BushLAN Distributed Wireless:

Transcription

1 Australian National University BushLAN Distributed Wireless: Spectrum efficient long range wireless extension of broadband infrastructure to remote areas July 24,

2 1 Abstract BushLAN is a distributed wireless network designed to provide long range wireless access toanarbitrarilylargenumber ofconnectedusersataconsistent onembpsperuser perone MHz of radio spectrum. The network can be deployed incrementally by adding one access point (AP) for each additional user without the need for an investment in cell towers. APs can be located in roof-mounted locations anywhere where there is a connection to high speed backhaul. 2 Introduction Traditional wireless networks are designed around cells. Typically each cell contains one base station that serves many users located geographically within the cell. Some of the problems of the cellular approach are, Net capacity is constrained by providing users with exclusive access to spectrum on a shared basis. Range is limited by design and large area coverage requires a multiple cell deployment. Deployments to sparsely populated areas are usually uneconomical. In BushLAN, APs connect users to the backhaul via a large scale multiple-input-multipleoutput (MIMO) wireless network. The number of users that can be served on the same spectrum andtherangeofthenetwork scales withthenumber ofapsdeployed andisonly limited by the availability of high speed backhaul. The main design goal of BushLAN is simplicity of implementation. Fig. 1 shows a typical deployment scenario. The APs (red) within the central area of a town are connected to a broadband fibre network and hence the Internet. The APs may be installed on homes if backhaul bandwidth is sufficient. 3 Performance Capacity: Fig. 2 shows capacity per user on a BushLAN network with 1000 APs compared to a cellular network as more users are added. We assume that each AP on the BushLAN network transmits the same power as the single base of the cellular network. BushLAN significantly outperforms the cellular network because it does not need to share spectrum amongst users. For a large network in which the number of APs and users are equal, the capacity in Mbps per user per MHz of bandwidth approaches 1. Range: Fig. 3 shows the ratio of the ranges of BushLAN to a cellular link for free-space (Friis) transmission, 1000 APs and various numbers of users as a function of the SNR 2 Copyright Gerard Borg (Australian National University)

3 Figure 1: A pictorial representation of a BushLAN network deployment in the Australian Capital Territory. (signal-to-noise-ratio) that would be seen at the receiver of the single link. Range is enhanced in BushLAN because more power becomes available for beamforming as APs are added to the network. How does BushLAN compare to LTE? In Fig. 4 we show the predicted LTE and BushLAN performance on 20 MHz of wireless spectrum versus the number of users on the network. The key feature of LTE is that it minimises self-interference. This allows it to exploit high order modulation and to achieve a capacity of 150Mbps for a single user. BushLAN achieves a consistent 20 Mbps independently of the number of users and therefore underperforms LTE when there is a small number of connected users. However for large numbers of users, the situation reverses. The LTE per-user capacity drops inversely as users are added to the network. With 100 connected users, LTE can only deliver 1.5Mbps per user whereas BushLAN can still deliver 20Mbps per user. This corresponds to a net throughput of 2000Mbps: much larger than 150Mbps for LTE. 3 Copyright Gerard Borg (Australian National University)

4 Figure 2: Capacity compared to a cellular network 4 What if we want more capacity per user than 1 Mbps per 1 MHz? Single user performance in BushLAN can be significantly enhanced by ganging more than one station per user and by sharing data over the parallel wireless links. A pictorial example of this is shown toward the east in Fig. 1 where four remote stations have been ganged in order to increase the capacity for a single user by a factor of 4. Ganging is more efficient than high order modulation because it scales linearly with the number of stations added. The bit rate of high order modulation on the other hand scales only logarithmically with SINR. 5 Case Study There are many applications of the BushLAN approach. Here we cite the most typical. BushLAN can be applied to the service of remote users located outside regional towns that are connected to the Internet. APs would be located within the town and connected to high speed backhaul. The APs would be deployed incrementally as needed with the nearest users added in the early phase of an expanding roll-out. Fig. 5 shows 1024 APs 4 Copyright Gerard Borg (Australian National University)

