manager figures out the answer by using the location information and other data such as the user’s preference and network status and sends a list of available RANs to the MUT. The user then selects a RAN from the list and connects to it.

entities of the network is a resource manager, which coordinates traffic distribution and selects the most appropriate RAN. It has a common database for managing users’ profiles through entries such as authentication, location, preferred access system, billing, policy, and users’ terminal capabilities. A network that meets these requirements is called a “common core network (CCN).”

C. Network Model

3) Multi-Mode Terminal

Various network models are possible under the MIRAI concept; a possible one is illustrated in Fig. 3. Base stations serve as access points and interface with CCNs. The CCNs are connected to the Internet via gateway routers. A CCN provides services for several RANs. In general, the RANs overlap, so an MUT can access several RANs in one location. The area covered by these wireless networks can be quite large. Mobile IPv6 is the envisioned protocol for connecting CCNs and providing global (macro) mobility management. In a CCN-managed area, fast handover between base stations requires local (micro) mobility management. MUTs attached to a base station use the IP address of the gateway as their mobile IP care-of address. Inside a CCN, MUTs are identified by their home address. Base stations are connected to (or integrated with) a regular IP forwarding engine [6].

For a single terminal to be able to access different RANs, the terminal should have multiple RAN modules or be a software-defined radio (SDR)-based reconfigurable terminal. Such a terminal is called a “multi-service user terminal.” B. Basic Access Signaling (BAS) and Network (BAN) To develop technologies to meet these requirements, we are focusing on three major technical components: basic access signaling (BAS), a common core network (CCN), and a multi-service user terminal (MUT). We describe here only the details of BAS because it is the most original aspect of MIRAI. The purpose of BAS is to provide a set of functions specific to heterogeneous wireless networks including RAN discovery, selection, vertical handover, location update, paging, and AAA [4]. A basic access network (BAN) is a wireless system that provides BAS between users and the network.


We have two alternatives in deploying a BAN. One is to use existing wireless systems such as wireless LANs, 2G cellular systems, and 3G cellular systems, over which BAS is overlaid while keeping their own services. We have been researching this choice. The other alternative is to provide a wireless system dedicated to BAS, either by using already existing systems like paging systems and 2G-cellular systems or developing a completely new system.

Region Register


term (inc. tion initia s ll a C RA N (1) ilable f ava o t is (2) L

BAC (3) Selection of a RAN








BAN MUT Radio Access Network ( RAN) Foreign Region Network

Home Region Network

GR: Gateway Router; SN: Signaling Node; ANG: Access Network Gateway; ANR: Access Network Router

Fig. 3 Conceptual view of MIRAI system. ) data ation

Resource Manager

BAN-BS (1) (2)


RAN1 (4)

(4) Connection setup


MUT (Multi-service User Terminal)




The BAN should have a broad coverage area, preferably larger than that of the RANs it supports, and a reliable communication means for signaling transmission, where a high data rate is not necessary [5]. We consider a BAN to have base stations and basic access components inside terminals. Example signaling when an MUT makes a call is illustrated in Fig. 2. To initiate the call, the MUT sends a packet including location information to a resource manager loc and inal ID


IPv6 network

Coverage of Basic Access Network (BAN)

Common Core Network (CCN)

Fig. 2 Example signaling of RAN discovery and selection using BAN.

We have developed a proof-of-concept demonstration system in which the fundamental functions of MIRAI are implemented. In general, BAS can be implemented in any wireless system, including paging systems, 2G-cellular systems, WLANs, and 3G cellular systems. We focused on a two-way paging system because it is currently the most possible platform on which BAS can be implemented, but we consider this a short-term solution. We have been considering middle- and long-term solutions using other wireless systems as well.

B. Configuration of an Experimental MUT We implemented all the functions of an MUT into a Linuxbased laptop PC with PHS and 802.11b cards (see Fig. 5). A basic access component (BAC) for communicating with the BAN-BS is connected to the PC. It has a GPS receiver for obtaining location information. The MUT supports RAN discovery, RAN selection, and vertical handover.

