Performance of Fixed Wireless Access with ...

Paper presented at VTC 2003, Fall, Orlando, FL

Performance of Fixed Wireless Access with cdma2000 1xEV-DO Eduardo Esteves, Mehmet I. Gurelli and Mingxi Fan QUALCOMM Incorporated 5775 Morehouse Drive San Diego, CA 92121-1714

Abstract— Fixed and portable wireless Internet access is becoming an important segment for the wireless industry. 1xEVDO, also known as IS-856, has been designed to offer increased user experience and high capacity packet data services on cdma2000 networks. The initial set of 1xEV-DO performance results in the literature has focused on mobile environments. In this paper, we address some of the fixed wireless aspects and the corresponding impacts on system performance. A network level simulation of both forward and reverse links including physical and MAC layer details of 1xEV-DO is used to quantify performance. In addition to average throughput per sector, we quantify the user performance on both links as a function of total attenuation. This provides insight on how performance varies within the coverage area – a very important aspect for fixed access. Moreover, we also show how different forward link data schedulers can be used to tradeoff system capacity for a more uniform performance regardless of the terminal distance to the serving base station. Index terms—cdma2000 1xEV, IS-856, Wireless Internet access, high-data rate cellular systems.

I.

INTRODUCTION

cdma2000 1xEV-DO, also known as IS-856, is a third generation solution to high-speed wireless Internet access [1][2]. The IS-856 air interface incorporates several advanced techniques such as adaptive modulation and coding, turbo codes and incremental redundancy, Hybrid-ARQ, fast channel feedback information and multiuser diversity just to name a few. These techniques, as detailed in [5], help improve forward link system capacity. On the reverse link, typical power control methods are used to minimize the amount of interference received at each base station. In addition, the reverse link employs a decentralized rate control algorithm used by the terminal to determine the data rate at which it may transmit [4]. For typical mobile environments, the throughput per sector was shown to be 550-900kbps and 180-220kbps for the forward and reverse links, respectively [3][4]. Another segment that is growing interest in the wireless industry is the fixed Internet access. These solutions addresses the broadband access demand of residential customers, SOHO and small/medium enterprises. In general, the computing platform includes desktop computers but also laptops and PDAs. Hence, we also

consider portable devices as part of fixed wireless access. Slowly varying fading and indoor propagation typically characterizes the wireless channel for fixed and portable applications. In this paper, we consider the impact of fixed wireless channels on the performance of 1xEV-DO. We show that, in this case, the nature of the wireless channel is more benign leading to improved throughput performance. Moreover, in a fixed access solution, one is concerned with how the user performance varies within the coverage area. We show how receive diversity and forward link data schedulers can be used to improve performance of users at the edge of the cell. II.

FIXED WIRELESS CHANNEL MODEL

Although the fixed or portable device is typically stationary when used, the fixed wireless channel is characterized by temporal and spatial fluctuations. These fluctuations account for scatter variations surrounding the device as well as slowly movements of the user. This results in a small scale fading process that is commonly represented as Ricean for indoor reception. In this paper, the Ricean K factor is assumed uniform over [0 11] dB. Moreover, we assume the fading process to be characterized by Doppler frequency uniformly distributed in (0 2.5] Hz. Multipath components may occur during indoor reception but, in this paper, we consider the most common case of a single path component Ricean fading channel. Indoor propagation results in building penetration losses that depend on several factors such as construction materials, floor height, distance from exterior walls, frequency, etc [6]. Building penetration loss varies widely from 5 to 20dB. In our studies, we assume an average penetration of 12dB. Poor indoor reception can be alleviated in practice by several techniques. Some of them include mounting an external antenna, use of RF repeaters and coupling the wide-area wireless broadband access with a wireless local area network solution. Further discussions on these techniques are beyond the scope of this paper.

