《OFDM技术开发系列》（英文版） Beyond 3G Wideband wireless data access based on OFDM and dynamic packet assignment

MULTIPLE ACCESS FOR BROADBAND WIRELESS NETWORKS Beyond 3G: Wideband wireless Data Access based on OFDM and dynamic Packet assignment Justin Chuang and Nelson Sollenberger, AT&T Labs-Research ABSTRACT sion capabilities, increasingly demanding Internet applications and user expectations have emerg The rapid growth of wireless voice sub- Experience with laptop computers and personal scribers, the growth of the Internet, and the digital assistants(PDAs) has shown that many Icreasing use of portable computing devices end users desire their portable equipment to pro- rapidly over the next few years. Rapid progress cations they enjoy at their desks with fer in digital and RF technology is making possible compromises. Experience with wireless access has highly compact and integrated terminal devices, demonstrated the singular importance of ere data software is making wireless Internet access access. Wireless packet data access in macrocell- more user-friendly and providing more value. lar environments at peak rates beyond 2 Mb/s is Transmission rates are currently only about 10 likely to be needed in the near future to provide kb/s for large cell systems. Third-generation users with an application environment with few wireless access such as WCDMA and the evolu- compromises from fixed environments. Chal ion of second-generation systems such as lenges for the high-speed wireless data access TDMA IS-136+, EDGE, and CDMA IS-95 will future are transmission speeds at 100-1000 times provide nominal bit rates of 50-384 kb/s in existing rates; costs of a few cents per minute for macrocellular systems. [1 This article discusses access; RF power transmission efficiency that is packet data transmission rates of 2-5 Mb/s in 10-20 dB better than existing systems; and sub- macrocellular environments and up to 10 Mb/s stantially increased spectral efficiency in microcellular and indoor environments as a Two important business drivers for comple complementary service to evolving second-and mentary packet data access at speeds above 2 hird-g tion wireless D Mb packet assignment for high-efficiency resource Integration of wireless data services across management and packet admission; OFDM macrocellular, microcellular, and private the physical layer with interference suppression, indoor systems, and with other services d fre diversity;as·High efficiency well as smart antennas to obtain good power Wireless service providers pay dearly to acqui and spectral efficiency are discussed in this pro- spectrum. Efficiency of spectrum usage is always posal. Flexible allocation of both large and a strong factor in a decision on wireless technol small resources also permits provisioning of ogy Spectrum efficiency becomes crucial for services for different delay and throughput very high-speed data services(e. g,>2 Mb/s) requirements. By taking advantage of improvements in digital signal processing(DSP)and radio frequency INTRODUCTION (RF) technologies, orthogonal frequency-divi- sion multiplexing(OFDM) provides the possibil- wireless Internet access is expected to grow rapid- ity to provide 2 Mb/s packet data at a cost an ly, because of the maturing of digital cellular, with a spectrum efficiency that allow wireles portable computing, and fixed Internet technolo- providers to compete with wireline carriers for gies Data transmission rates are growing rapidly data services Integrated services also provid in fixed networks with the use of wavelength-divi- significant billing advantages for both customers sion multiplexing(WDM) in backbone fiber net- and service providers. Based on customers' pref works and the introduction of cable modems and erences, telecommunications companies such high-speed digital subscriber line(HDSL) technol- AT&T are moving in the direction of delivering ogy in the fixed access networks In parallel with tegrated services which cover local residential the expanding availability of high-speed transmis- and business, long distance, and both wireline 0163-68040010002000IEEE IEEE Communications Magazine. July 2000

78 IEEE Communications Magazine • July 2000 Beyond 3G: Wideband Wireless Data Access Based on OFDM and Dynamic Packet Assignment 0163-6804/00/$10.00 © 2000 IEEE ABSTRACT The rapid growth of wireless voice sub￾scribers, the growth of the Internet, and the increasing use of portable computing devices suggest that wireless Internet access will rise rapidly over the next few years. Rapid progress in digital and RF technology is making possible highly compact and integrated terminal devices, and the introduction of sophisticated wireless data software is making wireless Internet access more user-friendly and providing more value. Transmission rates are currently only about 10 kb/s for large cell systems. Third-generation wireless access such as WCDMA and the evolu￾tion of second-generation systems such as TDMA IS-136+, EDGE, and CDMA IS-95 will provide nominal bit rates of 50–384 kb/s in macrocellular systems. [1] This article discusses packet data transmission rates of 2–5 Mb/s in macrocellular environments and up to 10 Mb/s in microcellular and indoor environments as a complementary service to evolving second- and third-generation wireless systems. Dynamic packet assignment for high-efficiency resource management and packet admission; OFDM at the physical layer with interference suppression, space-time coding, and frequency diversity; as well as smart antennas to obtain good power and spectral efficiency are discussed in this pro￾posal. Flexible allocation of both large and small resources also permits provisioning of services for different delay and throughput requirements. INTRODUCTION Wireless Internet access is expected to grow rapid￾ly, because of the maturing of digital cellular, portable computing, and fixed Internet technolo￾gies. Data transmission rates are growing rapidly in fixed networks with the use of wavelength-divi￾sion multiplexing (WDM) in backbone fiber net￾works and the introduction of cable modems and high-speed digital subscriber line (HDSL) technol￾ogy in the fixed access networks. In parallel with the expanding availability of high-speed transmis￾sion capabilities, increasingly demanding Internet applications and user expectations have emerged. Experience with laptop computers and personal digital assistants (PDAs) has shown that many end users desire their portable equipment to pro￾vide essentially the same environment and appli￾cations they enjoy at their desks with few compromises. Experience with wireless access has demonstrated the singular importance of widespread coverage and anywhere/anytime access. Wireless packet data access in macrocellu￾lar environments at peak rates beyond 2 Mb/s is likely to be needed in the near future to provide users with an application environment with few compromises from fixed environments. Chal￾lenges for the high-speed wireless data access future are transmission speeds at 100–1000 times existing rates; costs of a few cents per minute for access; RF power