A. Configuration of the System As illustrated in Fig. 4, the CCN is composed of a resource manager (RM), a BAN base station (BAN-BS), and five other nodes, which are implemented in Linux-based PCs. Within the CCN, cellular-IPv6-based micro-mobility is supported. All the components are located in an experimental room on the third floor of a seven-story building. A five-meter antenna for the BAN-BS is installed on top of the building and connected to the BAN-BS by coaxial cable. We set up a star-topological optical fibre-based network in the Yokosuka Research Park (YRP) area. Three access points are outside the building, and seven are inside the building. Each of the inside access points can have IEEE802.11a, 802.11b, the personal handy-phone system (PHS: Japanese digital cordless telephone system), and Bluetooth, while the outdoor access points can have 802.11b and PHS. All fibres connect to a media converter, enabling us to make any network configuration. In the experiments mentioned here, we used only 802.11b and PHS. Region Register Correspondent (RR) Node (CN) Antenna

Hub 1 Gateway (GW)

Resource Manager (RM)


Hub 2 Signaling Channel by Basic Access Network (BAN)

Intermediate Node (IN) Common Core Network (CCN) IN 2

PHS Access Server (PHS AS)

Fig. 5 Experimental MUT. C. Configuration of Basic Access Network (BAN) A BAN should meet three major requirements (high capacity, high reliability, and low-power consumption) because it should be a reliable signaling network that can cover the areas of all of the RANs at best while providing relatively low-speed wireless links. An FDD/TDMA-based BAN system (major parameters listed in Table 1) was developed for the 400-MHz band. Both uplink and downlink channels have a bandwidth of 25 KHz, providing a transmission rate of 19.2 kilo symbols/sec. The control packets (reservation, ACK, etc.) in the uplink channel are sent using 4-QAM in all situations to maintain reliability. Adaptive modulation is used in uplink data transmissions to meet the contrasting requirements for wider coverage and higher capacity using minimal power at the BAC. Thus, uplink data packets are sent using one of six modulation methods. In contrast, only 16-QAM is used for all packets (both control and data) in the downlink channel. Rate 1/2 convolutional coding is implemented using an industry standard encoder and Viterbi decoder. This gives a coding gain of 5.2 dB at a BER of 10-5.

IN 3

Table 1 BAN physical-layer parameters. Optical Fiber-Based Test-Bed Network

802.11b AP

Media Converter Unit

802.11b AP

BAC: 367.3375 MHz

Tx power

BS: < 1 W

BAC: 200 mW

Ant. gain

BS: 7 dBi

BAC: 2 dBi

Noise figure



Down: 16QAM



BAN Component (BAC)

Tx frequencies BS: 385.3375

PHS Card 802.11b Card Multi-service User Terminal (MUT)

Fig. 4 Configuration of demonstration system.

Up: Adaptive QAM(64/16/4) + Bi-orthogonal(32/16/8)

Tx rate

19.2 kilo symbols/sec in a 25 KHz channel


Dynamic TDMA / Dynamic TDM




A. Frame/Slot Configuration of BAN MAC The MAC (medium access control) protocol for BAN is based on dynamic TDMA for uplink and dynamic TDM for downlink. Each time frame on the downlink is divided into a 6-slot control field and an n-slot data field (Fig. 6). The control field is used to transmit control signals including frame start, broadcast, rate-switching indication, slot assignment, and acknowledgement. The signal formats of the downlink control field are shown in Fig. 7. The data field is used to transmit downlink data packets to users. Each time frame of the uplink channel includes two reservation request (RR) slots, two uplink acknowledgement (UA) slots, two reserved slots, and another n slots for uplink data transmission. Each RR and UA slot is divided into four mini slots. Thus, eight mini slots are reserved for RRs, and eight are reserved for UAs. The number of data slots, n, is a design parameter; it can be set according to traffic volume within a BAN cell, response time, Doppler frequency, etc. When n = 32 and there are 276 symbols/slot and 19.2 ksym/sec, one slot equals 14.375 msec. A 64-bit-long unique ID is designed for each BAC while a 16-bit-long temporary ID is used in the MAC layer. To improve reliability, a link-layer ARQ (automatic repeat request) scheme is used.




