Advanced Cellular Internet Service (ACIS) - CiteSeerX

TOPICS IN PCS

Advanced Cellular Internet Service (ACIS) Leonard J. Cimini, Jr., Justin C.-I. Chuang, and Nelson R. Sollenberger AT&T Laboratories — Research

ABSTRACT

The public’s desire for mobile communications and computing, as evidenced by the popularity of cellular phones and laptop computers combined with the explosive demand for Internet access suggest a very promising future for wireless data services. The key to realizing this potential is the development and deployment of high-performance radio systems. In this article we describe a basic service concept, Advanced Cellular Internet Service (ACIS), and the technologies for achieving reliable high-speed transmission to wide-area mobile and portable cellular subscribers with very high spectrum efficiency. Such a wireless service, optimized to meet the needs of a client-server model for information retrieval and Web browsing, and combined with evolutionary enhancements in second-generation technologies, can provide an attractive option for third-generation systems. The radio link design combines OFDM with transmit and receive antenna diversity and Reed-Solomon coding to overcome the link budget and dispersive fading limitations of the cellular mobile radio environment. For access, a dynamic packet assignment algorithm is proposed which combines rapid interference measurements, priority ordering, and a staggered frame assignment schedule to provide spectrum efficiencies of two-to-four times existing approaches.

T

he public’s desire for mobile communications and computing, as evidenced by the popularity of cellular phones, pagers, and laptop computers, combined with the rapid growth in demand for Internet access, suggest a very promising future for wireless data services. The key to realizing this potential is the development and deployment of highperformance radio systems. While many proposals for third-generation radio systems point to the recent rapid growth in Internet traffic and Web browsing as major motivators, the service definitions generally emphasize new radio technologies with provisions for a broad range of services. An alternative vision, proposed here, is to view third-generation systems as a combination of: • Second-generation systems, which have had great success in providing voice services • The evolution of these systems for voice and low- to modest-bit-rate data • A complementary high-speed packet data service In this article we present a proposal for an Advanced Cellular Internet Service (ACIS), providing reliable, high-speed (1–2 Mb/s) wireless packet access for mobile and portable cellular subscribers.

high-performance computer technologies and attractive network applications. Interest in wireless Internet access was initially generated by the opportunities for e-mail and messaging, but the recent explosion in popularity of the World Wide Web suggests broader long-term opportunities for wireless data. When you combine the success of cellular and paging services with the increased presence of laptop computers and the rapid growth in the number of Internet users, wireless data should have a very bright future. Nevertheless, today the number of wireless data subscribers is small, with the most formidable obstacle to user acceptance being the performance limitations of existing services and products, including link performance (data rate, latency, and quality); network performance (access, coverage, spectrum efficiency, and quality of service); and price. Therefore, it is timely to consider a synergistic combination of wireless, computer, and Internet technologies to provide a wide-area packet data service with large improvements in performance beyond existing approaches (Fig. 1).

CURRENT WIRELESS DATA SYSTEMS

WIRELESS DATA OPPORTUNITY

Current wireless data systems fall into two categories (for a good summary, see [1]): • Wide-area and microcellular services (designed for metropolitan-area coverage) offering limited bit rates, on the order of 10–100 kb/s. These include RAM Mobile Data, ARDIS, Cellular Digital Packet Data (CDPD), and the emerging digital cellular data services. • Products which provide much higher rates (1–10 Mb/s) but have small coverage areas, usually limited to within a building. These include WaveLAN and RangeLAN2. Below we briefly review these current and emerging wireless data options.

Current cellular radio and paging services have been very successful in providing untethered communications. Wireless access to the public switched telephone network has been growing at rates of 30 to 50 percent a year with the number of cellular subscribers in the United States now exceeding 40 million. The introduction of the Global System for Mobile Communications (GSM) and other digital cellular technologies has accelerated the growth of wireless communications in Europe and throughout the world. With the advent of new personal communications services (PCS), wireless access is expected to become even more popular, with predictions of more than 400 million subscribers worldwide by the year 2000. Personal computers and Internet services have experienced even more rapid growth than wireless services due to low-cost

Wide-Area Services — The ARDIS mobile data network, providing coverage in over 400 metropolitan areas in the United States, uses 25 kHz channels in the 800 MHz band and provides 4.8 kb/s transmission rates in most areas. A protocol for 19.2 kb/s has been defined and has been deployed in some areas. The RAM Mobile Data Network, based on the MOBITEX protocol, uses 12.5 kHz channels in the 800 and 900 MHz bands and provides 8 kb/s transmission rates. Both ARDIS and RAM networks are designed for messaging; their limited bandwidth and large packet transmission delays make interactive sessions infeasible. CDPD is an overlay of AMPS cellular networks using paired 30 kHz channels to provide a transmission rate of 19.2