Paper presented at VTC 2003, Fall, Orlando, FL LINK BUDGET AND NETWORK SIMULATIONS

In this paper, the cell radius is determined in order to satisfy the minimum link budget for 9.6 kbps transmission on the reverse link (the limiting link). One could impose a higher minimum transmit rate requirement at the edge of the cell, but we wanted it to be consistent with the assumptions commonly used in 2G and 3G mobile systems. In Table 1, we provide the basic 1xEV-DO link budget according to [7]. The typical link budget for mobile devices is 134.5 dB of maximum path loss assuming 5dB of reverse link load and 10dB of vehicle penetration loss. However, we observe that a fixed terminal is capable of achieving the same performance with building penetration loss of up to 18.5dB. The additional 8.5dB margin is a result of increased transmit power, reduced required Eb/Nt and lack of the body loss factor. Note that the maximum transmit power of 400mW can be easily achieved in fixed devices since no battery constraints exist. Also, up to 2.5dB improvement in the required Eb/Nt can be observed for slowly varying Ricean fading models. Table 1 Reverse Link Budget 1xEV-DO Reverse Link Budget

AT Tx Power (mW) AT Tx Power (dBm) AT Antenna Gain (dBi) Body Loss (dB) AT EIRP (dBm) BTS Rx Antenna Gain (dBi) BTS Cable Loss (dB) BTS Noise Figure BTS Thermal Noise (dBm/Hz) Data Rate (bps) Data Rate (dBHz) Required Eb/No per Antenna (dB) Load Margin (dB) BTS Receiver Sensitivity (dBm) Log Normal Stdev (dB) Log Normal Fade Margin (dB) Soft Handoff Gain (dB) Differential Fade Margin (dB) Building/Vehicle Penetration Loss (dB) Maximum Path Loss (dB)

Fixed Mobile Terminal Terminal 200 400 23.0 26.0 1.5 1.5 3.3 0.3 21.2 27.2 17.0 17.0 3.0 3.0 5.0 5.0 -169.0 -169.0 9600 9600 39.8 39.8 6.6 4.1 5.0 5.0 -117.6 -120.1 8.0 8.0 10.3 10.3 4.1 4.1 2.1 2.1 10.0 18.5 134.5 134.5

by the appropriate antenna gain factor due to non-peak azimuth and elevation angles. Moreover, a lognormal shadow factor of 10dB standard deviation is introduced to account for local diffractions surrounding the transmitter and/or receiver. The simulations include complete characterization of SINR, power control, forward link adaptation and reverse MAC layer controls as described in the IS-856 standard [1] and further discussed in [3,4,5]. The results presented in this paper refer to the center, or embedded, sector of the network. IV.

FORWARD LINK THROUGHPUT RESULTS

In this section we show 1xEV-DO forward link throughput results when fixed wireless channels are considered. In figure 1, the total sector throughput is presented as a function of the number of active users. By active, we mean transmitting data at enough rate to keep its data buffer full. In this plot we consider the so-called proportional fairness scheduler [5] as it attempts to take channel feedback information to maximize sector throughput subject to a fairness criteria for periods of time greater than 1.5 sec. In addition to the Ricean channel model described earlier, we also consider 1-path Rayleigh fading channel (ITU Pedestrian A model) for comparisons purposes. First, for 1 receive antenna devices, the Ricean throughput varies from 840kbps (1 user) to 1.24Mbps (16 users). This throughput improvement is referred to multiuser diversity gain. Sector Throughput vs Number of Users Pfair Scheduler 1700000 1600000 1500000

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Complete network level simulators for the 1xEV-DO forward and reverse links are used in this paper. The network consists of 37 sectorized cells, where each sector is represented as a hexagon with side equal to 2/3 of the cell radius. Each sector has a typical 17dBi, 65degree antenna with 4-degree downtilt. The users are randomly placed within the sector area. For each tx-rx path, the total RF attenuation is determined by the propagation loss using the ITU-R m.1225 model scaled

Figure 1 Forward Link Sector Throughput – Proportional Fairness Scheduler

Another important improvement shown in figure 1 is the receive diversity gain. Note that the Ricean throughputs improve to 1.24Mbps (1 user) and 1.57Mbps (16 users) once the second receive antenna is used. These show a remarkable 50% and 26% diversity gain, respectively.