S A D O W N 2


D D D D D D D D D D D D C C C C C C D D D D D D D D D D . . . n 1 2 3 3 4 4 1 2 3 4 5 6 7 8 9 1 0   Data Field Control Field n slots 6 slots

Uplink R R R U U s R R A A v d

R U U U U U U U U U U U s D D D D D D D D D D D . . . v 1 2 3 4 5 6 7 8 9 1 n d 0  


Frame start Broadcast Rate switching indication Slot assign. for reservation request Slot assign. based on previous status Dynamic assign. of available slots Downlink slot assign. ACK of uplink data packets Downlink data Reservation request ACK of downlink data packets Uplink data Reserved

Fig. 6 Frame /slot configuration of BAN. FS slot (Frame Start + Broadcast) Frame Start (96bit)

Padding (360bit) Frame Body(462bit)

B slot (Broadcast slot ) Broadcast (456bit) Frame Body(456bit)

SA-D1 slot (Slot Assignment of downlink data slot ) Rate Switching Indication1(64bit)

To establish a connection with the BAN-BS, users send their request packets in the reservation request (RR) slots. The response is not a packet to each user but a bitmap to all users, thereby saving radio resources. The bitmap is broadcast in the SAR (slot assignment for RR) sub-field.

Any UD slots left over after assigning UD slots to both requesting and communicating users are additionally assigned in the SADYNM (dynamic assignment of available slots) sub-field to users who still have many packets to be transmitted.

R S I 2


As mentioned above, adaptive modulation is used for uplink transmission. The BAN-BS decides which modulation should be used in the next timeframe and notifies the MUT by using rate switching indication (RSI) bits in the downlink control field. Thus, the number of bits in a MAC-layer frame varies from 75 to 720 while the number of symbols in the physical frame remains constant (see Fig. 8).

Another bitmap is also used in SAPRV (slot assignment based on previous status) to give information about the next available slot to already communicating users. This bitmap, however, does not indicate exact uplink data (UD) slots to be used. Users themselves compute their exact UD slots by using both SAR and SAPRV bitmaps.



B. Features of BAN MAC Protocol 1.


S S S A A R S A D D S A P O Y I R R W N 1 N V 1 M

SAR SAPRV (8) (32)

SADYNM (192bit=16bit× 12)

DACK (32)

SA DOWN1(DD1∼DD8) (128bit=16bit× 8)

Frame Body(456bit)

SA-D2 slot (Slot Assignment of downlink data slot ) SA DOWN2(DD9∼DD32) (384bit=16bit× 24)

Rate Switching Indication2(64bit)

Padding (8)

Frame Body(456bit)

Fig. 7 Signal format of downlink control field. Length (5 or 7 bit)

Frame Type (8 bit)

Control (8bit)

Frame body (variable)

CRC-16 (16bit)

padding (variable)

Tail Bit (6bit)

Mac Data frame (variable: 75 - 720 bits) FEC + interleave + adaptive mod. RampUp (3sym)

Preamble (20symbol)

Physical-frame Payload  (240-symbol data + 5-symbol pilot)

Guard (8sym)

Physical frame (276 symbols)

Fig. 8 Uplink MAC-layer frame and physical layer. 5.

Acknowledgement of a data packet sent in a downlink data (DD) slot is piggybacked on an uplink data packet. When a user has nothing to transmit but a NACK, the user transmits it in a UA slot.




Req. TA

Call Initiation

Req. auth. and TA





-Authentication -TA assignment

Req. connection G


Location data Available RAN list

Connects to a RAN Req. to release TA

TA release

We have introduced MIRAI, a solution to seamless integration of heterogeneous wireless networks towards 4G. MIRAI mainly concentrates on the development of technologies related to three areas: a basic

Available RAN list

Connects to a RAN

C. Preliminary Evaluation



Location data from GPS C

C3 and C4 in the downlink control field, which include vital information, are transmitted twice to increase reliability.