150

0163-6804/98/$10.00 © 1998 IEEE

IEEE Communications Magazine • October 1998

kb/s, with the user experiencing a maximum throughput of about half that rate. The number of CDPD subscribers is still small, but CDPD is only now reaching critical mass in terms of coverage and terminal availability. The Internet capability of CDPD, coupled with its strong synergy with cellular networks and cellular terminals, makes it a strong contender for wide-area wireless data users. GSM, the time-division multiple access (TDMA)-based European digital cellular standard, supports a variety of circuit-switched data modes using internetworking functions. The current transmission rates range from 2.4–9.6 kb/s, with planned upgrades to 14.4 kb/s. In 1996, there were about 1/2 million wireless data users on GSM digital cellular networks, with predictions of over 6 million subscribers by the end of the year 2000. Several higher-bit-rate services, with peak data rates of more than 100 kb/s, will be available in the near future, including a packet-switched data service, General Packet Radio Service (GPRS). IS-136, the TDMA-based North American digital cellular standard, supports 9.6 kb/s circuit data and fax access. By aggregating time slots and introducing higher-level modulation, such as 8-PSK (phase shift keying) and 16-QAM (quadrature amplitude modulation), data rates of 40–60 kb/s can be achieved. Personal Digital Cellular (PDC), the TDMA-based Japanese digital cellular standard, is very similar to IS-136 and is soon to deploy a packet data service. For IS-95, the code-division multiple access (CDMA)based North American digital cellular standard, developers and operators are planning to introduce variable-bit-rate access with peak rates of 9.6 kb/s and 14.4 kb/s. A packet data service is also planned that would use a CDPD network. However, CDMA has several significant obstacles to supporting high-speed packet data services: • A spreading factor of 85 (13 kb/s voice) or 128 (8 kb/s voice) is used with IS-95 to provide about 20 dB of processing gain. At much higher bit rates, CDMA systems must either reduce the processing gain or expand the bandwidth, but neither may be an attractive alternative. • CDMA requires precise power control so that the signals from all terminals arrive at a base station at about the same power level. The power control problem is more easily managed for circuit modes of operation by combining a channel probing procedure, which slowly raises the transmission power for initial access to avoid causing severe interference, with rapid feedback power control once a channel is established, to maintain the required transmission power levels. With packet transmission, channel probing is undesirable because it will cause delay; it is also not efficient for short packets. Microcellular Services — Recently, Metricom has deployed a microcellular packet data system (called Ricochet) in the San Francisco Bay Area and some other areas. It operates in the 902–928 MHz inductrial, scientific, and medical (ISM) band with 160 kHz channel spacing and uses frequency hopping to provide peak transmission rates of about 100 kb/s, with Internet connectivity. Other wireless data access systems that may provide some Internet capability include digital cordless or microcellular systems, such as the Personal Handyphone System (PHS), the Digital European Cordless Telecommunications System (DECT), and the Personal Access Communications System (PACS). These systems were designed primarily for voice service for pedestrians within buildings or over dense areas, but with the larger bandwidths available in these systems, data services in the 32–500 kb/s range could be supported.


Provide wide-area cellular Internet service to mobile subscribers 1–2 Mb/s packet-based Drivers: - Wireless growth - Internet growth - Computing technology - RF and digital technology

■ Figure 1. ACIS target applications and drivers.

Local Area Network Products — Wireless local area networks (WLANs) have been available for several years. Most operate in the unlicensed ISM bands (902–928 MHz, 2.4–2.4785 GHz, and 5.725–5.850 GHz); provide a limited transmission range, generally less than 100 m in an office environment; and use spread spectrum modulation. Current products include Lucent’s WaveLAN, which supports speeds up to 2 Mb/s, and Proxim’s RangeLAN2; a standard has also been recently completed by IEEE 802.11. While WLANs generally support Internet connectivity, so far they have been most successful in niche markets, such as inventory and LAN adjuncts in buildings where added wiring is particularly difficult. The current WLAN market is about US$200 million/year.

FUTURE WIRELESS DATA OPTIONS The number of wireless data subscribers today is small. To realize the enormous potential which has been predicted, the performance of current systems must be significantly improved. There have been many recent proposals for achieving higher bit rates and/or covering wider areas. There have been several recent standards activities addressing the need to push WLANs to a higher level of performance. The HIPERLAN standard defined by the European Community in 1995 operates in the 5 GHz band with transmission rates of over 20 Mb/s [2]; however, no products are available yet. To spur such activities in the United States, the FCC has allocated 300 MHz (5.15–5.35 GHz for indoor use and 5.725–5.825 for both indoor and outdoor use) for unlicensed operation, in what is being called the national information infrastructure (NII) band. In the last few years there has also been extensive research on providing wireless access to asynchronous transfer mode (ATM) networks (e.g., [3]). In an effort to cover wider areas, a multitier approach has been proposed which seeks to take wireless technologies that have been optimized for large-cell environments, such as cellular packet radio systems, and other technologies that have been independently optimized for small-cell environments, such as WLANs, and integrate them together in a seamless way. One example is the ANDREW project at Carnegie-Mellon University, which uses a combination of CDPD at 19.2 kb/s for large-cell environments and WaveLAN at 2 Mb/s for campus and indoor environments, to provide Internet connectivity. The challenge here is to make applications work well across a wide range of access speeds. The most extensive efforts to provide higher bit rates can be found among the so-called third-generation projects around the world. The European Universal Mobile Telecommunications Systems (UMTS) program has as its goal a capability of up to 2 Mb/s in a localized radio environment (e.g., in-building and microcell) and up to 144 kb/s in larger cells [4], and seeks to provide a broad range of services ranging

151

Transmitter

N symbols

N tones IFFT

RF

and Doppler), and interference. Furthermore, the high-bandwidth demand and packet nature of these systems make frequency reuse much more challenging.

RF

In the following section obstacles to achieving a high-bit-rate radio link are discussed, and a physical-layer solution to overcome the link budget and dispersion limits is proposed. Access and frequency reuse challenges are then discussed, and a new dynamic packet access algorithm is described. The article goes on to discuss some additional considerations, such as required spectrum and cost of service. The article is summarized in the last section.

• • M transmitters •

R-S encoder

IFFT

AN OUTLINE OF THE ARTICLE

Receiver

RF

FFT

Diff. det. R-S decoder

RF

FFT

Diff. det.

■ Figure 2. The OFDM-based ACIS transceiver architecture. from voice to video telephony and multimedia access. Initial research efforts concentrated on two approaches: • Advanced TDMA (ATDMA), an adaptive modulation scheme which is similar to GSM for large cells but achieves higher data rates using linear modulation for small cells • Code DIvision Testbed (CODIT), a direct-sequence spread-spectrum scheme with bandwidths of 1, 5, and 20 MHz and transmission rates up to 2 Mb/s More recently, efforts have centered around evolutionary extensions of GSM using wideband TDMA and a more revolutionary wideband CDMA approach. In the latter scheme, wider RF channels of 5 MHz, compared to the existing 1.25 MHz channel used in IS-95, are proposed to support higher data rates, with wide-area mobile transmissions up to 384 kb/s. It should be noted that such wideband schemes pose problems in terms of spectrum allocation, especially in the United States. In the same spirit as UMTS is the International Telecommunications Union (ITU) program called International Mobile Telecommunications in the year 2000 (IMT-2000), formerly called Future Public Land Mobile Telecommunications System (FPLMTS). Its goals are similar to those of UMTS for services, but IMT-2000 seeks to produce a global standard for third-generation wireless access using the frequency bands 1885–2025 MHz and 2110–2200 MHz. However, the 1850–1990 MHz band has been auctioned in the United States for PCS services, so the possibility of deploying IMT2000 in the United States raises spectrum questions. TDMA and CDMA schemes similar to those for UMTS have been proposed, along with multicarrier approaches (e.g., [5]).

