Mobility vs. bit rate.

Transcription

1 AN AIR INTERFACE FOR AN INTEGRATED WIRELESS BROADBAND SYSTEM FOR THE 60 GHZ INDOOR CHANNEL Branimir Stantchev, Jens Voigt, Volker Aue, and Gerhard P. Fettweis Dresden University of Technology, Mobile Communications Systems Abstract In this paper the air interface for an integrated broadband wireless system at 60 GHz is introduced. The system is targeted for accommodating dierent multimedia services such as voice, video, and high data rates up to 155 Mbit/s ATM at low cost and to allow for some mobility. The paper discusses the diculties of designing a multiple access scheme and choosing the right modulation techniques for such an integrated system. It is shown that due to the heterogeneity of the dierent demands regarding bit rate, bit error rate, burstiness, cost, and mobility, a modular system structure is needed. After the general problem statement of the basic properties and demands, a solution for the air-interface at 60 GHz is derived. I. Introduction Within the scope of the \Innovationskolleg Kommunikationssysteme" an integrated multimedia communications system at 60 GHz is currently under investigation at the communications laboratory of Dresden University oftechnology. In a today's oce or oce-like environment a great variety of communications and telecommunications networks can be found. Most commonly each oce is equipped with some kind of phone access. In some cases this service is provided through a wireless system such as DECT. In addition, one or more local area networks provide for data transmission and computer communications. The cost for installing and maintaining a variety of networks is high, but as each system provides only a limited number of services, the existence of more than one network is required. The system presented here has the design goal to provide the user with various services of dierent bandwidth requirements like speech, fax, and video communications, as well as the full features of a wireless local area network (WLAN) by one single wireless network. The system is targeted to operate at frequencies around 60 GHz. At 60 GHz sucient bandwidth is available which is required for accommodating services of very high rate. Further advantages can be taken from the propagation properties of electro-magnetic waves at this frequency, as the high attenuation of walls limits cell sizes to the sizes of rooms, and thus, this frequency is ideal for building a pico-cellular network structure. A pico cellular structure is needed for reaching the anticipated capacity. A drawback from using this frequency can be the restriction This work was partly sponsored by the German NSF (DFG) and the Free State of Saxony. mobility fast slow movable integrated wireless broadband system cellular/pcs WLAN 16 kbit/s Mbit/s 155 Mbit/s Fig. 1. Mobility vs. bit rate. WLAN/ATM bit rate to indoor use and the use for short distances between buildings. The system is targeted to integrate various multimedia services. That means we achieve convergence of speech, video, and data transmission. Moreover, our network is designed openly for future services and it is aimed to support the asynchronous transfer mode (ATM) standard to allow for compatibility to existing broadband data networks. The resources like bandwidth and transmission time are allocated dynamically depending upon the user's requirements. In order to cost-eectively oer high data rates and mobility through one system, we allow a trade-o between bit rate and mobility (see Fig. 1). Users who demand a high bit rate can only be portable, i.e., they have to be in a xed place during transmission. Users who demand services of lower bit rates like a plain voice call are allowed to move during transmission. The general system concept for such a network is introduced in [1]. The indoor radio propagation channel at 60 GHz is characterized by almost perfect attenuation of the radio waves by walls, doors, etc., so mutual interference between dierent cells is negligible in most cases and a small frequency reuse factor can be employed [2]. Hence, cell planning is facilitated. A way to get around frequency planning entirely is to use a dynamic algorithm to assign bandwidth to the dierent cells. In order to avoid the employment ofmany expensive s (one in each cell), one or more central s provide signals to multiple transponders. At the signals are upconverted to an intermediate frequency of 2.4 GHz. The and the transponders are interconnected by a ber optics backbone carrying the analog signals at the intermediate