As shown in Table 2, in the paging, if an MUT already had a TA, only about two seconds were needed for the BAN-BS to return a list of available RANs after paging (process B), although process A, which is necessary when an MUT has no TA, took a few seconds. Therefore, an MUT can reasonably be expected to connect to a RAN within a few seconds after being paged by the BAN-BS. In call initiation, an MUT with a TA received a list of available RANs from the BAN-BS in about two seconds (process F).




To improve reliability, a stop and wait ARQ scheme is used for both uplink and downlink packet transmission. Selective repeat ARQ is also used for downlink transmission.

We obtained sample delays for processes A to G in Fig. 9 both with experiments and simulations under the condition that down- and up-link channels had no bit errors. In the experiments, we obtained data for only processes A, B, and G because the system did not have a function to monitor the other processes. In the simulations, we tested two cases: the system had only one MUT or 1000 MUTs. Each MUT made a call or was paged once an hour on average.

GW or RM

A -TA assignment


DD slots are assigned in SADOWN (downlink slot assignment) to the users to whom downlink data packets are transmitted. Each user needs to listen to only the assigned slots. This helps save power.

We evaluated the delay performances of the BAS scheme depicted in Fig. 9. It can be divided into three parts: paging, call initiation and temporary address (TA) release. In paging, the BAN-BS broadcasts the 128-bit-long global address (GA) of an MUT, which has a BAC, in the B slot. The MUT then requests authentication and a 16-bit-long TA. After having received the TA, the MUT is paged by the BAN-BS. The MUT returns location data obtained by its GPS receiver to the resource manager (RM). The RM returns a list of available RANs. If the MUT already has a TA, the procedures indicated by dotted lines are not necessary. The TA release is needed to make efficient use of a limited number of TAs.

BAN-BS Broadcasts GA

Call initiation





A 64-bit-long unique ID is used to identify each BAC while a 16-bit-long temporary ID is used at the MAC layer to increase efficiency.

OK MUT: Multi-service User Terminal BAC: Basic Access Network Component GW: Gateway

RM: Resource Manager GA: Global Address TA: Temporary Address RAN: Radio Access Network

Fig. 9 Evaluated procedures of basic access signaling. access network (BAN), a common core network, and a multi-service user terminal. A MIRAI demonstration system was also presented. The feasibility of basic access signaling, necessary for seamlessness, over a newly developed BAN was demonstrated by the system. Future work includes improving the BAN and studying the feasibility of applying it to other wireless systems. REFERENCES [1] B.G. Evans and K. Baughan, “Visions of 4G,” Electronics & Commun. Eng. J., vol. 12, no. 6, pp. 293-303, Dec. 2000. [2] S. Ohmori, Y. Yamao and N. Nakajima, “The future generations of mobile communications based on broadband access technologies,” IEEE Commun. Mag., vol. 38, no. 12, pp. 134-142, Dec. 2000. [3] G. Wu, P. Havinga and M. Mizuno, “MIRAI Architecture for Heterogeneous Network,” IEEE Commun. Mag., pp. 126-134, Feb. 2002. [4] K. Mahmud, M. Hasegawa, G. Wu and M. Mizuno, “Basic access network assisted IP mobility and AAA in MIRAI architecture,” VTC02 Spring, Birmingham, USA, May 2002. [5] K. Mahmud, et al, “On the required features and system capacity of basic access network in the MIRAI,” WPMC’01, pp. 1199-1204, Sept. 2001. [6] M. Kuroda, et al, “Signaling services in wireless IP overlay networks,” in Wireless IP, Artech House, 2002.

Table 2 Processing delays of basic access signaling (BAS). (in sec) Experiments Simulation (1 MUT) Simulation (1000 MUTs)

A 3.065 5.099 5.496

B 1.637 2.128 2.181

C 1.639 2.687

D 1.218 3.031

E 2.185 2.269

F 1.639 2.060

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