transmission efficiency that is 10–20 dB better than existing systems; and sub￾stantially increased spectral efficiency. Two important business drivers for comple￾mentary packet data access at speeds above 2 Mb/s are: • Integration of wireless data services across macrocellular, microcellular, and private indoor systems, and with other services • High spectrum efficiency Wireless service providers pay dearly to acquire spectrum. Efficiency of spectrum usage is always a strong factor in a decision on wireless technol￾ogy. Spectrum efficiency becomes crucial for very high-speed data services (e.g., > 2 Mb/s). By taking advantage of improvements in digital signal processing (DSP) and radio frequency (RF) technologies, orthogonal frequency-divi￾sion multiplexing (OFDM) provides the possibil￾ity to provide > 2 Mb/s packet data at a cost and with a spectrum efficiency that allow wireless providers to compete with wireline carriers for data services. Integrated services also provide significant billing advantages for both customers and service providers. Based on customers’ pref￾erences, telecommunications companies such as AT&T are moving in the direction of delivering integrated services which cover local residential and business, long distance, and both wireline Justin Chuang and Nelson Sollenberger, AT&T Labs-Research MULTIPLE ACCESS FOR BROADBAND WIRELESS NETWORKS

include voice services, circuit data, and packer Pos amic packet assignment(DPA) has been pro and wireless services Integrated services also d sed, with the potential to provide 384 kb/s DM can largely data with transmission rates from 30 kb/s to a data services in macrocellular environments using few hundred megabits per second. Providing only I MHz of spectrum 3]. It is possible to eliminate the nomadic customers in areas such as airports, and this acis concept into a wideband con hotels, and other public areas with the same user text in 5 MHz while providing a complementary effects of experience they have in their office is the key service to third generation systems such as driver to deploy such high-rate complementary EDGE and WCDMA. This wideband OFDM packet data services. system would support an order of magnitude interference for Wideband code-division multiple access higher peak data transmission rate in macrocells (WCDMA)will use 5 MHz channels, and it is a at 2 to 5 Mb/s and up to 10 Mb/s in microcells leading candidate for third-generation wireless IS-136, GSM or WCDMA would provide circuit transmission rates access[1]. However, it will be limited to about voice and other circuit-based services and basic 384 kb/s(nominal)peak data rates for macro- data services. A complementary high-speed cellular wireless access(up to 2 Mb/s rates are packet data mode would provide fast wireless ed for indoor environments). Global S, acket data access to meet the demand for wire tem for Mobile Communications (GSM less data in the future that provides access pe and it readily enhancements based on enhanced data rates formance similar to wideband fixed access. Since for GSM Evolution(EDGE)using adaptive portable equipment is power-limited, strongly modulation will provide bit rates up to 384 kb/s net al traffic should be supported, and interference in the near future. [1] Second-generation wire link transmission rates should be allowed to less systems will evolve with complementary adapt downward as necessary to support the suppression and packet data solutions that generally use frequen ed link budgets. Wideband oFDM wire cy channels separated from circuit voice and cir less access might also be configured to introduce cuit data access. Time-division multiple access new broadband capabilities using OFDM only coding to (TDMA)and CDMA systems are being consid- on the downlink, which is then integrated wit ered in which circuit and packet access share a emerging wireless packet data systems such as common frequency channel and access modes General Packet Radio Service(GPRS), EDgE, are separated by time slots or spreading codes. or WCDMA to provide two-way access. An However, the expected demands for high peak- example of such a system with the EDGE uplink rate Internet access are motivating increasing is discussed in[4].2 consideration of complementary access based on There are a number of reasons to consider separate frequency channels to provide maxi- such a high-rate complementary packet data mum peak rates and to allow optimization for capability for downlinks. Wireless Internet packet data transmission alone OFDM was proposed for digital cellular sys- more, for data services, peak bit rate is very tems in the mid-1980s [2]. OFDM has also beer portant in determining overall system perfor shown to be effective for digital audio and digi- mance, because of the highly bursty nature of tal video broadcasting at multimegabit rates in Internet traffic. GPRS, EDGE, and WCDMA Europe, and it has been incorporated into stan- solutions will support transmission rates of dards by the European Telecommunications 144-384 kb/s in macrocellular environments. To Standards Institute(ETSI). The IEEE 802.11 achieve rates in the megabits-per-second range tandards group recently chose OFDM modula- for all environments using -5 MHz spectrum is ion for wireless LANs operating at bit rates up challenging for both the physical layer and radio to 30 Mb/s at 5 GHz. In this article, OFDM resource management design Single-carrier modulation combined with dynamic packet TDMA solutions are limited in supportable assignment with wideband 5 MHz channels is transmission bit rate by equalizer complexity proposed for high-speed packet data wireless Even though new techniques such as interfer access in macrocellular and microcellular envi- ence suppression and space-time processing are ronments, supporting a family of peak bit rates promising, the interactions of these techniques Peak rates exceeding I sive environments, and it readily supports inter- intercode interference at high bit rates limits considered for some ference suppression and space-time coding CDMA solutions. The use of oFDM with suffi- tems enhance efficiency. Dynamic packet assignment ciently long symbol periods of 100-200 us for can support excellent spectrum efficiency and packet data transmission addresses these issues. 2 In/4 we focused on the high pea It supports a high bit rate in time delay spread architecture of such a mm in a macrocullar WIDEBAND OFDM environments with performance that improves with increasing delay spread up to a point of system. This article pro- WCDMA is now recognized as one of the lead- extreme dispersion. Another reason to consider vides a detailed discussion ng candidates for third-generation wireless a complementary packet data solution is to use of the design considera- access. Based on direct-sequence spread-spec- optimized admission procedures for packet data tions under different con- trum with a chip rate of 3. 84 Chips/s, it occu- access that is fairly aggressive in order to achieve ditions. However, the pies a bandwidth of about 5 MHz. It will support high spectral efficiency. An aggressive admission numerical results shown ircuit and packet data access at nominal rates policy will result in high word error rates in /4 were based up to 384 kb/s in macrocellular environments, (WERs) that can generally be managed for improved radio link and provide simultaneous voice and data ser- Internet services using automatic repeat reques design using convolution (ACIS) concept based on OFDM signaling and (ARQ)techniques but are problematic for al codes to achieve even vices. An advanced cellular internet delay-sensitive voice services. Therefore, a com- better performance Magazine·Ju