2 frequency. The transponder nally converts the signal to the radio frequency of 60 GHz. The overall system concept is shown in Fig. 2. A serves dierent cells via multiple transponders. The s are connected via a backbone. Virtual cells exist for mobile users. This paper describes the air interface of our integrated wireless broadband system. In the following section we rst outline the concept of the air interface. In the sequel, we describe in more detail the modulation schemes used for transmitting high and low rate data, the frame format, and handover techniques. The concept of virtual cells is explained in Section IIB. Conclusions are presented in Section III. II. System outline In order to support higher data rates a high bandwidth per cell is necessary. The system's overall bandwidth is 21 GHz. As a certain frequency reuse factor (typically 5) is required [2], only a single frequency band of about 200 MHz can be assigned to each cell. Resources are allocated using time division duplex (TDD) to distinguish between and the user and time division multiple access (TDMA) to share resources between dierent users. TDMA has been preferred over code division multiple access (CDMA), since TDMA is less sensitive to nonlinearities. Furthermore the resolution of analogto-digital converters (ADC) and digital-to-analog converters (DAC) at high sampling rates limit the dynamic range of the system. For CDMA a higher dynamic range is required than for TDMA. Another advantage of TDMA is that it avoids the problem of chip synchronization and the necessity of fast power control. It can be shown that a separation in frequency of uplink and downlink using frequency division duplex (FDD) is not necessary in the 60 GHz indoor environment [2]. As hardware design of the and the mobile is facilitated for uplink and downlink using the same frequency, the system does not make use of FDD. For pure TDD, however, the design of higher layer's multiple access schemes is more complicated and synchronization of the signals is aggravated. A. Modulation schemes and data rate classes As several services with dierent transmission rates and mobility requirements are to be supported, the use of multiple modulation schemes adapted to the type of user is necessary. Generally, robust modulation schemes such as minimum shift keying (MSK) or dierentially quarternary phase shift keying (DQPSK) are to be used for mobile users at low data rates as well as for signaling information (network access and connectivitychannel NACCH) for securely builtup and maintained connections even during a mobile user's movement. For the considered cell sizes it is expected that the delay spread does not exceed 100 ns. This is why signals with burst rates of 6 Mbit/s using MSK or DQPSK can still BER HR2 / AWGN HR3 / AWGN HR2 / frequency selective measured IR Ebs/No Fig. 3. Performance of HR2/HR3 in AWGN channel and with a measured impulse response. be considered narrowband, i.e., the delay spread is less than the symbol duration. Thus, receivers can be built at low complexity without the expensive need for an equalizer. For higher rates using single carrier techniques the symbol duration shortens, and an equalizer is needed. This is why wechoose orthogonal frequency division multiplexing (OFDM) for high rate users which can cope with the delay spread and for which an equalizer is not needed [3]. For OFDM a more complex transmitter and receiver design is required than for the low rate single carrier techniques. Single carrier transmission and ordinary ISI equalization becomes infeasible for high data rates, since the necessary signal processing power is currently not available. As the carrier power of the transponders is xed, an increase in bit rate results in a decrease of bit energy. For transmitting with higher rates, a higher signal-to-noise ratio SNR is required at the receiver. Depending on the SNR conditions and the service a user demands, a user can adjust its burst rate to the channel conditions. We restrict the choice of possible modulation parameters by introducing four classes of burst rates of 6, 25, 100, and 200 Mbit/s referred to as LR, HR1, HR2, and, HR3, respectively. The LR class uses MSK or DQPSK. For the HR classes, OFDM is used. The dierent data rate classes and the corresponding modulation schemes and burst rates are shown in detail in Table I. Examples for the necessary signal-to-noise ratios for the high rate channels are given in Fig. 3. The proposed signaling classes have a dierence in bit energy of approximately 6 db going from one class to another. The steps between LR/HR1, HR1/HR2, and HR2/HR3 are 6 db, 6 db, and 3 db, respectively. Taking into account the loss of approximately 2 db between DBPSK-OFDM and DQPSK-OFDM modulation (HR2/HR3), the necessary SNRs for the dierent data rates at a given BER are separated by about 6 db each. Depending on the ii

