Introduction

This chapter aims to exploit the key finding of Chapter 3 and apply it to the TDD air interface of UMTS (UTRA-TDD). The significant finding of the previous chapter is that ideal synchronisation is not necessarily a prerequisite to obtaining the maximum capacity in a TD-CDMA/TDD network. This has led the author to develop a novel technique which is called the time slot (TS)-opposing method. In this chapter this method is used to develop a centralised dynamic channel assignment (DCA) algorithm.

The approach in this chapter is as follows: first, in section 4.2 a simple centralised DCA algorithm used in a single cell is studied. This investigation aims to find an upper bound of the network performance when combining the TS-opposing technique with a DCA algorithm. Second, in section 4.3 the TS-opposing algorithm is investigated in a cellular TD-CDMA/TDD network. For this approach it is assumed that a group of BSs (following the so called bunch concept (Mihailescu et al., 1999)) is connected to a radio network controller (RNC).

TS-opposing technique applied to a single cell

In this investigation an idealised deployment scenario is assumed to investigate the new TS-opposing mechanism. This means that a TS-opposing algorithm is employed with the aim of improving the capacity only with respect to a single cell. The capacity obtained thereby is then compared with the capacity of an equivalent FDD interface.

A cluster of seven hexagonal cells is assumed with the cell of interest (COI) in the centre.

Until now, the focus of the book has been on interference analysis and management for either a cellular TDD-CDMA system or an ODMA enhanced TDD-CDMA system. Interference management can be viewed as a form of resource management. In this chapter the issue of link adaptation is addressed with a focus on what metrics are appropriate to enable radio resource management in a cross-layer manner. In conventional systems, the decision on how to choose the ideal physical mode (PHY-mode) is primarily based on the knowledge of the interference encountered. This information is reported to higher-layer entities that deal with the radio resource management. Subsequently, the radio resource management entity makes a decision as to which physical resource will be used (e.g. by employing a DCA algorithms as described in Chapter 5 and 8) and this information is reported back to the physical layer. It is apparent that this process can take quite a long time. Meanwhile the channel conditions may have changed significantly. Hence, the previously chosen radio resource may no longer be the ideal choice. Therefore, new methods (e.g. resource metric estimation) are discussed here that (a) base the decision as to which radio resource to use, not only on interference, but also, for example, on the statistics of the channel state information, and (b) make the decision as to which radio resource to use already at the physical layer. For (a) it is shown that the TDD mode is ideally suited due to the reciprocity of the channel.

DCA techniques

Congestion-based routing, as developed in the previous chapter, is shown to require the lowest transmitted power, and in most cases achieves the highest capacity of all the routing algorithms examined in Chapter 5. All of these routing algorithms have allocated TDD time slots on a first-come-first-served basis and according to the rules outlined in section 6.4.1. This allocation only serves to ensure that the limitations of the TDD hardware are considered. It makes no attempt to optimise time slot allocation.

The allocation of time slots with regard to system performance has been shown to be an effective technique to mitigate interference (Haas, 2000). Integrating slot allocation, or DCA, into the routing algorithm would appear to be the most effective approach due to the interactive nature of interference. This approach will also need to conform to the extra limitations imposed by relaying. In addition to the rules in section 6.4.1, it is obvious that the slot allocation must be in the same order as the relays. A combined DCA will allow minimisation of the desired measure, in this case congestion, simultaneously in routing and slot allocation. This chapter develops a combined routing and resource allocation algorithm for TDD-CDMA relaying. It starts by reviewing one such algorithm applicable to TDMA and FDMA. A novel method of time slot allocation according to relaying requirements is then developed. Two measures of assessing congestion are presented based on matrix norms.

In this and the following two chapters the focus moves away from networks which are controlled centrally by a base station to a hybrid cellular network which permits cellular operation as well as peer-to-peer operation. Essentially we consider multi-hop wireless networks based on opportunity-driven multiple access (ODMA) which will be shown to reduce the overall transmission power in a system, to be resilient to shadowing and to potentially increase the coverage compared with single-hop transmission. However, for simple receivers and low user density, the actual capacity of UTRA-TDD may be marginally reduced from the maximum non-relaying capacity. This chapter begins the study of ODMA based systems by analysing the implications of relaying in a cellular scenario compared to a conventional nonrelaying system. Initially the interference is analysed by investigating the effect of reduced transmitted power resulting from reduced path loss for a link. The effect of shadowing is considered and it is shown that a relaying system is able to benefit from increased zero mean lognormal shadowing by utilising the diversity of paths available. A correlated shadowing model is developed from a previous model considering both distance and angle of arrival (Klingenbrunn and Mogensen, 1999) to include the shadowing correlation between all transceivers, as they may all be available to receive in a relaying environment. It is shown that while this affects the interference pattern the perturbation is not significant.