IEEE Communications Magazine • July 2000 79 and wireless services. Integrated services also include voice services, circuit data, and packet data with transmission rates from 30 kb/s to a few hundred megabits per second. Providing nomadic customers in areas such as airports, hotels, and other public areas with the same user experience they have in their office is the key driver to deploy such high-rate complementary packet data services. Wideband code-division multiple access (WCDMA) will use 5 MHz channels, and it is a leading candidate for third-generation wireless access [1]. However, it will be limited to about 384 kb/s (nominal) peak data rates1 for macro￾cellular wireless access (up to 2 Mb/s rates are proposed for indoor environments). Global Sys￾tem for Mobile Communications (GSM) enhancements based on Enhanced Data Rates for GSM Evolution (EDGE) using adaptive modulation will provide bit rates up to 384 kb/s in the near future. [1] Second-generation wire￾less systems will evolve with complementary packet data solutions that generally use frequen￾cy channels separated from circuit voice and cir￾cuit data access. Time-division multiple access (TDMA) and CDMA systems are being consid￾ered in which circuit and packet access share a common frequency channel and access modes are separated by time slots or spreading codes. However, the expected demands for high peak￾rate Internet access are motivating increasing consideration of complementary access based on separate frequency channels to provide maxi￾mum peak rates and to allow optimization for packet data transmission alone. OFDM was proposed for digital cellular sys￾tems in the mid-1980s [2]. OFDM has also been shown to be effective for digital audio and digi￾tal video broadcasting at multimegabit rates in Europe, and it has been incorporated into stan￾dards by the European Telecommunications Standards Institute (ETSI). The IEEE 802.11 standards group recently chose OFDM modula￾tion for wireless LANs operating at bit rates up to 30 Mb/s at 5 GHz. In this article, OFDM modulation combined with dynamic packet assignment with wideband 5 MHz channels is proposed for high-speed packet data wireless access in macrocellular and microcellular envi￾ronments, supporting a family of peak bit rates ranging from 2 to 10 Mb/s. OFDM can largely eliminate the effects of intersymbol interference for high-speed transmission rates in very disper￾sive environments, and it readily supports inter￾ference suppression and space-time coding to enhance efficiency. Dynamic packet assignment can support excellent spectrum efficiency and high peak-rate data access. WIDEBAND OFDM WCDMA is now recognized as one of the lead￾ing candidates for third-generation wireless access. Based on direct-sequence spread-spec￾trum with a chip rate of 3.84 Mchips/s, it occu￾pies a bandwidth of about 5 MHz. It will support circuit and packet data access at nominal rates up to 384 kb/s in macrocellular environments, and provide simultaneous voice and data ser￾vices. An advanced cellular Internet service (ACIS) concept based on OFDM signaling and dynamic packet assignment (DPA) has been pro￾posed, with the potential to provide 384 kb/s data services in macrocellular environments using only 1 MHz of spectrum [3]. It is possible to expand this ACIS concept into a wideband con￾text in 5 MHz while providing a complementary service to third generation systems such as EDGE and WCDMA. This wideband OFDM system would support an order of magnitude higher peak data transmission rate in macrocells at 2 to 5 Mb/s and up to 10 Mb/s in microcells. IS-136, GSM or WCDMA would provide circuit voice and other circuit-based services and basic data services. A complementary high-speed packet data mode would provide fast wireless packet data access to meet the demand for wire￾less data in the future that provides access per￾formance similar to wideband fixed access. Since portable equipment is power-limited, strongly asymmetrical traffic should be supported, and uplink transmission rates should be allowed to adapt downward as necessary to support the required link budgets. Wideband OFDM wire￾less access might also be configured to introduce new broadband capabilities using OFDM only on the downlink, which is then integrated with emerging wireless packet data systems such as General Packet Radio Service (GPRS), EDGE, or WCDMA to provide two-way access. An example of such a system with the EDGE uplink is discussed in [4].2 There are a number of reasons to consider such a high-rate complementary packet data capability for downlinks. Wireless Internet usage is likely to be downlink-limited. Further￾more, for data services, peak bit rate is very important in determining overall system perfor￾mance, because of the highly bursty nature of Internet traffic. GPRS, EDGE, and WCDMA solutions will support transmission rates of 144–384 kb/s in macrocellular environments. To achieve rates in the megabits-per-second range for all environments using ~5 MHz spectrum is challenging for both the physical layer and radio resource management design. Single-carrier TDMA solutions are limited in supportable transmission bit rate by equalizer complexity. Even though new techniques such as interfer￾ence suppression and space-time processing are promising, the interactions of these techniques with equalization significantly lower achievable bit rates in hostile operating environments for single-carrier solutions. Low spreading gain or intercode interference at high bit rates limits CDMA solutions. The use of OFDM with suffi￾ciently long symbol periods of 100–200 ms for packet data transmission addresses these issues. It supports a high bit rate in time delay spread environments with performance that improves with increasing delay spread up to a point of extreme dispersion. Another reason to consider a complementary packet data solution is to use optimized admission procedures for packet data access that is fairly aggressive in order to achieve high spectral efficiency. An aggressive admission policy will result in high word error rates (WERs) that can generally be managed for Internet services using automatic repeat request (ARQ) techniques but are problematic for delay-sensitive voice services. Therefore, a com￾OFDM can largely eliminate the effects of intersymbol interference for high-speed transmission rates in very dispersive environments, and it readily supports interference suppression and space-time coding to enhance efficiency. 1 Peak rates exceeding 1 Mb/s under limited condi￾tions for very few simulta￾neous users are also considered for some sys￾tems. 2 In [4] we focused on the architecture of such a system in a macrocullar system. This article pro￾vides a detailed discussion of the design considera￾tions under different con￾ditions. However, the numerical results shown in [4] were based on an improved radio link design using convolution￾al codes to achieve even better performance

