A Comparison of IEEE e Mobile WiMAX Deployments in 700 MHz and 2500 MHz Bands

Transcription

1 A Comparison of IEEE e Mobile WiMAX Deployments in 700 MHz and 2500 MHz Bands Francis E. Retnasothie, M. Kemal Ozdemir - Logus Broadband Wireless, Raj Jain Washington University in St. Louis, Yuefeng Zhou, Nader Zein NEC, Shyam Parekh Alcatel Lucent, Doug Gray WiMAX Forum Consultant, Hassan Yaghoobi-Intel Mostafa (Tom) Tofigh AT&T ABSTRACT The Federal Communications Commission (FCC) in the United States has made the 700 MHz band available for broadband wireless access. With television channels converting to digital transmission, similar spectrum is expected to be available in other countries since digital TV channels require less spectrum than analog channels. This paper compares the performance and cost of WiMAX deployments in the 700 MHz band with those in the 2500 MHz bands. We compare range, coverage, and capacity, and discuss deployment challenges and limitations. Issues in using advanced multiple antenna techniques, such as MIMO and beamforming to enhance range and channel capacity are discussed. Both time division duplex (TDD) and frequency division duplex (FDD) systems are considered. 1. INTRODUCTION The frequency band between 470 MHz and 862 MHz has traditionally been allocated worldwide for radio and TV broadcasting and encompasses the UHF TV channels. With the planned transition to digital radio and TV formats, portions of this band will become available for other services and applications. This has been commonly referred to as the Digital Dividend. The specifics and timing for revised allocations in this band will vary country by country but it is safe to conclude that regulators worldwide will give serious consideration to providing additional spectrum for fixed and mobile broadband services. For example, the ITU (International Telecommunications Union), WRC (World Radio Conference) 2007 identified frequencies in the 790 MHz to 862 MHz range for International Mobile Telecommunications (IMT) applications [WRC07]. In the USA, the Federal Communications Commission (FCC) has already made some of the newly recovered spectrum in 700 MHz band available for broadband access. Telecommunications service providers are, therefore, interested in understanding the issues and challenges in using this spectrum. This paper addresses this need with respect to Mobile WiMAX and shows how the performance in the 700 MHz band compares with that in the 2500 MHz band which is a commonly used band for WiMAX deployments in North America. The WiMAX Forum is an organization of service providers, equipment vendors, chip vendors and users of WiMAX. The WiMAX Forum develops interoperability specifications that allow equipment from different vendors to interoperate. The Mobile WiMAX Release 1.0 Profiles currently cover several frequency bands ranging from

2 2300 MHz to 3800 MHz. For comparison purposes these bands can be grouped into two categories, 2500 MHz and 3500 MHz. Using a hypothetical mid-sized metropolitan area comprising urban, suburban, and rural demographic regions, this paper provides a comparison of WiMAX deployments at 700 MHz and 2500 MHz from a range, coverage, capacity, duplexing, and performance perspective. Since a WiMAX profile in the UHF frequency band is still under development, we selected a set of parameter values that we believe will be representative of a WiMAX 700 MHz solution. This analysis provides insights into the deployment challenges of having limited spectrum for high population density regions as well as the advantages of having access to the 700 MHz band for range and coverage in the more sparsely populated rural areas. The organization of this paper is as follows. In Section 2, we present a system model that shows the key parameters that affect the performance along with the details of the 700 MHz frequency band in the United States. In Section 3, we present channel models, discuss building and vehicular penetration losses and other deployment considerations. Coverage and capacity simulations are presented in Section 4. Deployment considerations in dense urban, urban, suburban, and rural areas are discussed in Section 5. Finally, conclusions are summarized in Section SYSTEM MODEL For the analysis of the 700 MHz band performance for WiMAX applications, rural, suburban, and urban environments are considered in this paper. The model in Figure 1 shows various components of a WiMAX access system. P TxAnt_in G TxAnt Path Loss (PL) Building Penetration Loss h b h s G RxAnt X BS d Vehicle Penetration Loss Figure 1: System Model The key components are the base station (BS) with antennas high on a tower and the mobile station (MS) at or near ground level. The MS can be sitting inside a building, walking outdoors, or traveling at a high speed in a car. The path-loss and penetration losses for these fixed, nomadic, and mobile applications depend upon the height of the