Introduction

In this chapter a distributed dynamic channel assignment (DCA) algorithm applicable for the TDD mode of the UMTS terrestrial radio access (UTRA) is presented. It is closely related to the DCA used in the DECT (digitally enhanced cordless telecommunications) system (Punt et al., 1998). Once again, the discovery made in Chapter 3 is exploited; that is, that for certain scenarios opposed synchronisation of TSs between neighbouring cells is advantageous. The new distributed DCA algorithm is supported by the results of the investigation in section 4.2. In this section it was demonstrated that synchronous transmission and reception between neighbouring cells may not yield the greatest capacity that is attainable in a single cell. It was found, however, that when the centralised DCA algorithm developed in section 4.3 was applied to multiple cells it was not feasible to fully exploit the potential gains revealed by the capacity analysis of a single cell.

In this chapter it is demonstrated that by applying the novel distributed DCA algorithm, which utilises the TS-opposing idea, greater capacity can result than would be obtained by synchronous transmissions. Most importantly, this is shown to be valid for a TDMA-CDMA/TDD (TD-CDMA/TDD) network, which accounts for full spatial coverage. Channel asymmetry is assumed to be arranged by code pooling rather than TS pooling (3GPP, TSG, RAN, 2000c).

With 3G having seen lack luster performance in mobilizing the Internet, the media and communications industries are migrating to more advanced technologies with the intention of improving the underlying performance and economics for delivering broadband services. Advanced technologies such as 3G-LTE, UMB, mobile WiMAX, are being touted as the migratory technologies towards 4G, with the support of technologies like 802.11n, and WiFi-mesh. As these technologies come to market it remains to be seen how well they perform relative to service demands. The open mobile Internet environment created with 4G will challenge the traditional service provider business models and dramatically lower the barriers for new entrants to penetrate the market with a variety of innovative solutions. It is competition derived from these innovative solutions that will drive the 4G market.

Introduction

As the media and communications industries grapple with 3G, and the mobilization of the Internet, 4G is emerging on the horizon with the promise of enabling the economic delivery of high bandwidth services to a plethora of cellphone, consumer electronic and computing devices. Traditionally 4G has been considered the realm of the mobile service providers, with the notion that they would migrate their legacy 3G networks to 4G once the technology was available and market demand evident. However advanced technologies like WiFi-mesh, 802.11n and WiMAX provide opportunities for fixed and broadband service providers and new entrants to offer portable and mobile Internet solutions that challenge the traditional position of mobile operators.

Introduction

There are a variety of unlicensed wireless networks that operate in shared frequency bands. Many of these networks are installed in portable and hand-held devices. Given that these devices are portable it is not uncommon for devices using one unlicensed network to be in close proximity to another device using another unlicensed wireless network. These unlicensed wireless networks may operate in the same frequency band and may even operate in the same channel within the band. This raises the issue of whether these two networks can coexist in the same location. This chapter describes this coexistence issue.

Section 20.2 gives an overview of the various unlicensed frequency bands in the United States. Many of these bands are also unlicensed bands in other countries. Section 20.3 is a short survey of some of the unlicensed wireless networks that share these frequency bands. Section 20.4 gives a short history of coexistence of unlicensed wireless networks. Section 20.5 illustrates how to evaluate coexistence of two unlicensed wireless networks. Section 20.6 describes several methods of improving coexistence. Sections 20.7 and 20.8 give example of evaluating coexistence of wireless networks, using two IEEE 802 standards as case studies. And finally Section 20.9 discusses the issue of dynamic spectrum access in which unlicensed wireless networks identify and use unused portions of licensed frequency bands.

Overview of Unlicensed Frequency Bands

Unlicensed wireless networks can operate in a variety of frequency bands.

Introduction

If you are reading this book, I probably do not have to convince you that wireless LANs (WLANs) are experiencing dramatic growth and are rapidly becoming an entrenched technology in every day life. Today, we see that WLANs have become a competitive differentiator for hotels, multi-tenant dwellings, coffee shops and other establishments where customers are expected and encouraged to stay for more than a few quick moments. Conversely, it is quickly becoming a negative distinction to not have WLAN services in those circumstances.