lementary high-peak-rate packet dat ili- the application of multiple transmit antennas for With the wider ty designed with non-delay-sensitive sending adjacent subchannel signals to achieve a priority is attractive. In this article lurIn bandwidth OFDM to overcome physical layer for hopping or interleaving in the time domain 9 requency diversity without red iscussed in this attaining high bit rates, and we consider DPA to which introduces delay. More advanced trans- enable aggressive packet access with high s nitter diversity based on space-time odin trum efficiency. In addition, we will also discuss can further enhance spectrum efficiency provid subchannels are a frame structure which allows flexibility to ed accurate channel estimation is available. Sim accommodate low-delay services with small fied transmitter diversity can be achieved by available which resources, so potential benefits of multimedia transmitting the same OFDM symbols on multi- services can be realized ple antennas with delayed transmission times The re manae of this article is organized as With the wider bandwidth discussed in this arti follows. We discuss OFDM-based physical layer cle, many subchannels are available, which pro- techniques and DPA-based medium access con- vides a possibility to achieve good performance achieve goo trol(MAC)techniques for realizing the proposed by exploiting time and frequency diversity with- wideband oFDM system. Through a combination out multiple transmit antennas. of OFDM, DPA, adaptive modulation and cod Assume a bandwidth of 5 MHz is divided into exploiting time ing, smart antennas, and space-time coding, dif- about 20 radio resources of 200 kHz each with 1 erent bit rates can be provided with varying MHz reserved for guard bands. Every 200-kHz and frequency efficiency and robustness We describe a possible radio resource can be constructed by grouping a diversity without frame structure in which all these techniques can cluster of (25)8-kHz subchannels. Frequency be implemented for both large-resource high-rate diversity can be achieved by hopping over differ data services and small-resource low-delay ser- ent clusters on different time slots. The same transmit vices. Simulation results based on the large hopping pattern is repeated once every frame of resource assignment procedure are shown to 8 slots. Up to 20 users can be simultaneously antennas demonstrate the potential performance achiev- assigned, one resource each, using different hop- ble in macrocellular environments. We conclude ping patterns that are free from collisions. High this article by outlining important attributes of rate users can be assigned multiple or al this proposal and areas for further study resources. Date rates equivalent to a fraction of nominal radio resource can also be assigned by cheduling transmission in the time domain. w PHYSICAL AND MAC LAYER will discuss assignment of large and small TECHNIQUES AND DEPLOYMENT esources for different applications. a key fea- ture of a 5 MHz bandwidth is the availability of ScENARIOS diversity and interleaving in both time and fre quency domains, which enables high coding gain This section discusses how wideband OFDM can to achieve performance enhancement using a be implemented in both macrocells and micro- single transmit antenna. cells to provide ubiquitous broadband services OFDM has been proposed for the physical Most of the techniques discussed next for macro- layer for ACIS in macrocells with 1-2 b/s/Hz cells are also applicable to enable wideband channel coding using mode adaptation with OFDM in microcells with potential for even quadrature phase shift keying(QPSK)and 8- Igher rates. PSK modulation to support peak bit rates up to 1 Mb/s channe WIDEBAND OFDM IN MACROCELLS ls[3」.This allows for various overheads to account for up to Physical Layer Techniques- In typical wire- 50 percent of the total available bandwidth. With line applications, communication channels are a 4 MHz bandwidth, similar to WCDMA, generally static over the connection period. In 5 Mb/s can be achieved. OFDM provides this case, OFDM subchannel power and bit allo- support for interference suppression and s cation can be optimized through measurement antennas [7] because the effects of dispersion and feedback in the initial link setup process. can be removed at a receiver easily by first pro- Measurement errors and feedback delay signifi- cessing each antennas signal with a discrete cantly reduce the performance of this technique Fourier transform(DFT) before combining with in time-varying wireless fading channels. In wire- an interference suppression algorithm Packet less channels, good link performance can be data wireless access tends to be dominant-inter achieved by OFDM when combined with diversi- ference-limited, so linear interference suppre ty, interleaving, and coding [2]. OFDM inherent- sion techniques are effective to increase capacity ly provides frequency diversity over subchannels, with a two-branch receiver. These technique which introduces an opportunity for interleaving support operation near 0 dB signal-to-interfc in the frequency domain. However, adjacent ence(S/T)and at about 5 dB signal-to-noise ratio subchannels may still be highly correlated. Sony (SNR) for 1 b/s/Hz coding 7] has proposed an OFDM-based scheme [5]using One of the strong challenges of providing up time-domain interleaving combined with fre- to 5 Mb/s transmission rates on downlinks for quency hopping to enhance performance. This packet data in macrocells is the link budget. RF ystem also uses frequency hopping to achieve power amplifier cost is a major factor in base station cost, and it is a major contributor to when high peak rate is desired power supply requirements, heat management, while bandwidth is limited, there may generally and equipment size. An IS-136 channel deliver not be enough"clusters"of subchannels to use about 24 kb/s of coded user data with acceptable for frequency hopping Reference [ 3] proposed quality on a fading channel at about 17 dB SNR IEEE Communications Magazine. July 2000