3 BS antenna (h b ), power transmitted by the BS antenna (PTxAnt_in), gain of the transmitting antenna (GTxAnt), gain of the receiving antenna (GRxAnt), height of the MS or subscriber antenna (h s ), and the speed of the MS. These are the key parameters that, along with the environment and frequency band, determine the performance MHz frequency allocation by FCC The main factor that affects capacity is the spectral width of available channels. Therefore, we first describe the 700 MHz spectrum made recently available by the FCC. In the United States, the FCC has announced availability of several licenses ranging from 2 MHz of spectrum to 22 MHz of spectrum per license in the 700 MHz band [FCC07, FCC07b]. The US 700 MHz band allocation is designated as Lower 700 MHz Band and Upper 700 MHz Band. The licenses available in these bands are shown in Figure 2 and listed in Table 1 [FCC07]. Figure MHz band allocation in the US. Note that there are two licenses in the upper 700 MHz band that have only 2 MHz spectrum. These will not be considered further in this paper since the spectrum is considered to be insufficient for a deployment offering broadband services. Traditional cellular networks use frequency division duplexing (FDD) in which the downlink (BS to MS) uses one frequency channel and uplink (MS to BS) uses another frequency channel. WiMAX allows FDD but also allows time division duplexing (TDD) in which downlink (DL) and uplink (UL) transmissions use the same frequency channel but alternate in time. TDD allows the data rate in the two directions to be asymmetric, as is, generally required for data traffic. Assuming 700 MHz WiMAX equipment is available with either 5 or 10 MHz channel bandwidths, the US licenses in the 700 MHz band will support base station configurations as shown in Table 2 for TDD operation. It is assumed that the BSs and MSs have one transmit antenna and two receive antennas. This is called 1x2 single input

4 multiple output (SIMO) configuration. The average DL sector capacity is based on a DL to UL traffic ratio of 3:1 and a frequency reuse factor of 1 (the same frequency channels are reused in each BS sector and in all adjacent cells). Table 1: 700 MHz Channels in the US License Bandwidth Channelization Lower 700 MHz Band: A-A 12 MHz 2x6 MHz paired channels B-B 12 MHz 2x6 MHz paired channels C-C 12 MHz 2x6 MHz paired channels D 6 MHz Unpaired channel E 6 MHz Unpaired channel Lower 700 MHz Band: C-C 22 MHz 2x11 MHz paired channels D-D 10 MHz 2x5 MHz paired channels A-A 2 MHz 2x1 MHz paired channels B-B 2 MHz 2x1 MHz paired channels Table 2: Possible BS Configurations for US 700 MHz Band Plan License Channel BW Maximum Channels per Sector Average DL Sector Capacity Lower 700 MHz Band D or E 5 MHz Mbps Lower 700 MHz Band A-A, B-B, or C-C 5 MHz Mbps Upper 700 MHz Band C-C 10 MHz Mbps Upper 700 MHz Band D-D 5 MHz Mbps Upper 700 MHz Band A-A, B-B Not suitable for broadband deployment 3. COMPARATIVE ANALYSIS In this section we present simulation parameters and results for the 700 MHz and the 2500 MHz frequency bands. We first describe the channel models and then discuss the effect of building and vehicular penetration losses, and antenna configurations. 3.1 Channel Models A number of empirical models have been developed for the wireless channel between the BS and MS. For 700 MHz applications, we based our simulations on the Hata model [Hata80]. For the 2500 MHz band, we used the COST 231 model which is an extension of the Hata model [COST231]. Figure 3 shows cell radius as a function of the path loss for the 700 MHz and 2500 MHz frequency bands using the Hata and COST231 models. The Stanford pedestrian model [Hari2000] result is included for comparison as it is also a model often used in the higher frequency bands.