3 hallway hallway hallway Fig. 2. System concept. TABLE I Data rate classes and corresponding modulation schemes. Data rate class Modulation Burst rate (Mbit/s)/ Symbol duration (s) LR (low rate) /4-DQPSK or MSK (dierential 6/0.33 quaternary phase shift key- ing or minimum shift keying HR1 (high rate 1) OFDM/DBPSK with 50 carriers (orthogonal frequency division multiplexing/dierential binary phase shift keying 25/2 HR2 (high rate 2) OFDM/DBPSK with /2 carriers HR3 (high rate 3) OFDM/DQPSK with /2 carriers capabilities of a user's terminal, the desired data rate, the available SNR, and the available bandwidth, a user is allowed to select any of these modulation classes for transmitting its data. If a user is transmitting with a high rate modulation scheme, and the channel condition worsens, e.g., due to user mobility, or insucient SNR at the receiver, and the modulation does not perform satisfactory, a fall-back to a class of lower rate with a more robust modulation scheme is possible. Thus, a connection can be maintained at a lower data rate class rather than dropping it. Likewise, if a user demands a higher rate and the conditions allow for it, a switch to a modulation class of higher rate is possible. B. Frame format In order to enable the support of the anticipated diversity of services with their individual bandwidth demands, a exible frame format is needed. The shortest period a user is allowed to transmit or receive data is referred to as a slot. A frame consists of 20 slots. The rst two slots are used for transmitting signaling information. The subsequent slots are used for transmitting data in the uplink and downlink. A specic user can allocate one or more slots per frame, depending on its demands and the system capacity currently available. Some services do not require the same data rate in the uplink as in the downlink. Since these requirements can vary over time the border between uplink and downlink can be adjusted to suit the actual needs. The frame period is set to be 3 ms and every frame consists of 18 slots occupying 5% of the frame duration. The initial 10% of the frame are used for signaling information (NACCH), where signaling from the to the user prevails. The time slot scheme is shown in detail in Fig. 4. Every 150 s slot includes a guard interval of about 10 s taking into account the switching time between up and downlink. The maximum propagation time of 200 ns in every direction (corresponding to a maximum cell radius of 6 m) is negligible. A larger guard interval can be avoided, if and user do not use consecutive time slots. In one time slot 880 bits can be transmitted resulting in an overall burst transmission rate of about 6 Mbit/s. The 5% time slots are chosen to satisfy several criteria, namely a short overall delay time, transmission of at least one ATM cell within one slot, small iii

4 super frame = 9 frames guard- interval frame (100%) R (up/down) : : : : : : R (up/down) signaling (10%) downlink uplink L L uplink << downlink H : : L L H : S 5% slot transmit time slot duration 150 µ s guard interval (10%) frame duration 3ms H - high rate L - low rate Fig. 4. Frame format. overhead for robust time and frequency synchronization, handover requirements (scanning of adjacent cells), and the ability to realize the lowest (6 Mbit/s) data rate using an inexpensive modem structure. C. Multichannel-Simulcast-Handover Handover is performed by the concept of virtual cell extension. As described in Section I, at frequencies around 60 GHz the cell dimensions are typically determined by the indoor environment such as walls, doors, etc. For these small cell sizes, even for the speed of a pedestrian, a handover has to be carried out almost instantaneously within the fraction of seconds. Consequently, mobile assisted handover or mobile controlled handover concepts as used in GSM or DECT are not applicable, where handover procedures take time up to 3 seconds [4]. We therefore propose to simultaneously transmit (simulcast) the information from the transponders in those adjacent cells that can directly be reached from the current location of the mobile user. In doing so a handover has to be carried out only on the physical layer by tuning to the frequency of the closest transponder. The network then has time to recongure the new simulcast environment, i.e., to determine the new user location and to nd the new neighboring cells. For this kind of handover the topography has to be initially known by the system. Learning the topography to the network, however, has to be done only once during setup either by programming it in, or by learning it adaptively during setup mode. Full mobility is guaranteed only for