80 IEEE Communications Magazine • July 2000 plementary high-peak-rate packet data capabili￾ty designed with non-delay-sensitive services as a priority is attractive. In this article we consider OFDM to overcome physical layer barriers for attaining high bit rates, and we consider DPA to enable aggressive packet access with high spec￾trum efficiency. In addition, we will also discuss a frame structure which allows flexibility to accommodate low-delay services with small resources, so potential benefits of multimedia services can be realized. The remainder of this article is organized as follows. We discuss OFDM-based physical layer techniques and DPA-based medium access con￾trol (MAC) techniques for realizing the proposed wideband OFDM system. Through a combination of OFDM, DPA, adaptive modulation and cod￾ing, smart antennas, and space-time coding, dif￾ferent bit rates can be provided with varying efficiency and robustness. We describe a possible frame structure in which all these techniques can be implemented for both large-resource high-rate data services and small-resource low-delay ser￾vices. Simulation results based on the large resource assignment procedure are shown to demonstrate the potential performance achiev￾able in macrocellular environments. We conclude this article by outlining important attributes of this proposal and areas for further study. PHYSICAL AND MAC LAYER TECHNIQUES AND DEPLOYMENT SCENARIOS This section discusses how wideband OFDM can be implemented in both macrocells and micro￾cells to provide ubiquitous broadband services. Most of the techniques discussed next for macro￾cells are also applicable to enable wideband OFDM in microcells with potential for even higher rates. WIDEBAND OFDM IN MACROCELLS Physical Layer Techniques — In typical wire￾line applications, communication channels are generally static over the connection period. In this case, OFDM subchannel power and bit allo￾cation can be optimized through measurement and feedback in the initial link setup process. Measurement errors and feedback delay signifi￾cantly reduce the performance of this technique in time-varying wireless fading channels. In wire￾less channels, good link performance can be achieved by OFDM when combined with diversi￾ty, interleaving, and coding [2]. OFDM inherent￾ly provides frequency diversity over subchannels, which introduces an opportunity for interleaving in the frequency domain. However, adjacent subchannels may still be highly correlated. Sony has proposed an OFDM-based scheme [5] using time-domain interleaving combined with fre￾quency hopping to enhance performance. This system also uses frequency hopping to achieve interference averaging. However, when high peak rate is desired while bandwidth is limited, there may generally not be enough “clusters” of subchannels to use for frequency hopping. Reference [3] proposed the application of multiple transmit antennas for sending adjacent subchannel signals to achieve frequency diversity without requiring frequency hopping or interleaving in the time domain, which introduces delay. More advanced trans￾mitter diversity based on space-time coding [6] can further enhance spectrum efficiency provid￾ed accurate channel estimation is available. Sim￾plified transmitter diversity can be achieved by transmitting the same OFDM symbols on multi￾ple antennas with delayed transmission times. With the wider bandwidth discussed in this arti￾cle, many subchannels are available, which pro￾vides a possibility to achieve good performance by exploiting time and frequency diversity with￾out using multiple transmit antennas. Assume a bandwidth of 5 MHz is divided into about 20 radio resources of 200 kHz each with 1 MHz reserved for guard bands. Every 200-kHz radio resource can be constructed by grouping a cluster of (25) 8-kHz subchannels. Frequency diversity can be achieved by hopping over differ￾ent clusters on different time slots. The same hopping pattern is repeated once every frame of 8 slots. Up to 20 users can be simultaneously assigned, one resource each, using different hop￾ping patterns that are free from collisions. High￾rate users can be assigned multiple or all resources. Date rates equivalent to a fraction of a nominal radio resource can also be assigned by scheduling transmission in the time domain. We will discuss assignment of large and small resources for different applications. A key fea￾ture of a 5 MHz bandwidth is the availability of diversity and interleaving in both time and fre￾quency domains, which enables high coding gain to achieve performance enhancement using a single transmit antenna. OFDM has been proposed for the physical layer for ACIS in macrocells with 1–2 b/s/Hz channel coding using mode adaptation with quadrature phase shift keying (QPSK) and 8- PSK modulation to support peak bit rates up to 1 Mb/s in about 800 kHz channels [3]. This allows for various overheads to account for up to 50 percent of the total available bandwidth. With a 4 MHz bandwidth, similar to WCDMA, up to 5 Mb/s can be achieved. OFDM provides good support for interference suppression and smart antennas [7] because the effects of dispersion can be removed at a receiver easily by first pro￾cessing each antenna’s signal with a discrete Fourier transform (DFT) before combining with an interference suppression algorithm. Packet data wireless access tends to be dominant-inter￾ference-limited, so linear interference suppres￾sion techniques are effective to increase capacity with a two-branch receiver. These techniques support operation near 0 dB signal-to-interfer￾ence (S/I) and at about 5 dB signal-to-noise ratio (SNR) for 1 b/s/Hz coding [7]. One of the strong challenges of providing up to 5 Mb/s transmission rates on downlinks for packet data in macrocells is the link budget. RF power amplifier cost is a major factor in base station cost, and it is a major contributor to power supply requirements, heat management, and equipment size. An IS-136 channel delivers about 24 kb/s of coded user data with acceptable quality on a fading channel at about 17 dB SNR. With the wider bandwidth discussed in this article, many subchannels are available, which provides a possibility to achieve good performance by exploiting time and frequency diversity without using multiple transmit antennas

Therefore, 2.5 Mb/s would require 100 times as cuit services. As a result, the dCa gain is limit much transmit power(20 dB)unless additional ed to somewhat better traffic resource utiliza One of the echniques are introduced. Smart antenna tech- tion, which may be achieved at the cost of nology using four switched 30" beams in a 120 nonoptimal interference management. To benefits of dpa sector is now a well-developed technology with achieve the potential of DCA gain, channel reas- some early deployment. This technology pro- signments must take place at high speed to avoid vides up to 6 dB in link budget improvement rapidly changing interference. DPA, based on and also improves capacity. Terminal two-branch properties of an OFDM physical layer, is pre receiver diversity combined with concatenated posed, which reassigns transmission resources on onvolutional/Reed-Solomon coding supports a packet-by-packet basis using high-speed receiv it is relatively receiver sensitivities of less than 5 dB SNR with er measurements to overcome these problems 1 b/s/Hz coding Space-time coding can provide [9 Having orthogonal subchannels well defined Insensitive to SNR gain based on transmit diversity By com- in time-frequency grids, OFDM has a key advan bining smart antenna technology at base stations e here with the ability to rapidly measure errors In power with terminal receiver sensitivities of less than 