5 3.2 Building and Vehicular Penetration Loss Measurements conducted by Ferreira et al [Ferreira07] show that for 900 MHz GSM systems, the path loss inside the buildings is significantly higher than the outdoors. The difference has a mean of 5.7 db and standard deviation of 11.1 db. The effect at 700 MHz is expected to be similar. Attenuation for 1800 MHz GSM and 1900 MHz UMTS can be obtained by adding an additional 1.9 db loss. Since subscribers will often be located in vehicles, penetration loss for these applications also must be considered [Kostanic98]. 7 6 Cell Radius in km Path Loss in db 700 MHz Hata 2500 MHz Stanford Ped MHz COST 231 Figure 3: Suburban Outdoor Path Loss Comparison 3.3 Antenna Requirements and Deployment Considerations Emerging wireless systems require the use of multiple antennas at either the receiver or the transmitter or both. Advanced multiple antenna techniques such as multiple input multiple output (MIMO) with space time coding (STC) and spatial multiplexing and beamforming can provide transmit and receive diversity to enhance both range and channel capacity. In order to fully realize the benefits of these multi-antenna systems, the antennas must be separated far apart to make their signals uncorrelated. For a lower correlation between antenna elements of a BS, the BS antennas must be separated by several wavelengths [Andersen97] which at 700 MHz amounts to several meters. In the 2500 MHz band on the other hand, the antenna spacing requirement for MIMO systems are less due to the shorter wavelength. Thus, it is easier to use MIMO at 2500 MHz. Due to antenna spacing limitations, it is challenging to realize uncorrelated antennas via spatial separation at MSs. Alternatively, uncorrelated antennas can be realized by using cross polarized antennas. This can allow the use of multiple antenna techniques, and especially collaborative 2x2 MIMO.

6 3.4 High Mobility Support Based on Doppler Spread WiMAX uses orthogonal frequency division multiple access (OFDMA) which divides the channel into 1024 subcarriers for a 10 MHz channel and 512 subcarriers for a 5 MHz channel. High-speed mobility causes a shift (Doppler shift) in the frequency of these subcarriers resulting in inter-carrier interference (ICI). Upper and lower bounds have been developed for the ICI power as a function of velocity and symbol period for different time varying models [WF07]. We extended these bounds for the 700 MHz and 2500 MHz systems. We found that the ICI for the 700 MHz OFDMA system will be, on average, approximately 11 db better than the 2500 MHz system. Considering that the required SINR for Convolutional Turbo Code (CTC) 64QAM ¾ at the bit error rate (BER) of 10-6 is 20 db in an AWGN (additive white Gaussian noise) channel, the 2500 MHz systems may have difficulty operating at 64QAM ¾ at higher velocities, such as >250 km/hr, in some environments. The 700 MHz systems will exhibit better performance in these high velocity applications. 3.5 Other Parameter Differences Cable Losses: Network operators typically prefer base-mounted transmitter power amplifiers rather than tower-mounted amplifiers for ease of maintenance. The amplifiers transmit power, therefore, must be sufficient to overcome cable losses. In the 2500 MHz band, cable losses are almost twice the losses at 700 MHz. To achieve the same transmit power at the base station antenna port, therefore, 700 MHz deployments can use lower power base-mounted amplifiers or, alternatively, lower-cost cables. In either case, this cost savings would help to partially mitigate the cost impact of the larger antennas and associated mounting structures required in the 700 MHz band. Line of Sight: True line-of-sight (LOS) is defined as a path free of obstructions within the first Fresnel zone to minimize the simultaneous reception of reflected out-of-phase signals and excess losses due to signal diffraction. The Fresnel zone is an ellipsoid with a radius proportional to the square root of the wavelength. Therefore, higher base station height at 700 MHz is necessary to achieve the same level of Fresnel zone clearance that can be achieved at 2500 MHz. Other Relevant Parameters: Although we have not explicitly analyzed the effect of other parameters and factors that impact range such as mobile station antenna gain, transmit power, noise figure, etc., it can be assumed that these are comparable for the two bands. 4. COVERAGE AND CAPACITY SIMULATIONS We conducted several system level simulations to compare coverage, interference, and channel capacity performance between 700 MHz and 2500 MHz WiMAX Systems. The simulation time is about 100 seconds, which contains 20,000 5-ms frames. We assumed a 7-cell configuration with a wrap around model. This makes the effective simulation period to be much longer. Table 3 shows the key parameters used for these simulations.