users of the low data rate class for which the burst rate is 6 Mbit/s. Using a code with code rate r = 0:5, about 150 kbit/s can be transmitted in each time slot. A low data rate user, e.g., voice transmission at 16 kbit/s requires only every ninth time slot. The parameters have been chosen such that a mobile can always scan all possible frequencies for the NACCH between consecutive trac bursts. This is guaranteed even if the network is not synchronized as shown in Fig. 5. Between two trac bursts each mobile user has the time of eight frames to scan for the adjacent cell's NACCH. If all cells are asynchronous, i.e., the beginnings of all frames in the neighboring cells are aligned, the mobile user needs two frames time for safely capturing a signaling burst for each frequency. With ve possible frequencies, the mobile can scan for all NACCHs of all neighboring bands between adjacent frequency bands. For a frequency reuse factor of nine, 18 frames are necessary for scanning all possible frequencies. Analysis in [2] indicates that a frequency reuse factor of ve is sucient for frequencies at 60 GHz. Scanning the adjacent cells the mobile user has to detect the corresponding cell's receive power level, the signal-to-interference ratio (SIR), and the time slots allocated for trac in this cell. The latter information is carried in the signaling burst. In case the adjacent cell's receive level is higher or the SIR is better, a mobile user can simply switch to this cell by sending his next iv

5 signaling downlink uplink f f 1 f 2 f 3 f 4 f 5 f t 9 frames = 1 super frame Fig. 5. Capturing of signaling bursts in asynchronous mode. uplink burst using the frequency of the cell and in the assigned time slot. After the mobile user has changed its cell using the described procedure the system can perform handover procedures in higher layers such as switching the trac burst of this user to all neighboring cells of this new cell. A drawback of the proposed simulcast handover procedure is that it reduces the overall system capacity. When not in handover the data transmitted in the neighboring cells is not needed, but bandwidth is used. Investigations on trac capacities in atypical oce environment showed, however, that (at least) two dierent cell groups need to be considered. The rst group are hallways, stair halls, and corridors. Cells belonging to this group have to support the simulcast transmission of many low bit rate users, since these cells usually have a large number of neighboring cells. The second group are oces where high rate users mostly occupy the available bandwidth. Fortunately, cells belonging to this group have less neighboring cells than cells belonging to the rst group, and thus the overhead introduced by the simulcast handover is less. Simulations conrmed a system balance in both cases. III. Conclusions In this paper, a concept for the air interface of an integrated broadband mobile communications system in the indoor radio propagation channel at 60 GHz has been presented. Both, the multiple access scheme and the modulation techniques have been explained. The inuence of the radio channel's properties and the heterogeneity of the services to be transmitted have been taken into account toprovide the most system exibility. A new handover procedure well suited for pico cellular indoor environments has been introduced. Due to the channel properties and the cost of hardware in the 60 GHz range, time division multiple access is preferred. We have investigated modulation techniques and introduced data rate classes. These data rates are adaptively used depending upon the user's requirements and the channel conditions. Even for poor channel conditions a connection with low data rate is possible. A common network access and connectivity channel (NACCH) is transmitted with this low rate to guarantee a stable system behavior even at a high mobility and with a low SIR. Our current work involves improvements on the medium access control and further performance evaluations. References [1] V. Aue, G. Fettweis, A. Finger, E. Forster, J. Hubner, H. Kaluzni, K. Kojucharow, R. Lehnert, W. Nowak, H. Nuszkowski, F. Poegel, M. Riedel, M. Sauer, B. Stantchev, J. Voigt, J. Wagner, S. Zimmermann: A Wireless Broadband Integrated Services Communications System at 60 GHz, ACTS Mobile Communications Summit '96, Granada, November 1996, pp [2] J. Voigt, A. Wernicke, and G. P. Fettweis: Investigations on Cochannel Interferences in an Indoor TDMA-System at 60 GHz, this workshop. [3] J. Bingham, Multicarrier Modulation for Data Transmission: An idea Whose Time Has Come, IEEE Communications Magazine, May 1990, pp [4] R. Steele, Mobile Radio Communications, Pentech Press Ltd, v