TARGET APPLICATION AND CHALLENGES In summary, strong wireless voice growth, strong portable computing growth, plus strong fixed Internet growth lead to a strong wireless data opportunity. The target of the work presented here is to provide a high-bit-rate packet data service (with 1–2 Mb/s peak data rates) in macrocellular environments to mobile as well as portable users, with good spectrum efficiency (Fig. 1). The higher bit rates will provide user experiences for existing services, such as Web browsing with a portable computer, that are similar to LAN connectivity and will allow new mobile-specific applications, such as digital maps and driving directions. The desire to provide these high peak data rates with widearea coverage is a very challenging problem. One of the main challenges is the mobile radio environment itself, specifically, path loss, multipath fading (characterized by the delay spread

152

ACIS: DOWNLINK RADIO DESIGN PHYSICAL LAYER CHALLENGES Radio link performance in a mobile environment is primarily limited by propagation loss, fading, delay spread, and interference. Here, we concentrate on the link budget and dispersive fading limitations. Link Budget Limitations — Because of the large path loss encountered in serving wide areas, the link budget is challenging for these data rates and the desired performance level. Modeling path loss accurately is a difficult task requiring different, often empirical, models for different environments. For the purposes of this article we consider a simple model for path loss, P 1 (1) L= r =K γ Pt d – where P r is the local mean received signal power, P t is the transmitted power, and d is the distance between the transmitter and receiver. The path loss exponent γ = 2 in free space, and is between 2 and 4 for typical environments. The average received signal-to-noise ratio (SNR) is P 1 Pr (2) = K γt Pn d No B where No is the one-sided noise power spectral density, B is the signal bandwidth, and K is a constant independent of distance, power, and bandwidth. Given the performance requirement SNR ≥ SNR o , it is clear that the path loss imposes limits on the bit rate, KPt (3) B≤ γ d N o SNRo SNR =

and the signal coverage,   KPt d≤   N o BSNRo 

1γ

(4)

For example, consider the target data rate of 1 Mbaud, which is about 30 times that of an AMPS cellular voice circuit. Since the SNR is inversely proportional to the baud, this corresponds to a 15 dB increase in the required transmitted power to achieve the same bit error performance and cover the same area as a typical cellular voice circuit. Clearly, the coverage and performance of such systems will be severely limited without the introduction of new techniques. This is especially true for the uplink, where a mobile terminal cannot overcome the link budget limitations and still maintain a reasonable level of complexity and power consumption. Multipath Fading Limitations — In addition to the link budget limitations, the bit rate is also limited by the multipath


nature of the radio environment. Physically, the received signal can be considered as the summation of a large number of signals which have traveled different paths to the receiver and, therefore, have different time delays, amplitudes, and phases. Depending on the extent of the channel impulse response, called the delay spread, and the resulting intersymbol interference (ISI), the maximum data rate can be severely limited. This delay spread in a macrocellular environment could be as large as 40 µs, limiting the data rate to about 5–10 kbaud if no measures are taken to counteract the resulting ISI. In addition, the channel is time-varying, with Doppler rates as high as 200 Hz if operating in the 2 GHz PCS band.

A PHYSICAL LAYER SOLUTION Asymmetric Service — Since portable terminals must be powered by batteries and their transmit power is limited to about 1 W, achieving transmission rates beyond 10–100 kb/s in large-cell environments is impractical. On the other hand, base stations are usually powered from commercial main power systems and can transmit with higher power; subsequently, bit rates of over 1 Mb/s may be possible. Therefore, we propose an asymmetric service: a high-speed downlink with about 1–2 Mb/s peak data rates and a lower-speed 50–100 kb/s uplink. This alleviates the severe power problem at the mobile terminal and should be suitable for the most attractive new applications which would be supported by ACIS. In particular, Web browsing and information access, which have caused the recent explosion in Internet usage, are highly asymmetrical in transmission requirements. Only the transmission path to the subscriber needs to be high-speed. Many other services provided over the Internet can be provided with low to moderate bit rates transmitted from subscribers. Only video telephony and large file transfers in the direction from a terminal toward the network may require high-speed transmission from terminals. Multicarrier Modulation — One possibility for overcoming the delay spread limitations on the downlink is to use a singlecarrier system modulated at 1 Mbaud with equalization and coding (e.g., [6]). This equalizer could require 20–40 taps and must be updated at the highest Doppler rate. In addition, the extensive period required to train the equalizer could be a major source of inefficiency in a packet-based system. An alternative approach, and the one taken here, is to use a multicarrier system. The basic concept is to divide the total bandwidth into many narrowband subchannels which are transmitted in parallel. The subchannels are chosen narrow enough so that the effects of multipath delay spread are minimized. The particular multicarrier technique used here is called orthogonal frequency-division multiplexing (OFDM) [7–8] and is the standard for digital audio broadcasting (DAB) in Europe [9] and for asymmetric digital subscriber line (ADSL) in the United States [10], and has been proposed for many other applications. To ensure a flat frequency response and achieve the desired bit rate, 100–200 subchannels are required, each modulated at 5–10 kbaud. With 5–10 kbaud subchannels and guard periods of 20–40 µs, delay spreads as large as 40 µs can be accommodated with little or no intersymbol interference (ISI). Since no equalization is required, OFDM alleviates the need for a long training period. Diversity and Coding — To reduce the link budget shortfall in the downlink, techniques for reducing the required SNR must be incorporated. To eliminate some of the 15 dB link budget shortfall, multiple base station transmit antennas are used, with each antenna transmitting a subset of the total