5 interference or path loss parameters in parallel dB SNR, the downlink for wideband OFDM can on all candidate channels, either directly or upport peak transmission rates of 2-5 Mb/s with based on pilot tones. One of the benefits of about the same transmit power and coverage as Dpa based on interference avoidance is that it performance even a single transceiver for IS-136 TDMA or analog is relatively insensitive to errors in power con ellular technolog rol, and provides good performance even with without power out power control. Reference 8 shows that control MAC-Layer Techniques- Very high spec Ca without power control decreases capacity trum efficiency will be required for wideband to a factor of 2. however. even without OFDM, particularly for macrocellular opera- control, interference avoidance can outperform tion First-generation cellular systems used interference averaging with power control. This fixed channel assignment. Second-generation is particularly advantageous for packet transmis- cellular systems generally use fixed channel sion where effective power control is problemat- assignment or interference averaging with due to the rapid arrival and departure of pread spectrum. WCDMA will also use inter- interfering packets. ference averaging. Interference avoidance ol The basic protocol for a downlink comprises dynamic channel assignment(DCA)has been four basic steps used in some systems, generally as a means of . A packet automatic channel assignment or local capacity minal page from a base station to a ter- enhancement, but not as a means of large sys-. Rapid measurements of resource usage by a temwide capacity enhancement. Some of the tial capacity gain of DCA are the difficulties .A short report from the terminal to the base introduced by rapid ch station of the potential transmission quality intensive receiver measurements required by a (a unit of high-performance DCA or interference avoid width that is separately assignable) ance algorithm. OFDM pre mises to overcome Selection of resources by the base and trans- these challenging implementation issues. It was mission of the data shown by Pottie 8] that interference averaging This protocol could be modified to move some techniques can perform better than fixed chan- of the over-the-air functions into fixed network nel assignment techniques, whereas interfer- transmission functions to reduce wireless trans ence avoidance techniques can outperform mission overhead at the cost of more demand- interference averaging techniques by a factor of ing fixed network transmission requirements. 2-3 in spectrum efficiency The frame structures of adjacent base stations For existing second-generation systems, the are staggered in time(i.e. neighboring ba ntially b/s/Hz/sector(assuming 3 sectors/cell) is much DPA functions outlined above with a predeter lower than that shown in [8 which was obtained mined rotation schedule). This avoids collisions under idealized conditions. IS-136 TDMA today of channel assignments(i.e, the possibility for provides a spectrum efficiency of about 4 per- adjacent base stations to independently select cent(3 x 8 kb/30 kHz x 1/21 reuse). GSM also the same channel, thus causing interference provides a spectrum efficiency of about 4 per- when transmissions occur). In addition to cent(8 x 13 kb/200 kHz x 1/12 reuse) IS-95 achieving much of the potential gain of a rapid CDMA provides a spectrum efficiency of 4 per- interference avoidance protocol, this protocol cent to 7 percent(12 to 20 x 8 kb/1250 kHz x 1 provides a good basis for admission control and reuse x 1/2 voice activity). DCA combined with mode(bit rate)adaptation based on measured circuit-based technology(which is the approach generally taken to date)can provide some bene Figure l shows the performance of this algo- fits. However, it cannot provide large capacity rithm with several modulation/coding schemes gains, because of the dynamics of interference in and with either two-branch maximal-ratio-com a mobile system as well as the difficulty in imple- bining or two-branch receiver interference sup menting rapid channel reassignments In circuit- pression using packet traffic models based on based systems channel variations, especially Internet statistics [9]. Results with interferenc those caused by the change of shadow fading, suppression for space-time coding are not includ are frequently faster than what can be adapted ed because each transmitted signal appears as by the slow assignment cycle possible in the cir- multiple signals, which significantly limits the

IEEE Communications Magazine • July 2000 81 Therefore, 2.5 Mb/s would require 100 times as much transmit power (20 dB) unless additional techniques are introduced. Smart antenna tech￾nology using four switched 30˚ beams in a 120˚ sector is now a well-developed technology with some early deployment. This technology pro￾vides up to 6 dB in link budget improvement and also improves capacity. Terminal two-branch receiver diversity combined with concatenated convolutional/Reed-Solomon coding supports receiver sensitivities of less than 5 dB SNR with 1 b/s/Hz coding. Space-time coding can provide SNR gain based on transmit diversity. By com￾bining smart antenna technology at base stations with terminal receiver sensitivities of less than 5 dB SNR, the downlink for wideband OFDM can support peak transmission rates of 2–5 Mb/s with about the same transmit power and coverage as a single transceiver for IS-136 TDMA or analog cellular technologies. MAC-Layer Techniques — Very high spec￾trum efficiency will be required for wideband OFDM, particularly for macrocellular opera￾tion. First-generation cellular systems used fixed channel assignment. Second-generation cellular systems generally use fixed channel assignment or interference averaging with spread spectrum. WCDMA will also use inter￾ference averaging. Interference avoidance or dynamic channel assignment (DCA) has been used in some systems, generally as a means of automatic channel assignment or local capacity enhancement, but not as a means of large sys￾temwide capacity enhancement. Some of the reasons for not fully exploiting the large poten￾tial capacity gain of DCA are the difficulties introduced by rapid channel reassignment and intensive receiver measurements required by a high-performance DCA or interference avoid￾ance algorithm. OFDM promises to overcome these challenging implementation issues. It was shown by Pottie [8] that interference averaging techniques can perform better than fixed chan￾nel assignment techniques, whereas interfer￾ence avoidance techniques can outperform interference averaging techniques by a factor of 2–3 in spectrum efficiency. For existing second-generation systems, the achieved spectrum efficiency measured in b/s/Hz/sector (assuming 3 sectors/cell) is much lower than that shown in [8], which was obtained under idealized conditions. IS-136 TDMA today provides a spectrum efficiency of about 4 per￾cent (3 x 8 kb/30 kHz x 1/21 reuse). GSM also provides a spectrum efficiency of about 4 per￾cent (8 x 13 kb/200 kHz x 1/12 reuse). IS-95 CDMA