Branimir Stantchev, Jens Voigt, Volker Aue, and Gerhard P. Fettweis. Dresden University of Technology, Mobile Communications Systems. Abstract|In this paper ...
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 di erent 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 di erent 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 of Technology. 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 di erent 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.
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Fig. 1. Mobility vs. 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-e ectively o er 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 di erent 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 di erent cells. In order to avoid the employment of many expensive base stations (one in each cell), one or more central base stations provide signals to multiple transponders. At the base station signals are upconverted to an intermediate frequency of 2.4 GHz. The base station and the transponders are interconnected by a ber optics backbone carrying the analog signals at the intermediate
frequency. The transponder nally converts the signal to the radio frequency of 60 GHz. The overall system concept is shown in Fig. 2. A base station serves di erent cells via multiple transponders. The base stations 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.
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In order to support higher data rates a high bandwidth per cell is necessary. The system's overall bandwidth is 2 1 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 base station and the user and time division multiple access (TDMA) to share resources between di erent 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 base station and the mobile station 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 di erent 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 di erentially 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 connectivity channel 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
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 we choose 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 di erent 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 di erence 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 di erent data rates at a given BER are separated by about 6 dB each. Depending on the ii
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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 (di eren6/0.33 tial quaternary phase shift keying or minimum shift keying HR1 (high rate 1) OFDM/DBPSK with 50 carri25/2 ers (orthogonal frequency division multiplexing/di erential binary phase shift keying HR2 (high rate 2) OFDM/DBPSK with 200 100/2 carriers HR3 (high rate 3) OFDM/DQPSK with 200 200/2 carriers locate 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 base station 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 base station 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
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 speci c user can aliii
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