number of subchannels, and up to two-branch antenna diversity is used at the mobile. In the scheme proposed here, the cluster of subchannels on each antenna is composed of a subset of widely spaced subchannels covering the entire transmission bandwidth. Alternatively, each subset can include widely spaced clusters of subchannels (e.g., two or three adjacent subchannels). Spreading the subchannels over the transmit antennas randomizes the fading across the OFDM bandwidth. To realize the full potential of the diversity, this is combined with Reed-Solomon coding across subchannels, using a combination of erasure correction, based on signal strength, and error correction. As shown in the next subsection, with four transmit antennas at the base and two receive antennas at the mobile, the required SNR can be reduced by 10 dB. In addition, since a wider bandwidth and multiple transmit antennas are being used, the total transmitted power can be increased to make up the rest of the link budget shortfall. By transmitting at a power level about equal to that of an ordinary analog channel on each of four antennas, cellular link budgets can be achieved. Switched-beam smart antennas could also be used; in particular, using electronically switched beams, with four beams per antenna, another 6 dB of gain could be obtained without increasing transmitter power. Which beam is used for any given packet will be part of the channel assignment scheme. Simulation Model — The transmitter and receiver architectures are shown in Fig. 2. In the results presented here, QPSK modulation is considered with differential detection on each branch of the mobile receiver. Differential detection avoids the additional training required to recover the carrier phase; however, the error performance is inferior to ideal coherent detection. The differentially detected signals are then added together to provide an approximation to maximal-ratio combining. Ideal coherent detection is also considered for comparison. However, coherent detection requires a reliable and accurate method of channel estimation. Finally, the performance of this technique in the presence of a single co-channel interferer is presented. In the radio link simulations, Rayleigh fading is assumed with Doppler rates as high as 200 Hz, along with a two-ray, equal-amplitude, delay spread profile, with impulse separations as large as 40 µs. In the example here, the OFDM signal is composed of 120 subchannels. Thus, data modulates each tone with a 160 µs symbol period. An additional 40 µs guard interval is used to eliminate any ISI due to the channel delay spread. This results in subchannels which are spaced by 6.25 kHz, block rates of 5 kbaud, and a total data rate of 600 kbaud, or, equivalently, channel bit rates of 1.2 Mb/s for QPSK. Reed-Solomon coding with 6-bit code symbols, corresponding to grouping three QPSK symbols in time, is used for error correction. The Reed-Solomon codeword is then formed across subchannels. For QPSK, a (40, 20) code, which corrects 10 erasures, based on signal strength, and five randomly errored symbols, is used. With the reduction in the delivered bit rate due to the 1/2-rate code, the peak rate for the sample QPSK system is 600 kb/s. Results — The corrected word error rate (WER) versus the average SNR is shown in Figs. 3–5 for the sample system described above. Here, word refers to one Reed-Solomon codeword, or a block of 240 bits for QPSK. In Fig. 3 the performance of QPSK modulation using four transmit antennas at the base and two receive antennas at the mobile is shown in a channel with a 10 Hz Doppler rate (i.e., very slowly varying, as might be expected for pedestrian traffic) and a 200 Hz Doppler rate (i.e., around 100 km/hr at 2

153

required compared to 8.5 dB for four transmit and two receive antennas. Clearly, diversity is needed at the mobile; even with four transmit antennas, an SNR of about 14.5 dB is still needed. However, the number of transmit antennas can be reduced with a much smaller resulting penalty. For example, for two transmit antennas less than a 2 dB penalty is encountered. In Fig. 5 the performance of OFDM with diversity and coding is shown in the presence of a single co-channel interferer. WER is plotted versus the average signal-tointerference ratio (SIR) for 40 µs delay spread, 10 Hz Doppler, and QPSK with differential detection and 1/2-rate Reed-Solomon coding. In this example it is assumed that there is no noise. Results are shown when there is no diversity and for the case of four transmit antennas and two receive antennas. The performance is very similar to the noise-only situation; in particular, a 1 percent WER can be achieved with an average SIR of only 9 dB — roughly a 10 dB improvement over the no diversity case. By optimally combining the signals at the two antennas to maximize the signal-to-(interference+noise) ratio (SINR), a significant improvement beyond these results should be possible. In the next section, the QPSK - Differential detection 4 transmit/2 receive antennas results in Fig. 5 are used to determine an access 1/2-rate R-S coding (40,20) threshold for the proposed dynamic packet assignment algorithm. Delay spread = 1.25 µs and 40 µs

GHz) and varying amounts of delay spread, including flat fading. For a target WER of 1 percent, less than 8.5 dB SNR is required, rather than the 17–20 dB typically needed for cellular systems. This represents about a 10 dB reduction in the link budget shortfall discussed earlier. The additional 2 dB needed in a flat fading environment is due to the reduced diversity effect. The penalty due to the increased time variation for a 200 Hz Doppler is only about 1 dB. Results are also shown using ideal coherent detection, with 40 µs delay spread and 10 Hz Doppler. In this case, the required SNR is reduced by another 3 dB. This would, of course, require a very reliable and accurate method of channel estimation. The use of four antennas at the base station and two at the mobile may be undesirable in some applications. In Fig. 4 the trade-offs in using fewer antennas at either the transmitter or receiver are evaluated. WER is plotted versus SNR for one particular case of delay spread (40 µs) and Doppler (10 Hz) and for QPSK with differential detection. At a 1 percent WER with no diversity, an SNR of about 18.5 dB is

1

Word error rate

.3

Doppler = 10 Hz Doppler = 200 Hz

.1

Ideal coherent detection (delay spread = 40 µs)

.03

.01

OTHER ISSUES Uplink Transmission — The uplink, or terminalto-base transmission direction, could also use OFDM by dividing the wideband channel into clusters of 10–20 tones for use by individual terminals to achieve bit rates of about 100 kb/s while requiring acceptable transmit power levels. An alternative to OFDM is the use of multiple narrow carriers, which must then be demodulated using a filter bank at the receiver.

Flat fading

.003

.001

4

5

6

7

8 9 SNR (dB)

10

11

12

13

■ Figure 3. ACIS link performance.

1 QPSK - Differential detection 1/2-rate R-S coding Delay spread = 40 µs Doppler = 10 Hz

Word error rate

.3 .1

4TX

.03

4TX

.01

.003 .001

1 receive antenna 2 receive antennas 0

2

4

6

8

1TX

2TX

1TX

10 12 SNR (dB)

14

■ Figure 4. Transmitter and receiver diversity trade-offs.