7 Table 3: Key Simulation Parameters Key Simulation Parameters Parameter Value Note FFT Size 1024 Bandwidth 10 MHz Carrier Frequency 700 MHz, 2500 MHz Permutation PUSC Cyclic Prefix Size 1/8 Duplexing TDD DL:UL Ratio 3:1 Modulations QPSK1/2, QPSK ¾, 16QAM1/2, 16QAM3/4, 64QAM1/2, 64QAM2/3, 64QAM3/4 BS Tower HAAT (m) 32 m HAAT=Height above average terrain CPE or Mobile Height (m) 1.5 m Building Loss (db) 12.8 db Body Loss (db) 0 db BS Max TX Power (Watt) 10 Watt CPE TX Power (Watt) 0.2 Watt BS Antenna Gain (dbi) 15 db CPE Antenna Gain (dbi) 1 db BS Rx Implementation loss 5 db NF = Noise figure including NF (db) CPE Rx Implementation loss 7 db including NF (db) Fading Channel Model ITU Pedestrian B Velocity for Pedestrian Users 3 km/hr Packet Scheduler PF (Proportional Fair) Traffic Model Cell deployment Full Buffer 3 sectors cell with segmentation; Cell radius = 2 km; 10 users per sector Number of tiers 2 7 cells (3*7=21 sectors), Wrap around model Path loss channel model COST System Gain and Range For this analysis, a 1x2 SIMO base station and mobile station implementation is assumed for the 700 MHz band and a base station beamforming array is assumed for the 2500 MHz band. The relative range, with 2500 MHz as a reference, for a suburban deployment with indoor mobile terminals for the two bands was analyzed. We found that the UL link budget is the determining factor in the 700 MHz band whereas the DL MAP range, not the beamforming gain, determines the link budget in the 2500 MHz band. Beamforming, therefore, does not appreciably impact the range but does have significant impact on DL

8 channel capacity resulting from the increased link margin and improved interference control. The lower path loss in the 700 MHz band provides a significant range advantage. In range-limited (noise limited) deployments it would take considerably more base stations in the 2500 MHz band to achieve the same area coverage as a 700 MHz deployment. It should be noted however, that this range analysis takes into account only AWGN and does not include the impact of the inter-cell interference that would normally be encountered in a typical multi-cellular deployment. 4.2 Average Interference Margin Figure 4 shows the average downlink interference margin, which is defined as the difference between signal-to-noise ratio (SNR) and signal-to-interference plus noise ratio (SINR) in the simulation. To obtain the average trend, we have plotted linear fitting lines, which show that the 700 MHz systems have a higher interference margin because the interference signal experiences a lower path loss. It is also observed that carrier-tointerference plus noise ratio (CINR) for data subcarriers vs. distance from the BS is much better at 700 MHz. Average Downlink Interference Margin vs. Distance from BS 12 Interference margin (db) MHz, cell radius 2km 2500MHz, cell radius 2km Distance(m) Figure 4: Downlink Interference Margin 4.3 Coverage We analyzed the Cumulative Distribution Function (CDF) curves of the received DL data subcarrier CINR and found that 90% of MSs have a DL data subcarrier CINR greater than -4 db in a 2500 MHz system, while in the 700 MHz system, 90% of MSs have a DL data subcarrier CINR greater than 0 db. This indicates the 700 MHz system will achieve greater coverage.