provides a spectrum efficiency of 4 per￾cent to 7 percent (12 to 20 x 8 kb/1250 kHz x 1 reuse x 1/2 voice activity). DCA combined with circuit-based technology (which is the approach generally taken to date) can provide some bene￾fits. However, it cannot provide large capacity gains, because of the dynamics of interference in a mobile system as well as the difficulty in imple￾menting rapid channel reassignments. In circuit￾based systems channel variations, especially those caused by the change of shadow fading, are frequently faster than what can be adapted by the slow assignment cycle possible in the cir￾cuit services. As a result, the DCA gain is limit￾ed to somewhat better traffic resource utiliza￾tion, which may be achieved at the cost of nonoptimal interference management. To achieve the potential of DCA gain, channel reas￾signments must take place at high speed to avoid rapidly changing interference. DPA, based on properties of an OFDM physical layer, is pro￾posed, which reassigns transmission resources on a packet-by-packet basis using high-speed receiv￾er measurements to overcome these problems [9]. Having orthogonal subchannels well defined in time-frequency grids, OFDM has a key advan￾tage here with the ability to rapidly measure interference or path loss parameters in parallel on all candidate channels, either directly or based on pilot tones. One of the benefits of DPA based on interference avoidance is that it is relatively insensitive to errors in power con￾trol, and provides good performance even with￾out power control. Reference [8] shows that DCA without power control decreases capacity up to a factor of 2. However, even without power control, interference avoidance can outperform interference averaging with power control. This is particularly advantageous for packet transmis￾sion where effective power control is problemat￾ic due to the rapid arrival and departure of interfering packets. The basic protocol for a downlink comprises four basic steps: • A packet page from a base station to a ter￾minal • Rapid measurements of resource usage by a terminal using the parallelism of an OFDM receiver • A short report from the terminal to the base station of the potential transmission quality associated with each resource (a unit of band￾width that is separately assignable) • Selection of resources by the base and trans￾mission of the data This protocol could be modified to move some of the over-the-air functions into fixed network transmission functions to reduce wireless trans￾mission overhead at the cost of more demand￾ing fixed network transmission requirements. The frame structures of adjacent base stations are staggered in time (i.e., neighboring base sta￾tions sequentially perform the four different DPA functions outlined above with a predeter￾mined rotation schedule). This avoids collisions of channel assignments (i.e., the possibility for adjacent base stations to independently select the same channel, thus causing interference when transmissions occur). In addition to achieving much of the potential gain of a rapid interference avoidance protocol, this protocol provides a good basis for admission control and mode (bit rate) adaptation based on measured signal quality. Figure 1 shows the performance of this algo￾rithm with several modulation/coding schemes and with either two-branch maximal-ratio-com￾bining or two-branch receiver interference sup￾pression using packet traffic models based on Internet statistics [9]. Results with interference suppression for space-time coding are not includ￾ed because each transmitted signal appears as multiple signals, which significantly limits the One of the benefits of DPA based on interference avoidance is that it is relatively insensitive to errors in power control, and it provides good performance even without power control

oles and building walls. In addition, high bit rates are desirable to provide a capability as near to that of wired access as possible. For 8 indoor and private system access, unlicensed pectrum at 5 GHz or higher may be desirable, delay div, int sup where large bandwidths are available. For ≥ these environments. small antennas are equired. Because of the large angular spread 2 b/s/Hz experienced at radio ports located in the clut ter of buildings and trees, simple omnidirec tional or low-gain antennas are appropriate. In 920 that environment. antenna beam switchi provides limited gains in performance, but 15 adaptive antenna arrays and/or space-time coding can be very effective. For example, in a 5 MHz channel, peak rates of 10 Mb/s could 1 b/s/Hz 5 be supported using two transmit and two eceive antennas for the radio link with space ding of 16-quadrature 0 lation(QAM) to achieve a 4 b/s/Hz codis 0 Occupancy (% rate while allowing for about 50 percent over head Mode adaptation to 5 or 2 Mb/s would a Figure 1. Performance as a function of occupancy for different modulation support appropriate link budgets for robust and diversity schemes Microcell radio ports could be implemented that provide little more than radio modem func- tions to allow for very small radio ports. One suppression of interference. These results are possible approach is to use a combination of based on an ofdm radio link with a bandwidt dual antennas at each port and multiport pro- of about 800 kHz, and the bit rates in the follow- cessing per user at a centralized headend For ng discussion are scaled up for an occupied example, if a user delivers, on average, a strong bandwidth of 4 MHz. A system is considered signal to M ports, the dual-branch signals ba with three sectors per base station, each having a hauled from the M "best" ports can be transceiver. All base stations share one wideband cessed at the central site using selection or FDM RF channel by using DPA to avoid co- combining techniques. Simulation studies have annel interference. DPA enables frequency shown that grouping of microcell ports in this use in the time domain among all radio esults transceivers Occupancy is defined to be the frac- ty and capacity due to macroscopic diversity tion of slots being used. As traffic intensity Moreover, this approach requires a minimal increases, occupancy increases, which results in amount of processing at the ports, thus keeping higher interference and more retransmissions. them simple. The processing at the central site Power control was not used to obtain these can also be fairly simple if the signals being esults. Simulation results based on the wideband combined are not dispersed by significant multi- set of parameters will be presented following a path propagation. The grouping approach i description of a possible frame structure. These therefore compatible with the use of OFDM, results show that good performance is obtained herein each frequency (or subgroup of fre with 1 b/s/Hz coding even at an average occupan-qu s)can be processed with paramete cy per base station of 100 percent(33 percent per optimized for that frequency. This kind of pro sector). With two-branch interference suppression cessing works best with time-division duplexing and 1 b/s/Hz coding, the