154

16

18

20

Synchronization — Any realistic burst or packet transmission system must achieve reliable frame synchronization. Several techniques have been proposed for OFDM systems [11]. In addition, OFDM is more sensitive to frequency offset and timing mismatch than single-carrier systems. One goal of our current research is a synchronization technique which can achieve fast frame synchronization while simultaneously providing reliable estimates of both the frequency offset and the multipath channel response. Sensitivity to Transmitter Nonlinearities — Any multicarrier signal, such as an OFDM signal, can have a large peak-to-mean envelope power ratio (PMEPR). For example, a baseband OFDM signal with N subchannels has a PMEPR = N. For N = 128, PMEPR is approximately 21 dB. When passed through a nonlinear device, such as a transmit power amplifier, the signal may suffer significant spectral spreading and in-band distortion. The conventional solutions to this problem are to use a linear amplifier or to back off the operating point of a nonlinear amplifier; both result in a significant power efficiency penalty. Two alternatives have been proposed to reduce the PMEPR of the signal input to the amplifier: • Nonlinear block coding [12], which avoids the transmission of large amplitude sequences • Deliberately clipping the OFDM signal before amplification In [13] it is shown that digital clipping and filter-


1

ing can reduce the PMEPR to 6 dB with only a slight impact on performance. This is comparable to singlecarrier QPSK with raised-cosine pulse shaping. Word error rate

Implementation — If ACIS is to be a viable service, a low-cost, low-power implementation of these receivers must be developed. One important advantage of OFDM, as shown in Fig. 2, is that both the transmitter and receiver modulation can be implemented as fast Fourier transforms (FFTs), allowing for very efficient digital signal processing (DSP) implementations.

.3 .1 4TX 2RX .03 QPSK - Differential detection 1/2-rate R-S coding

.01

Delay spread = 40 µs Doppler = 10 Hz

.003

ACIS: DOWNLINK MEDIUM ACCESS CONTROL MEDIUM ACCESS CONTROL CHALLENGES

1TX 1RX

.001

0

2

4

6

8

10 12 SNR (dB)

14

16

18

20

ACIS is targeted for applications such as Web brows■ Figure 5. Performance in co-channel interference. ing with a peak downlink rate on the order of 1–2 Mb/s using the wide-area cellular infrastructure. In order to provide bandwidth on demand using scarce and achieve efficiencies (measured in bits per second per spectrum, the downlink medium access control (MAC) protoHertz per sector) as high as 50 percent [14, Figs. 3 and 5]. col must: DCA combined with circuit-based technology (which has • Handle dynamic and diverse traffic with high throughput generally been the approach to date) can provide some ben• Efficiently reuse limited spectrum with high peak rates efits, but it cannot provide the large capacity gains predictand good quality ed in [14] because of the dynamics of interference in a It is clear that wireline-based MAC protocols cannot work mobile system. Channel reassignments must take place at well without addressing spectrum reuse issues, and circuithigh speed to avoid rapidly changing interference. With cerbased wireless access techniques are insufficient for handling tain assumptions, measurement-based DCA schemes can highly dynamic and asymmetric packet traffic. Therefore, an provide very high performance, particularly if channel selecefficient packet channel assignment algorithm is required and tion is based on a combination of both mobile and base stamust be carefully designed. tion measurements. CHANNEL ASSIGNMENT METHODOLOGY Based on the arguments above, DCA, for which any carrier is allowed to be used in any time slot, is considered here. This As discussed, very high efficiency will be required for 1–2 method provides significantly higher spectrum efficiency than Mb/s macrocellular Internet access. In [14] a comparison is fixed assignment except when the traffic load is heavy everypresented of various techniques to improve spectrum effiwhere. Even though this method imposes higher complexity, ciency. The results show that interference averaging techsuch as base station time synchronization and a rapid frequenniques, such as provided by CDMA systems, can perform cy switching requirement, it appears to be a good approach to better than fixed channel assignment techniques, and that efficiently reuse spectrum for ACIS without requiring higher interference avoidance techniques can perform even better. total bandwidth. In the following, we refer to the DCA algoIn particular, it is shown that dynamic channel assignment rithm proposed here for packet access as dynamic packet (DCA) with power control can provide a capacity that is assignment (DPA). two to three times higher than synchronous spread-spectrum For channel selection in the DPA algorithm, we combine attributes of two existing methods: 2. BS broadcasts destination •Interference sensing, or meaMS IDs, return channels, and numbers of packets to be surement-based DCA, which delivered avoids selecting channels already in use in the neighborhood of a given base station, Neighboring 4. BS assigns thus adapting to the interferBS performs channels to all ence environment procedures packets and •Channel segregation (CS) [15], with broadcasts MS staggered IDs, assigned which provides an adaptive time frame channels learning process to form good reuse patterns in the neighborhood of a given base station, thus making assignment more 3. MS scans all robust against possible mea1. MS locks to the 5. BS delivers downlink pilots and strongest BS surement error in the rapidly packets on the sends list of acceptable assigned changing environment channels to BS channels and Clearly, CS and measurementsends pilots based DCA are complementary. To introduce memory for preferred ■ Figure 6. A dynamic packet assignment algorithm.


155

Frequency

Pilots

Traffic carriers

implement when OFDM modulation is considered. Figure 7 shows an example of pilots sent by different base stations. These pilots can also be sent during a short time slot in each frame. Clearly, monitoring pilots at mobiles yields a direct downlink interference measurement, but mobiles must inform base stations of acceptable channels without significant delay to make these channels useful. Narrowband feedback channels are sufficient to carry the necessary information without taking excessive uplink bandwidth; however, these feedback channels require higher frequency reuse to reduce feedback errors. To reduce the effect of setup latency due to the feedback process, a staggered frame assignment procedure is introduced in the next subsection.

Unused channel BS 3

BS 1

BS 2

BS 1

BS 2

BS 2

BS 1

BS 1

BS 3

BS 2

BS 2

BS 1: BS 2: BS 3:

BS 3

Time

Algorithm: Turn on pilot [N(i-1) + j] whenever the ith slot of the jth frequency is used N: Number of traffic carrier frequencies used; N=3 is considered in the example

■ Figure 7. An example of interference monitoring using pilots. channels, a priority value is associated with each available channel for all radios at all base stations. For example, consider a system having three downlink frequencies, three sectors per base station, two radios per sector, and nine time slots per radio. There are six channel lists per base station, each associated with a radio, in which each of the 27 (time slot, frequency) choices are assigned different priority values (from 1 to 27). Initially each radio is given a randomly shuffled channel priority order. An adaptive learning process is then used to continuously update the channel priority order following each channel assignment process. This is easily performed by swapping the priority positions of the first channel having acceptable quality with the first channel failing the quality test (which is the highest-ranking unused channel before updating priority ranking). Compared with the original CS algorithm, this method learns preferred channels while employing interference look-ahead, thus avoiding using high-ranking bad channels.