5 Figure 3: Ratio of BushAN to cellular range located in a town of square side 6kms: the square black spot 100kms east of centre. The bases operating on 150 MHz in the town have vertically polarised antennas and are uniformly distributed throughout the town. These serve 512 users located to the west of the town. Propagation is assumed to be Friis (line-of-sight) on a flat terrain and fully deterministic (fixed wireless access). The resultant raw bit error rate leads to packet losses below 1% for 1024 byte packets using Forward Error Correction. BushLAN performance improves with the ratio of the number of APs to user terminals. The similarity between the pair-wise roll-out model of BushLAN and that of ad-hoc WiFi infrastructure networks suggests that BushLAN could be an alternative to the WiFi baseddeployments used by wireless ISPs. Such ISM band-based Internet services have proven very popular for broadband delivery in regional areas. BushLAN is much better suited to this application due to its 1Mbps per 1MHz per user spectrum reuse and its range. As urban fibre roll-outs become more prevalent world-wide, BushLAN should become the network of choice for this application. 6 Competition There are three obvious competitors: satellite, cloud RAN and cellular Massive MIMO. 5 Copyright Gerard Borg (Australian National University)

6 6.1 Satellite Figure 4: BushLAN versus LTE capacity performance. Satellite is a form of cellular network in which the design of the infrastructure directly limits the user base. Consider a specific example: the NBNCo satellite service. The phase I specs state that Australians can satisfactorily share the interim satellite broadband service at 6 Mbps speed. However currently the interim satellite service is at capacity and NBNCo has had to introduce a fair use policy. A replacement long term satellite solution (LTSS) is planned for The LTSS for the last 7% of Australians involves two Ka band (20 30 GHz) satellites worth $620 million and each delivering 90 Gbps over 100 spot beams through 15 gateways. The satellites will provide broadband services to approximately 200, 000 premises. Services offered are up to 25 Mbps download, 5 Mbps upload for premises that sit outside of NBN Co s fibre and LTE fixed-wireless footprint. However satellite is not scalable: at 25 Mbps, 7200 customers (Australia wide) can receive this service implying a contention ratio of about 28 to 1. Assumptions of contention ratios of to 1 are universal in Internet deployments. Contention is not an issue in BushLAN. The problem of large latency resulting from the use of geosynchronous orbits 6 Copyright Gerard Borg (Australian National University)

7 Figure 5: BushLAN deployment with 1024 bases (black spots) serving 512 users (red). also remains an unavoidable obstacle for satellite. BushLAN is designed for long range propagation. Comparison of Figs. 5 and 6 reveals that much of the populous terrain in Australia covered by satellite could also be covered by BushLAN and operation at VHF. 6.2 Cloud RAN (Artemis pcell) We consider Artemis pcell 1 (previously known as DIDO) as an example of cloud RAN or C-RAN. An array of base stations known as pwave radios create a personal cell that tracks mobiles. The physical phenomenon that produces the personal cell has not been revealed. In cloud RAN, all network signal processing is performed in the cloud. For this to be possible, two things must happen. Firstly, the landline pipe into the cloud must have enough bandwidth to transport the complex baseband samples of the entire wireless network. Secondly, joint wireless signal processing requires enormous computing power in the data centre. These are formidable challenges to overcome if the data centre is serving hundreds of base stations Copyright Gerard Borg (Australian National University)

8 Figure 6: Spot beams of the long term NBN satellite service. In BushLAN, these problems are overcome by distributed backhaul transport of wireless samples and distributed wireless signal processing. Latency considerations in cloud RAN also dictate that the data centre be located near to the base stations. The data centre adds significantly to cost and appears to preclude a do-it-yourself deployment model. 6.3 Cellular Massive MIMO Cellularmassive MIMOisahotresearchtopic. Massive MIMOisanetworkinwhichthere are many more bases than remote users. In some schemes this can allow backhaul traffic to be minimised. There are no plans to make such systems distributed and decentralised so cell tower deployments are the only option. In this case massive MIMO is really a form of multiuser-mimo. All the problems listed above for cellular networks apply equally here. 8 Copyright Gerard Borg (Australian National University)

9 7 Conclusion The BushLAN project aims to develop distributed wireless technologies to provide long range terrestrial wireless Internet to large populations of regional and remote users. BushLAN has scalable sum capacity and range and obviates the need for cellular deployments and cell towers. BushLAN s major breakthroughs are, 1. A solution to the wireless spectrum crunch. 2. A significant improvement in range and coverage compared to existing terrestrial wireless approaches. 3. The possibility of incremental deployment by users without deleterious interference to others. 9 Copyright Gerard Borg (Australian National University)