Paper presented at VTC 2003, Fall, Orlando, FL 8 Active Users, Ricean A Model, 1 RX Antenna PFair Scheduler 1.0E+07

Avg. user Throughput (bps)

In figure 2, we show how the single user throughput varies within the embedded sector coverage area. That is, a scatter plot of the average throughput experienced by a single user in a sector as a function of the total attenuation is shown. The total attenuation includes path loss, shadowing and sidelobe effects of the base station antennas. We can see that a single user can obtain over 1Mbps download speeds with attenuations up to 130dB, corresponding to a major portion of the cell area.

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Figure 3 User Throughput vs. Total Attenuation – Prop. Fairness Scheduler with 8 Users

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Figure 2 User Throughput vs. Total Attenuation – Single User Case

In figures 3 and 4, we show how 8 simultaneously active users in the sector affect user throughput. The two plots correspond to different schedulers that are responsible for assigning time slots to users. Figure 3 shows the proportional fairness scheduler that attempts to optimize sector throughput (1.24Mbps) while maintaining each user’s performance proportional to their average signal strength. Figure 4 shows the GOS=1 scheduler that attempts to equalize performance across different users regardless of their location. The penalty in this case is a loss in sector throughput (700kbps). This may be an important tradeoff for fixed wireless implementations. Note that while the maximum average user throughput is around 200kbps for GOS=1 instead of 400kbps for prop. fairness, the probability of getting throughputs below 50kbps even at 135dB of attenuation is much lower for GOS=1 than for proportional fairness.

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Figure 4 User Throughput vs. Total Attenuation GOS=1 Scheduler with 8 Users

In figure 5, we show the sector throughput performance as a function of number of active users for 1 antenna devices. Note the GOS=1 scheduler performance degrades as the number of users increase beyond 2 for both Ped A and Ricean channels. This is a consequence of the fact that guaranteeing a uniform grade of service is increasingly more difficult as the number of users increase. So, non-optimized serving times have to be selected hence reducing the overall sector throughput. The Pedestrian A curve using proportional fairness scheduler presents the interesting characteristic of being lower than the Ricean curve for less than 6 active users while better beyond this load. To understand this, one has to recall that the Ricean fading process produces less significant envelope fluctuations than the Rayleigh process. The better behavior of the Ricean channel leads to improved performance for low number of users.

Paper presented at VTC 2003, Fall, Orlando, FL However, as the number of active users increase, the proportional fairness scheduler is capable of taking advantage of the larger fading fluctuations of the Pedestrian A model to select optimum serving times maximizing sector capacity. Hence severe fading, while considered a deleterious effect in single user systems, can be used to improve system capacity in a multiuser environment. Sector Throughput vs Number of Users 1 Receive Antenna 1400000

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Figure 5 Forward Link Sector Throughput – Prop. Fairness vs. GOS=1 Schedulers

In figure 6, the cdf of the average user throughput is shown for both schedulers and channel models. This is another way of showing the effects of running the GOS=1 scheduler as opposed to proportional fairness for 8 active users. We can clearly see the tails of the distribution being pulled together as a result of the GOS=1 scheduler. User Throughput Distribution 8 simultaneous FTP users per sector - Dual antennae 1 0.9 0.8 0.7

V.