9 4.4 Modulation & Coding Scheme Utilization The probability of DL MCS (modulation and coding scheme) usage from QPSK ½ to 64QAM ¾ was also analyzed. It was found that higher order modulation schemes would be more likely in the 700 MHz systems thus resulting in a higher spectral efficiency. In the simulation, the MCS was chosen based on the required CINR at a PER (packet error rate) of 10%. 4.5 User Throughput Distribution The user throughput distribution for a 2 km radius cell between 700 MHz and 2500 MHz indicates that higher throughput per user is achieved for the 700 MHz band. It is also observed that, for a 2 km radius cell, the area over which 64QAM ¾ is supported is greater in the 700 MHz system than that in 2500 MHz system. This again confirms the observation that 700 MHz systems have both higher user throughput and better area coverage than 2500 MHz systems. 5. Metropolitan Area Deployment Comparisons In this section we look at various deployment scenarios to provide additional insights about these frequency bands in a typical wireless metropolitan area network (WMAN). We assumed a hypothetical mid-sized metropolitan area having a total population of approximately 1.75 million people over a demographically varied area of 1,500 km 2. The demographic regions for this assumed metropolitan area are broken down as shown in Table 4. The table also provides some market and usage assumptions to estimate average DL data density requirements. For any broadband wireless access deployment it is important to plan the network to meet the projected peak busy hour (PBH) demand. This is a function of population density, market penetration, and the desired performance during the period when the network is most heavily loaded. Table 4: Estimating Data Density Requirements Dense Urban Urban Suburban Rural/Open Space Area 100 km km km km 2 Population 800, , ,000 50,000 Addressable Market 70% 70% 75% 75% Population Growth 1%/yr 1%/yr 2%/yr 2.5%/yr Net Customers in Year 10 76,000 48,000 39,000 5,000 Estimated PBH Activity 1 out of 5 1 out of 6 1 out of 7 1 out of 7 DL Duty Cycle 25% Desired DL Data Rate During PBH 30 kilobytes per second per user for casual subscribers to 75 kilobytes per second for professional/high-end subscribers Required Data ~20 ~5.5 ~1.5 ~0.1 Density in Year 10 Mbps/km 2 Mbps/km 2 Mbps/km 2 Mbps/km 2

10 In addition to different population densities, each of the demographic regions will have region-specific propagation conditions due to the differences in number of buildings, building heights, and terrains. 5.1 Range for Indoor Mobile Stations and Deployment Summary The range predictions based on the Hata and COST 231 path loss models are summarized in Figure 5, assuming indoor mobile stations. Range Estimate for Demographic Regions Urban Suburban Rural MHz 2500 MHz Range in km Figure 5: Range Predictions for Indoor Mobile Station Table 5: Deployment Scenarios and Summary for Comparative Analysis Case 1 Case 2 Case 3 Case 4 Case 5 Frequency Band 700 MHz 2500 MHz Available Spectrum 6 MHz 10 or 12 MHz 22 MHz 20 MHz 30 MHz Channel BW 5 MHz 10 MHz 10 MHz Duplex TDD DL to UL Ratio 3:1 Total BS requirements with (1x2) SIMO Total BS requirements with BF in the 2500 MHz Band n/a n/a n/a Table 5 provides a summary of the BS deployment requirements for the hypothetical metropolitan area for different spectrum and channel alternatives. The channel capacity is based on simulations assuming a mixed usage model [WF06] with a 3:1 DL to UL traffic