DOWNLINK DYNAMIC PACKET ALLOCATION ALGORITHM Basic Algorithm — The DPA algorithm, shown in Fig. 6, gives good downlink performance with low implementation complexity. This method requires mobile stations (MSs) to perform interference measurements, based on the channel priority order, once informed of pending packets at the base station. The first unused channel with acceptable quality (here, this corresponds to an SIR greater than a threshold) is selected. To reduce measurement delay, narrowband pilot tones corresponding to the downlink traffic channels could be transmitted simultaneously by individual base stations. Mobiles need only scan the pilots, similar to spectrum analysis using techniques (such as FFT) which can be completed in a short time; this is convenient to

156

Staggered Frame DPA — Three key questions must be addressed: • How do adjacent base stations with concurrent traffic avoid selecting the same channel? • Is it possible to reduce channel quality degradation as time progresses when a selected channel is assigned to new mobiles by nearby base stations? • How do mobiles and base stations exchange information about channel selection and packet delivery? We introduce a staggered frame DPA method to address these issues. The frame structure, shown in Fig. 8, uses a staggered schedule for DPA among neighboring base stations. The purpose of a staggered frame is similar to the concept of token passing (but without specifically passing tokens since centralized coordination of base stations is not required; the only requirement is time synchronization among base stations) to allow only a single base station to perform the assignment

Superframe BS 1

BS 2

Superframe BS K

•••••

BS 1

BS 2

BS K

•••••

•••••

Frame Slot 1 Slot 2 BS 1 turns off ••••• pilots and broadcasts paging information

•••••

Slot J

•••••

Slot I

Slot 1 Slot 2

•••••

BS 1 BS 1 receives broadcasts preferred assigned channel channels lists for the next from superframe mobile #1

•••••

BS 2 performs channel assignment BS 1 starts transmitting regular traffic and sending corresponding pilots

At mobile #1 with pending packets: Slot 1 ••••• Measures pilots from other BSs

Slot J-1 ••••• Transmits preferred channel list

Slot I Receives assigned channel (T,F)

Slot 1

•••••

Slot T ••••• Receives packet

■ Figure 8. The staggered DPA frame structure.


6 No beam forming, no power control Four beams per sector, no power control Four beams per sector, power control

Retransmission probability (%)

Retransmission probability (%)

procedure in a small geographical 5 area at one time. Similar to the concept of frequency reuse, different 4 base stations can perform assignments simultaneously as long as they 3 are sufficiently separated in distance. SIR threshold = 10 dB 36 base stations This ensures that any base station 2 carriers performing interference sensing can 2 3 radios per base station detect all used channels in its neigh9 slots/frame borhood without any blind spots, 2 ms/slot 1 3 sector antennas thereby avoiding any co-channel interference (inter-base collision) 0 once the assigned channel is used to 0 10 20 30 40 50 deliver pending packets. FurtherSpectrum utilization (%) more, since the assignment is performed once every superframe,1 the ■ Figure 9. Single-channel DPA performance. degradation as time progresses is thus bounded if the superframe duration is reasonably short. In Fig. 8 other information need12 No beam forming, ed to be exchanged between base 11 no power control stations and mobiles is also specified: Four beams per sector, 10 • Base stations must inform all no power control 9 Four beams per sector, mobiles which are running power control 8 applications, such as Web browsing, of pending traffic. 7 • Once paged, a mobile must per6 SIR threshold = 10 dB form a scanning procedure 36 base stations 5 2 carriers using an FFT to determine a list 4 3 radios/base station of acceptable channels and feed 9 slots/frame 3 the information back to its base 2 ms/slot 2 station. A list rather than only 3 sector antennas 1 one channel is sent so that the base station can assign different 0 channels to different mobiles in 0 10 20 30 40 50 case multiple mobiles select the Spectrum utilization (%) same channel. • Upon getting lists from all ■ Figure 10. Multichannel DPA performance. mobiles having pending packets, a base station must assign chanThe co-channel-interference-limited case is considered; nels to deliver packets and inform the corresponding that is, noise is ignored in the simulation. In the propagation mobiles of its decision before the start of the next supermodel, the average received power decreases with distance as frame. d–4 and the large-scale shadow-fading distribution is log-norNotice that this approach also gives the additional flexibility of assigning more than a single server for a given mobile mal with a standard deviation of 10 dB. Rayleigh fading is according to its quality-of-service requirement. A designated ignored here, which approximates the case where sufficient control channel with a higher reuse factor is probably required antenna diversity is employed. Automatic repeat request to perform these information exchanges. It is important to (ARQ) is employed, assuming perfect feedback, to request note that, during the assignment frame, a base station must base stations for retransmissions. If a packet cannot be sucturn off the downlink pilots so that its mobiles can sense all cessfully delivered in 3 s, it is dropped from the queue. interference from other base stations by monitoring the pilots. Results — Figures 9 and 10 show the probability of packet Simulation Model — To characterize the DPA perforretransmission as a function of spectrum utilization, defined mance, a system of 36 base stations arranged in a hexagonal as the percentage of total available spectrum used in each secpattern is assumed, each having three sectors using idealized tor. The results of Figs. 9 and 10 correspond to the cases antennas with 120° beamwidths and a 20 dB front-to-back where each mobile is assigned one or multiple channels to ratio. The mobile antennas are assumed to be omnidirectiondeliver packets, respectively, resulting in different trade-offs al. In each sector, one radio provides nine 2 ms traffic slots, of quality versus delay. For utilization higher than 33.33 pereach of which can be used to deliver a downlink traffic packet cent, the same spectrum is, on average, used more than once using any of the two available carrier frequencies. The same at the same base station. Retransmission is performed if a channel can be used in different sectors of the same base stapacket is received in error, which is simulated based on the tion as long as the SIR measured at the DPA admission proWER curve obtained in Fig. 5. A 3–4 percent retransmission cess exceeds 10 dB. probability generally results in successful packet delivery after a few ARQ attempts without incurring excessive (greater than 3 s) packet delay. 1 A superframe is composed of K frames, where K = 4 in the results shown Additional performance enhancement is achieved by employing downlink beamforming and power control. Both in the following sections.