Like the personal computer revolution before it, as WLAN products permeate applications from the least significant to the mission critical, product lines are stratifying to serve those new and different market segments. However, with this increased popularity and diversity, comes increased deployment at the hands of those that do not understand, and should not need to, the intricacies of RF propagation and network management, often resulting in poor performance and disillusionment with the technology.

To date, two architectures have emerged as the leading approaches to WLAN implementation: independent and dependent. The independent architecture was first to arrive and is a logical extension of standard bridging practices followed in the wired LAN (Ethernet) world. With this approach, access points (APs) are treated as individually managed edge devices. Each AP is configured and managed independently from other peer APs irrespective of whether or not the APs belong to a specific administrative domain.

Introduction

A Wi-Fi “Hot Spot” is the common term used to describe locations that provide public Internet access using IEEE 802.11a,b or g wireless Ethernet, otherwise known as “Wi-Fi” (Wireless Fidelity). Wi-Fi uses a wireless Ethernet protocol that comes in various flavors including the most popular 802.11b/g, both of which operate in the 2.4 GHz frequency band of unlicensed spectrum. 802.11b can transmit/receive up to 11 megabits/second and 802.11g can transmit/receive up to 54 megabits/second. The new proposed 802.11n standard uses multi-path capabilities to transmit/receive at up to 155 megabits/second. 802.11a is a close cousin to 802.11g, with transmit/receive speeds of up to 54 megabits/second, but 802.11a runs in the 5 GHz unlicensed band. Most hotspots provide 802.11b and 802.11b/g (dual-mode capable) access – that is, the access points have radios that are able to function with either 802.11b or 802.11g connections. Some access points have tri-mode radios that run in 802.11a/b/g modes.

Wi-Fi has become extremely popular. Whereas 8 years ago, 802.11b was on the fringe edge of techno-geek toys with only a few thousands of devices sold, now more than 90% of the mobile computers shipped by Dell, HP and IBM (Lenovo) come with 802.11b/g built in (many also with 802.11a forming tri-mode radios). Hence, there are hundreds of millions of laptop computers with Wi-Fi built in. Those that do not have it built in can add it on with a PCMCIA card or USB drive device for less than $30.00.

In this chapter we present the requirements, architecture and design challenges for municipal wireless broadband access networks. We first provide an overview of the requirements and services anticipated for municipal Wi-Fi networks based on five recent municipal RFPs. Next, we present design guidelines for a layered network using access, mesh, injection and backhaul tiers to create a cost-effective municipal broadband network utilizing equipment conforming to the 802.11 family of standards in the mesh and access tiers. To illustrate the challenges involved in creating mesh networks we explore the costs and benefits of building single versus multi-radio mesh networks and of creating a network capable of supporting session-persistence roaming across a municipality. We provide design equations supported by measured data for multi-hop mesh networks using a single mesh radio configuration. Our roaming solution is designed to operate in the context of a routed mesh network and offers seamless integration with network access controllers, and in conjunction with dedicated network branch routers, session-persistent roaming within metro-scale Wi-Fi networks.

Introduction

Envisioned as a replacement for traditional Ethernet Local Area Networks in home and office environments, 802.11-based broadband access networks have seen an explosive growth in the public space over the last five years. Beginning with small scale hotspot deployments in high use public areas such as airports, cafes and businesses, cities have begun to embrace Wi-Fi technology as a catalyst for attracting customers and revitalizing downtown areas.

With all the ongoing commotion about Wi-Fi access gravitating towards free since the inception of the hotspot industry, it might come as a surprise that most networks in high profile locations remain for-fee, commercial hotspots. Among the top airports, fewer than 10% have chosen to deploy free Wi-Fi. Even within the airline club rooms, most of the clubs charge for service – or outsource the service to an operator who charges for service.

Similarly, outside of the business economy hotels (think about the second and third tier offerings from the prominent hotel brands), Wi-Fi access in major hotels remains feebased, with some frequent traveler programs subsidizing this service for their members. Some hotels are returning to fee-based access after enduring what they consider a failed experiment in providing free access.

While there will always be a place in the market for free Wi-Fi, the explosive growth and exponentially increasing usage of fee-based Wi-Fi demonstrates the viability of a market that was projected to be on its last legs in 2003.

In order to capitalize on this opportunity, it is best to assess the factors that will maximize the chances of success, which generally means generating meaningful revenue. Fee-based Wi-Fi depends heavily on business travelers – users with a real need for Internet access – who are willing to pay for service.

In this regard, the first rule of real estate applies to commercial hotspots as well, “Location, Location, Location.”