average retransmission (TDD), which requires using the same carrier probability is only about 3 percent throughout the frequency for transmission and reception. This system with the average delivered bit rate of onsistent with the planning for very high about 2.5 Mb/s per base station Using ARQ at speed micro- and picocellular services in third the radio link layer will permit Internet service at generation systems. this retransmission probability with good quality Backhaul could be a significant cost of service (QoS). Higher retransmission probabili- microcellular systems. Various innovative way ty may be acceptable at the expense of longer to use fiber, coax, microwave radio, and millime packet delay. Peak rates up to 5 Mb/s are possible ter-wave radio can be envisioned to make this with lower occupancies using 2 b/s/Hz coding. part of the system reliable. The key require Finally, in addition to interference suppression at ments are to deploy microcells only in areas the receiver, beam switching smart antenna tech- where there is a strong expectation of high niques, performed by the transmitter, can also be demand and provide wide-area applied to reduce interference, thus achieving coverage with a compatible technology. y per bas at 5 Mb/s good performane DPA requires low delay between the air inter- face and resource assignment function, so any architecture that minimizes radio port function- WIDEBAND OFDM IN MICROCELLS ality would need to consider that constraint. This For microcell deployment, very compact radio also means that DPA should allow able to permit convenient siting on existing equipmen, delay in microcellular ports with low power requirements are desir timing for IEEE Communications Magazine. July 2000

82 IEEE Communications Magazine • July 2000 suppression of interference. These results are based on an OFDM radio link with a bandwidth of about 800 kHz, and the bit rates in the follow￾ing discussion are scaled up for an occupied bandwidth of 4 MHz. A system is considered with three sectors per base station, each having a transceiver. All base stations share one wideband OFDM RF channel by using DPA to avoid co￾channel interference. DPA enables frequency reuse in the time domain among all radio transceivers. Occupancy is defined to be the frac￾tion of slots being used. As traffic intensity increases, occupancy increases, which results in higher interference and more retransmissions. Power control was not used to obtain these results. Simulation results based on the wideband set of parameters will be presented following a description of a possible frame structure. These results show that good performance is obtained with 1 b/s/Hz coding even at an average occupan￾cy per base station of 100 percent (33 percent per sector). With two-branch interference suppression and 1 b/s/Hz coding, the average retransmission probability is only about 3 percent throughout the system with the average delivered bit rate of about 2.5 Mb/s per base station. Using ARQ at the radio link layer will permit Internet service at this retransmission probability with good quality of service (QoS). Higher retransmission probabili￾ty may be acceptable at the expense of longer packet delay. Peak rates up to 5 Mb/s are possible with lower occupancies using 2 b/s/Hz coding. Finally, in addition to interference suppression at the receiver, beam switching smart antenna tech￾niques, performed by the transmitter, can also be applied to reduce interference, thus achieving good performance at 5 Mb/s even at 100 percent occupancy per base station. WIDEBAND OFDM IN MICROCELLS For microcell deployment, very compact radio ports with low power requirements are desir￾able to permit convenient siting on existing poles and building walls. In addition, high bit rates are desirable to provide a capability as near to that of wired access as possible. For indoor and private system access, unlicensed spectrum at 5 GHz or higher may be desirable, where large bandwidths are available. For these environments, small antennas are required. Because of the large angular spread experienced at radio ports located in the clut￾ter of buildings and trees, simple omnidirec￾tional or low-gain antennas are appropriate. In that environment, antenna beam switching provides limited gains in performance, but adaptive antenna arrays and/or space-time coding can be very effective. For example, in a 5 MHz channel, peak rates of 10 Mb/s could be supported using two transmit and two receive antennas for the radio link with space￾time coding of 16-quadrature amplitude modu￾lation (QAM) to achieve a 4 b/s/Hz coding rate while allowing for about 50 percent over￾head. Mode adaptation to 5 or 2 Mb/s would support appropriate link budgets for robust coverage. Microcell radio ports could be implemented that provide little more than radio modem func￾tions to allow for very small radio ports. One possible approach is to use a combination of dual antennas at each port and multiport pro￾cessing per user at a centralized headend. For example, if a user delivers, on average, a strong signal to M ports, the dual-branch signals back￾hauled from the M “best” ports can be pro￾cessed at the central site using selection or combining techniques. Simulation studies have shown that grouping of microcell ports in this way can yield impressive results in link reliabili￾ty and capacity due to macroscopic diversity. Moreover, this approach requires a minimal amount of processing at the ports, thus keeping them simple. The processing at the central site can also be fairly simple if the signals being combined are not dispersed by significant multi￾path propagation. The grouping approach is therefore compatible with the use of OFDM, wherein each frequency (or subgroup of fre￾quencies) can be processed with parameters optimized for that frequency. This kind of pro￾cessing works best with time-division duplexing (TDD), which requires using the same carrier frequency for transmission and reception. This is consistent with the planning for very high￾speed micro- and picocellular services in third￾generation systems. Backhaul could be a significant cost issue in microcellular systems. Various innovative ways to use fiber, coax, microwave radio, and millime￾ter-wave radio can be envisioned to make this part of the system reliable. The key require￾ments are to deploy microcells only in areas where there is a strong expectation of high￾speed service demand and to provide wide-area coverage with a compatible technology. DPA requires low delay between the air inter￾face and resource assignment function, so any architecture that minimizes radio port function￾ality would need to consider that constraint. This also means that DPA should allow some margin in timing for delay in microcellular transmission equipment. ■ Figure 1. Performance as a function of occupancy for different modulation and diversity schemes. 0 Retransmission probability (%) Occupancy (%) 1 b/s/Hz 2 b/s/Hz 0 5 10 15 20 25 30 35 40 45 10 20 30 40 QPSK, space-time coding QPSK, delay diversity QPSK, delay div, int sup 8PSK, delay div, int sup