11 ratio. The 2500 MHz results are provided for a 1x2 SIMO as well as a Beamforming BS configuration. As expected, the dense urban deployment is constrained by capacity rather than range regardless of the frequency band. Due to its range capability, the 700 MHz solution, when deployed to utilize its maximum range, is capacity constrained not only in the urban and suburban regions but also in the rural regions since at full range in these areas the resulting DL data density is only 0.01 Mbps/km2 while the desired DL data density is 0.1 Mbps/km 2. The simulation results clearly show the advantage of having more available spectrum. Having a metropolitan area-wide 30 MHz license in the 2500 MHz band enables a deployment with fewer base stations than having only 6 or 10 MHz of spectrum in the 700 MHz band. On the other hand with 22 MHz in the 700 MHz band, the lower band solution proves to be more BS-efficient than 2500 MHz with 1x2 SIMO base stations and is comparable when beamforming is assumed for 2500 MHz. Deploying beamforming solutions in the 2500 MHz band in the higher density urban regions can result in a 40 to 50% increase in DL channel capacity [WF07]. For a more complete business case analysis differences in spectrum cost must also be taken into account, especially in markets such as the US where spectrum licenses are generally awarded by an auction process. Simulations were also performed to reveal the base station deployment requirements for each of the demographic regions. It was observed that 700 MHz systems have a deployment advantage, even with limited spectrum availability, in the lower population density regions. Despite being capacity-constrained, the range advantage of 700 MHz does come into play in reducing the number of base stations required to cover these regions. 5.2 TDD or FDD? Since all but two of the licenses allocated in the US lower and upper 700 MHz band are paired, they will support either FDD or TDD. Generally the attributes of TDD make it the preferred duplexing approach when the traffic is expected to be asymmetric and spectrum is limited [WF06]. With asymmetric traffic, one of the links will be underutilized with FDD whereas TDD can adapt DL and UL subframes to match actual traffic conditions. Table 6 provides a comparison of the attributes for the two duplexing approaches. We analyzed a metropolitan area deployment using both TDD and FDD. The analysis shows that TDD requires considerably fewer base stations to meet data density requirements for DL to UL traffic asymmetries ranging from 3:2 to 3:1. On the other hand, if the traffic is projected to be symmetric or nearly so, FDD, with its more flexible interference control may prove to be a better choice. This will be an important consideration with respect to coexistence with high power TV and public safety systems in the 700 MHz band.

12 Table 6: Attributes of TDD and FDD TDD Adaptive DL to UL ratio for better spectral efficiency with asymmetric traffic Channel reciprocity for easy support of closed loop advanced antenna systems Greater flexibility with frequency reuse schemes with two independent paired channels Easy adaptation to varied global spectrum assignments Simpler transceiver design FDD Dedicated DL and UL channels Single transceiver to cover two paired channels Does not require Tx-Rx transition gap with full duplex FDD mobile stations More flexibility in dealing with interference issues 5.3 Other Mobile WiMAX Usage Models In assessing the business opportunity for a Mobile WiMAX deployment, an operator may also want to consider alternative usage models. The previous analysis assumed mobile handheld WiMAX devices and indoor operation. An operator can also elect to limit customers to outdoor operation thus eliminating the building penetration loss or address a market that only includes fixed roof-mounted outdoor subscriber antennas. The latter usage model adds the benefit of an outdoor mounted subscriber station with a high gain directional antenna. When considering this usage model however, an operator must also consider the added expense of a truck-roll and professional installation for the fixed outdoor subscriber terminals. Either of these options will increase the range capability in any of the bands being considered with the fixed outdoor model providing a 3.5 to 5 times range advantage over an indoor mobile station. 5.4 The 700 MHz Advantage in Rural Deployments The previous analysis clearly shows the advantage of a WiMAX deployment in the 700 MHz band in lower population density regions. In these areas, the range capability is more effectively utilized, whereas in the more heavily populated areas, base station capacity is more important. Looking at the results of the previous analysis for the suburban and rural demographic regions in more detail (Table 7) helps to better quantify the deployment benefits of 700 MHz in these lower population density regions. Although the demographic assumptions used for this specific analysis result in a capacity-constrained deployment for 700 MHz in the rural region there will certainly be cases in which the demographic factors result in a significantly lower data density requirements. Deployments in these regions will enable a greater use of the WiMAX range capability in the 700 MHz band. Flat open terrain characteristics, often encountered in rural environments, can also enable line-of-sight (LOS) to many customers to further increase the range potential. True LOS however, may be difficult to achieve in the 700 MHz band due to the size of the first

