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
`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
`
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`
`IEEE Communications Magazine • July 2000
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`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
`
`IEEE Communications Magazine • July 2000
`
`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.
`
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`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.
`
`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.
`
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`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.
`
`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
`
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`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.
`
`QPSK, space-time coding
`QPSK, delay diversity
`QPSK, delay div, int sup
`8PSK, delay div, int sup
`
`2 b/s/Hz
`
`1 b/s/Hz
`
`0
`
`10
`
`20
`Occupancy (%)
`
`30
`
`40
`
`45
`
`40
`
`35
`
`30
`
`25
`
`20
`
`15
`
`10
`
`05
`
`Retransmission probability (%)
`
`■ Figure 1. Performance as a function of occupancy for different modulation
`and diversity schemes.
`
`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
`
`82
`
`IEEE Communications Magazine • July 2000
`
`Facebook's Exhibit No. 1042
`Page 5
`
`

`
`528 tones divided into
`
`22 24-tone clusters
`
`x
`
`Three control
`channels
`
`22 packet data channels
`
`x
`
`x
`
`Pilot channel
`
`Paging channel
`
`Assignment channel
`
`Frequency
`
`24 OFDM blocks
`
`104 OFDM blocks in 8 slots
`
`■ Figure 2. Division of radio resources in time and frequency domains to allow
`DPA for high-peak-rate data services; small radio resources, needed for low-
`delay services, occupy only one slot, which is further divided into four mini-
`slots to allow coding across different frequency clusters.
`
`achieve this frequency diversity for small resources
`a slot is divided into mini-slots, at the cost of
`reduced efficiency due to TDMA overhead.
`
`HIGH-PEAK-RATE DATA SERVICES:
`LARGE RADIO RESOURCES
`
`528 subchannels (4.224 MHz) are organized into
`22 clusters of 24 subchannels (192 kHz) each
`and 8 time slots of 13 OFDM blocks each within
`a 20-ms frame of 128 blocks. Figure 2 shows this
`resource allocation scheme. The control channel
`functions are defined in [3]. This allows flexibili-
`ty in channel assignment while providing 24
`blocks of control overheard to perform the DPA
`procedures.
`This arrangement of tone clusters is similar
`to the arrangements in the band-division multi-
`ple access (BDMA) proposal by Sony. Figure 3
`depicts this operation. Each tone cluster would
`contain 22 individual modulation tones plus 2
`guard tones, and an OFDM block would have a
`time duration of 156.25 ms with 31.25 ms for
`guard time and ramp time to minimize the
`effects of delay spread up to about a 20-ms span.
`Of the 13 OFDM blocks in each traffic slot, two
`blocks are used as overhead, which includes a
`leading block for synchronization (phase/fre-
`quency/timing acquisition and channel estima-
`tion) and a trailing block as guard time for
`separating consecutive time slots. A single radio
`resource is associated with a frequency-hopping
`pattern, by which the packets are transmitted
`using eight different tone clusters in each of the
`eight traffic slots. Coding across eight traffic
`slots for user data, as shown in Fig. 3, exploits
`frequency diversity which gives sufficient coding
`gain for performance enhancement in the fading
`channel. This arrangement supports 22 resources
`in frequency that can be assigned by DPA. Tak-
`ing into account overhead for OFDM block
`guard time, synchronization, slot separation, and
`DPA control, a peak data rate of 2.1296 (3.3792
`x 22/24 x 11/13 x 104/128) Mb/s is available for
`packet date services using all 22 radio resources,
`each 96.8 kb/s.
`
`3 The grouping can be
`configured similar to con-
`ventional frequency plan-
`ning for reuse factor 4
`using a regular and repeti-
`tive pattern, for example,
`with timing groups 1 and
`2 alternating in the odd
`rows and groups 3 and 4
`in the even rows.
`
`4 The word “resource” is
`used to emphasize that the
`assignment of radio chan-
`nels for traffic bearers can
`be a general combination
`of time slots, frequency
`sub-car