REVERSE LINK TROUGHPUT RESULTS

In this section, we show reverse link throughput results under the assumption of the fixed wireless channel model described by a Ricean fading process. In table 2, the embedded sector throughput is shown as a function of the number of active users. For the usual base stations equipped with two spatially separated receive antennas, the throughput can reach 429kbps with 16 users. Beyond this point, the sector throughput is expected to be reduced given the additional overhead of Pilot and DRC channels transmitted by the active users. This capacity is obtained with rise-over-thermal of 5dB. Another interesting result is obtained when a 4-way receive diversity system is employed at each base station. Four-way diversity can be obtained by an array of antennas spatially separated or by using cross-polarized antenna panels instead of the regular vertically polarized panel. The result of the additional receive diversity at the base station is an increase in reverse link sector throughput in the order of 90% (16 user case). The additional capacity gain can be of significant interest in case of fixed wireless operations, where the reverse link may become loaded with a small number of active users. Consider the case of of 8 simultaneous users downloading large files (e.g. FTP). The average per user throughput is in the order of 53kbps and 80kbps for 2 and 4 rx antennas, respectively. Table 2 Reverse Link Sector Throughput (kbps) Number of Users Per Sector

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Figure 6 User Throughput CDF – 8 active users

In figures 7-9 the average user throughput on the reverse link is shown as a function of total attenuation similar to the results presented in Section IV. Figure 7 shows that single user data rates of 153.6kbps are achievable even with attenuations in the order of 130dB. Only beyond this point that the device power limit is achieved reducing the maximum transmit data rate. Figures 8 and 9 show average user throughputs between 80-100kbps and 40-80kbps for 4 and 8 active users, respectively.

Paper presented at VTC 2003, Fall, Orlando, FL Prop Loss vs. Throughput 160

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Figure 7 1user, Ricean, 5dB ROT, 2 RX antennas Figure 10 Ricean, 8 users, Rot=5dB, 4 RX antennas

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Figure 8 Ricean, 4 users, Rot=5dB, 2 RX antennas Throughput vs. Prop Loss 160

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CONCLUSIONS

In this paper we have shown the performance of 1xEVDO for fixed wireless channels. We have show that sector throughputs of 1.24Mbps and 430kbps can be obtained for the forward and reverse links, respectively. If receive diversity is used on the terminal (2 rx) and BTS (4 rx), the throughputs can reach 1.57Mbps and 814 kbps, respectively. We also show the distribution of the per user throughputs as a function of total propagation loss and the effects of forward link schedulers on its distribution. With the GOS=1 scheduler, sector capacity can be traded off for a more uniform user throughput distribution over the coverage area. REFERENCES

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Figure 9 Ricean, 8 users, Rot=5dB, 2 RX antennas

The 4-way receive diversity at the base station can improve the average user throughput within the coverage area as depicted in Figure 10. In this case, 8 simultaneous users, the average rate per user is better than 80kbps up to 130dB of total attenuation. Even at close 140dB, the average throughput is still 40kbps. The equivalent figures for the 2 antenna case in Figure 9 show an average 50kbps at 130dB and less than 20 kbps at the very edge of the sector.

[1] 3rd Generation Partership Project 2 (3GPP2) “cdma2000 High Rate Packet Data Air Interface Specification”, Technical Report C.S20024 v2.0, Oct 2000. [2] E. Esteves, “The High Data Rate Evolution of the cdma2000 Cellular System”, in Multiaccess, Mobility and Teletraffic for Wireless Communications: Volume 5, pp. 61-72, Kluwer Academic Publishers, December 2000 [3] P.J.Black and M.I.Gurelli, “Capacity Simulation of cdma2000 1xEV Wireless Internet Access System,” Proceedings of MWCN 2001, August 2001. [4] S.Chakravarty, R.Pankaj and E.Esteves, “An Algorithm for Reverse Traffic Channel Rate Control for cdma2000 High Rate Packet Data Systems,” Proceedings of Globecom 2001, Phoenix, November 2001. [5] E. Esteves, P.J.Black and M.I.Gurelli, “Link adaptation techniques for high-speed packet data in third generation cellular systems,” Proc. of European Wireless, Florence, February 2002. [6] T. Rappaport, Wireless Communications Principles and Practice, Prentice Hall, New Jersey, 1996. [7] P.J.Black and Q.Wu, “Link Budget of cdma2000 Wireless Internet Access System,” Proceedings of PIMRC 2002, September 2002.