13 Fresnel zone. Nevertheless, the non-los and near-los range at 700 MHz with favorable terrain characteristics and strategic antenna tower locations should be well over 10 km with an indoor MS and over 30 km with fixed outdoor mounted subscriber antennas. The data density, of course, would be low but may be sufficient for baseline services in sparsely populated areas. Table 7: Base Stations Required for Suburban and Rural Regions Case 1 Case 2 Case 3 Case 4 Case 5 Band 700 MHz 2500 MHz Available Spectrum 6 MHz 10 or 12 MHz 20 MHz 20 MHz 30 MHz Channel BW 5 MHz 10 MHz 10 MHz # of BSs required for a suburban region with DL data density 60 of 1.5 Mbps/km 2 # of BSs required for a rural region with DL data density of 0.1 Mbps/km CONCLUSIONS There is a global interest in allocating portions of the spectrum between 470 MHz and 862 MHz for broadband wireless services. Compared to fixed or mobile WiMAX solutions in the 2500 MHz frequency band, 700 MHz deployments provide a considerable range benefit. The range benefit does not always translate to a deployment advantage in regions with high population density unless operators have access to at least 20 MHz of spectrum. On the other hand, there is a significant deployment advantage with 700 MHz deployments in the lower population density regions with as little as 6 MHz of spectrum. Although many spectrum allocations will support either time division or frequency division duplexing, the asymmetric traffic expected in broadband data-oriented networks will generally favor WiMAX solutions based on TDD. This enables optimal spectral efficiency resulting in higher DL base station capacity and a more cost-effective deployment with fewer base stations. In some regions however, with a paired channel assignment, FDD may prove to be a better approach to effectively address interference issues due to coexistence with high power digital TV, high power mobile TV, and public safety systems. We believe that the 700 MHz and other bands in the UHF range will be very important bands for WiMAX deployments. Even with limited spectrum assignments, WiMAX in the UHF band can provide a cost-effective solution for providing services to residents in areas that would be uneconomical to serve with conventional wire-line or other wireless access technologies. WiMAX deployments in this band can be expected to play a key role in helping to bridge the digital divide.

14 REFERENCES [FCC07] Second Report and Order, FCC , August 10, 2007, [FCC07b] Erratum to Second Report and Order, DA , October 25, 2007, [WF06] Mobile WiMAX Part II: A Comparative Analysis, WiMAX Forum Website, May 2006, alysis.pdf [WF07] A Comparative Analysis of Mobile WiMAX Deployment Alternatives in the Access Network, WiMAX Forum Website, May 2007, ives.pdf [Ferreira07] Lúcio Ferreira, Martijn Kuipers, Carlos Rodrigues, Luís M. Correia, Characterization of signal penetration into buildings for GSM & UMTS, Proc. of Conftele th Conference on Telecommunications, Peniche, Portugal, May 9-11, 2007, available at [Hata80] M. Hata, Empirical Formula for Propagation Loss in Land Mobile Radio Services, IEEE Trans. On Vehicular Tech., Vol. VT-29, No. 3, Aug [Kostanic98] I. Kostanic, C. Hall, and J. McCarthy, Measurements of the Vehicle Penetration Loss Characteristics at 800MHz, Vehicular Technology Conference, vol. 1, pp. 1-4, Ottawa, Canada May [Andersen97] R. G. Vaughan and J. B. Andersen, Antenna Diversity in Mobile Communications, IEEE Trans. On Vehicular Tech. Vol. VT-36, No. 4, Nov [WRC07] ITU-R World Radio Communication Conferences 2007, [COST231] European Cooperative in the Field of Science and Technical Research EURO-COST 231, "Urban transmission loss models for mobile radio in the 900- and 1,800 MHz bands (Revision 2)," COST 231 TD(90)119 Rev. 2, The Hague, The Netherlands, September 1991, available at [Hari2000] K.V. S. Hari, K.P. Sheikh, and C. Bushue, Interim channel models for G2 MMDS fixed wireless applications, IEEE c-00/49r2, November 15, 2000, available as