157

techniques are idealized for the results shown here. For beamforming, each 120° sector is simply divided into four 30° beams (with the same 20 dB front-to-back ratio and idealized antenna pattern), and a packet is delivered using the beam that covers the desired mobile station. For power control we assume that there is sufficient time for each mobile station, upon each new packet assignment and before packet delivery, to adjust five times with a 1 dB step size and 30 dB dynamic range, based on a 10 dB target SIR. The iterative power control process is performed independently by each mobile assuming perfect SIR measurements. This provides an upper bound on the performance of the algorithm. With a 3–4 percent target retransmission probability, this DPA algorithm can achieve between 25 and 50 percent spectrum utilization with only two frequencies if beamforming is applied. If beamforming is not employed, a similar retransmission probability is achieved using three frequencies, at the cost of reducing spectrum efficiency. Integration of the power control algorithm with the DPA process to achieve the significant improvement shown here requires further study. The results shown here are consistent with the idealized performance predicted in [14], which is significantly superior to the efficiency provided by current cellular systems. Summary — In this section, an access protocol has been described, based on a dynamic packet assignment scheme, which allocates spectrum on demand with no collisions and low interference to provide high downlink throughput. To reduce the probability that adjacent bases assign the same channel at the same time, all radios have independent channel priority orders. However, simultaneous assignment in a local neighborhood results in blind spots. This latter problem is handled by a staggered frame assignment. In addition, the quality degrades as new channels are assigned at other bases. This is handled by reassigning channels every superframe. Finally, rapid measurements are made by using pilots and the existing FFT structure (from the OFDM receiver).

ADDITIONAL ISSUES

with a peak of 400 kb/s, on the downlink and perhaps an order of magnitude lower average rate on the uplink. This corresponds to about one Web object/s at the median size, but only about one Web object/10 s at the mean size. If service were launched with 1 MHz x 2 of spectrum, an efficiency of 4 percent per sector would support only 40 kb/s per sector or two active users. In order to support 10–20 active users per sector, a spectrum efficiency of 20–40 percent is required, which is possible using DPA, as indicated above. One of the advantages of the approach described in this article is that service can be started with limited spectrum. An initial introduction could use one to three carriers with 1–3 MHz for the downlink. This could be accommodated in the cellular or PCS bands. Full deployment with a reasonable level of interference would require a few channels per base. For future growth a block of 10–30 MHz may be needed.

ECONOMIC CONSIDERATIONS For fixed access, Internet users have shown a willingness to pay on the order of US$3/hr of service or 5 cents/min, although many Internet providers have switched to a flat rate service at US$15–20/month while providing 10–20 hr/month of usage, which is a rate of only US$1–2/hr or 1.7–3.5 cents/min. This again suggests that spectrum efficiency for high-speed packet service must be very high in order to provide low-cost service that will be attractive to end users.

CIRCUIT VOICE ACCESS The service described so far, with, as an example, 80-ms superframes, would be unsuitable for voice applications because of the delay associated with DPA. One solution to this problem is simply to use second-generation systems and their evolutionary systems to provide voice services. An alternative is to have OFDM as the fundamental physical layer and then adapt the network layer protocol for packet DCA or for circuit voice and data with low delay. The spectrum would be split with different bands for packet data and circuit voice and data. One proposal for using OFDM for voice traffic employs tone-hopping and interference averaging [5].

SYNERGY WITH SECOND-GENERATION SYSTEMS

In the second and third sections a high-bit-rate wide-area packet data service is described, and physical and MAC layer techniques have been proposed for providing this service. However, before such a service can become viable, several issues must be addressed, including: • Spectrum requirements • Economic viability • Compatibility with voice services • Synergy with second-generation systems

A significant advantage of ACIS is its synergies with secondgeneration systems. For example, since it is designed to be used with the same coverage (i.e., link budget) as second-generation systems, the base station infrastructure can be shared. In addition, common RF circuits and frame and clock rates can be employed in the base stations, with terminals supporting multiple modes of operation.

SPECTRUM REQUIREMENTS

In this article, we show that a wireless Internet service to support high-speed Web browsing that performs comparably to emerging fixed access network solutions is possible for widearea cellular systems. We propose an asymmetric wireless packet data service for mobile users in macrocells, with peak downlink bit rates of 1–2 Mb/s, while requiring channelization of about 1 MHz. Thus, service can be deployed with a limited amount of spectrum, and hierarchical cell structures are feasible. Such a wireless service, optimized to meet the needs of a client-server model for information retrieval and Web browsing and combined with evolutionary enhancements in secondgeneration TDMA technologies, can provide an attractive option for third-generation systems. In addition, there is a possible synergy with second-generation systems, sharing base station and network facilities and circuits and frame clock rates in terminals.

IS-136 TDMA provides a spectrum efficiency of about 4 percent measured in bits per second per Hertz per sector (3 x 8 kb/30 kHz x 1/21 reuse); GSM also provides a spectrum efficiency of about 4 percent (8 x 13 kb/200 kHz x 1/12 reuse); and IS-95 CDMA provides a spectrum efficiency of 4–7 percent (12 to 20 x 8 kb/1.25 MHz x 1 reuse x 1/2 voice activity). Service launch in as little as 1 MHz x 2 is desirable for a highspeed packet data system to enable refarming of spectrum that is currently in use for cellular and PCS access. Therefore, much higher spectrum efficiency will be required. A typical Web object has a median size in the range of 1.5–3 kbytes with a mean size of 10–20 kbytes. Assuming a Web object of 3 kbytes, a transmission speed of 240 kb/s is required to achieve a transmission time of 100 ms. Suppose an Internet user is modeled as demanding an average of 20 kb/s,

158

SUMMARY


The proposed scheme combines OFDM with transmit and receive antenna diversity and Reed-Solomon coding to overcome the link budget and dispersive fading limitations of the cellular mobile radio environment. In addition, a dynamic packet assignment process based on rapid interference measurements was proposed which can provide spectrum utilization efficiencies of two to four times those existing approaches and which would support initial deployment with as little as 1 MHz x 2 of spectrum.

REFERENCES [1] D. C. Cox, “Wireless Personal Communications: What Is It?” IEEE Pers. Commun., vol. 2, no. 2, Apr. 1995, pp. 20–35. [2] ETSI TC-RES 06921, “High Performance Radio Local Area Network (HIPERLAN); Functional Specification,” draft prETS 300 652, Sophia Antipolis, France, July 1995. [3] D. Raychaudhuri, “Wireless ATM Networks: Architecture, System Design and Prototyping,” IEEE Pers. Commun., vol. 3, no. 4, Aug. 1996, pp. 42–49. [4] J. S. DaSilva, D. Ikonomou, and H. Erben, “European R&D Programs on Third-Generation Mobile Communication Systems,” IEEE Pers. Commun., vol. 4, no. 1, Feb. 1997, pp. 46–52. [5] ARIB, “Report on FPLMTS Radio Transmission Technology Special Group,” v. E1.2, Jan. 1997. [6] J. G. Proakis, “Adaptive Equalization for TDMA Digital Mobile Radio,” IEEE Trans. Vehic. Tech., vol. 40, no. 2, May 1991, pp. 333–41. [7] S. B. Weinstein and P. M. Ebert, “Data Transmission by Frequency-Division Multiplexing Using the Discrete Fourier Transform,” IEEE Trans. Commun. Tech., vol. COM-19, no. 5, Oct. 1971, pp. 628–34. [8] J. A. C. Bingham, “Multicarrier Modulation for Data Transmission: An Idea Whose Time Has Come,” IEEE Commun. Mag., vol. 28, no. 5, May 1990, pp. 5–14. [9] M. Alard and R. Lassalle, “Principles of Modulation and Coding for Digital Broadcasting for Mobile Receivers,” EBU Tech. Rev., no. 224, Aug. 1987, pp. 168–90. [10] P. S. Chow, J. C. Tu, and J. M. Cioffi, “A Multichannel Transceiver System for Asymmetric Digital Subscriber Line Service,” Proc. GLOBECOM ’91, pp. 1992–96. [11] L. Hazy and M. El-Tanany, “Synchronization of OFDM Systems over Frequency Selective Fading Channels,” Proc. VTC ’97, pp. 2094–8. [12] T. A. Wilkinson and A. E. Jones, “Minimisation of the Peak to Mean Envelope Power Ratio of Multicarrier Transmission Schemes by Block Coding,” Proc. VTC ’95, pp. 825–29. [13] X. Li and L. J. Cimini, Jr., “Effects of Clipping and Filtering on the Performance of OFDM,” IEEE Commun. Letts., vol. 2, no. 5, May 1998, pp. 131–33. [14] G. J. Pottie, “System Design Issues in Personal Communications,” IEEE Pers. Commun., vol. 2, no. 5, Oct. 1995, pp. 50–67. [15] F. Furuya and Y. Akaiwa, “Channel Segregation, A Distributed Adaptive Channel Allocation Scheme for Mobile Communications Systems,” Trans. IEICE, vol. E74, June 1991, pp. 1531–37.


BIOGRAPHIES L EONARD J. C IMINI , J R . [SM ‘89] ([email protected]) received B.S.E. (summa cum laude), M.S.E.. and Ph.D. degrees in electrical engineering from the University of Pennsylvania in 1978, 1979, and 1982, respectively. During his graduate work he was supported by a National Science Foundation Fellowship. Since 1982, he has been employed at AT&T, where his current research interests are in wireless communications systems. He is a member of Tau Beta Pi and Eta Kappa Nu. He has been very active in the IEEE Communications Society and is currently serving as Area Editor for Wireless Communications for the IEEE Transactions on Communications and Editor-in-Chief of the IEEE JSAC Wireless Communications Series. He is also an adjunct professor at the University of Pennsylvania. JUSTIN C.-I. CHUANG [F ‘97] ([email protected]) received his B.S. degree (1977) from National Taiwan University, and his M.S. (1980) and Ph.D. (1983) degrees from Michigan State University, all in electrical engineering. From 1979 to 1982 he conducted thesis research on transient electromagnetics for radar target discrimination. From 1982 to 1984 he was with GE Corporate Research and Development, Schenectady, New York, where he studied personal and mobile communications. From 1984 to 1993 he was with Bellcore, Red Bank, New Jersey, as a member of the Radio Research Department. At Bellcore, his work on delay-spread effects, low-complexity modem design, and radio resource management led to key system parameter specifications in the Personal Access Communications System (PACS) which is now a personal communications system standard. From 1993 to 1996 he was with the Electrical and Electronic Engineering Department of the Hong Kong University of Science and Technology (HKUST), where he established the teaching and research program in wireless communications. In June 1996 he returned to the United States and joined AT&T Laboratories — Research in New Jersey, where he is now a technology consultant in the Wireless Systems Research Department, involved in creating technologies to provide reliable services on wireless platforms. He continues to serve as an adjunct professor of HKUST. He has published widely on various aspects of wireless communications, including radio techniques, system architecture, resource management, and prototype implementation. He is a member of Phi Kappa Phi and editor of Wireless Access Techniques for IEEE JSAC’s Wireless Communications Series. He was an editor for IEEE Transactions on Communications and a guest editor for two special JSAC issues on wireless personal communications. Currently, he is chair of the Technical Committee on Personal Communications of the IEEE Communications Society. NELSON R. SOLLENBERGER [F] ([email protected]) heads the Wireless Systems Research Department at AT&T. His department performs research on next-generation wireless systems concepts and technologies, including highspeed transmission methods, smart antennas and adaptive signal processing, system architectures, and radio link techniques to support wireless multimedia and advanced voice services. He received his Bachelor’s degree from Messiah College (1979) and his Master’s degree from Cornell University (1981), both in electrical engineering. From 1979 through 1986 he was a member of the cellular radio development organization at Bell Laboratories, where he investigated spectrally efficient analog and digital technologies for secondgeneration cellular radio systems. In 1987 he joined the radio research department at Bellcore, and was head of that department from 1993 to 1995. At Bellcore he investigated concepts for PACS. In 1995 he joined AT&T.

159

Advanced Cellular Internet Service (ACIS) - CiteSeerX

Advanced Cellular Internet Service (ACIS) - CiteSeerX

Suggest Documents