Wireless and RF design - octoScope Tech Trends

FMC, now a definite industry trend is not just a solution for cellular coverage indoors, but a market struggle between mobile and fixed operators. The overriding driver for most of us is the desire to use a single phone indoors and out. As operators are working to expand cellular coverage indoors, and thereby taking business away from fixed operators, they are also studying two enabling technologies – Wi-Fi cell phones and Femtocells to leverage their existing infrastructure.

What drives FMC

FMC is being promoted by the fixed carriers who are rapidly loosing market share to mobile operators as more and more users elect to have only a cell phone and no fixed phone line at home.

Figure 1: US wireline subscribers are declining. Mobile subscribers continue to grow.

The fixed operators such as British Telecom are setting up Mobile Virtual Network Operator (MVNO) partnership with mobile operators like Vodafone and are deploying FMC services to the home and enterprise. The fixed operators can attract calling minutes to the WiFi media at home while the user enjoys the cost savings of WiFi calls plus the convenience of a single phone for calls made inside and outdoors.

The mobile operators are drawn reluctantly into the FMC game because they don’t want to miss out on the emerging MVNO business opportunities.

Cable TV operators and broadband service providers, who have been competing in the fixed telephone business for years, are also backing FMC to attract local minutes of voice services to their broadband networks.

The WiFi market is maturing and getting ready to support FMC. Here is some data from In-stat:

VoIP is another significant driver for FMC with 102 million WiFi cellular handsets expected to be in operation in 2009, growing to 206 million combo handsets by 2010 (Source: In-Stat, 3/06).

Figure 4: Worldwide WiFi cellular Handset Forecast: $17 Billion VoIP market in 2009 without FMC

Those involved with FMC deployment include major manufacturers such as Motorola and Nokia. Recently BT has announced a significant FMC deployment into the SOHO and Enterprise environments. Alcatel is their integration partner and Vodafone is their MVNO partner.

WiFi to cellular handoff

To enable FMC, the industry needs a protocol for WiFi cell phones to switch between WiFi and cellular infrastructure automatically. This process is called handoff. The handoff protocol involves all the layers of the complex cellular infrastructure including call setup and billing.

FMC handoff could work between WiFi and a number of different cellular interfaces. First on the market is GSM-based GAN/UMA WiFi-cell service, but CDMA2000 SIP-based handoff scheme may be available soon with the backing of Verizon and other major carriers.

Next Generation WLAN - 802.11n

The emerging IEEE 802.11n WLAN (Wireless LAN) transmission technology based on MIMO (Multiple Inputs/Multiple Outputs) guarantees throughput of at least 100 Mbps but can deliver up to 600 Mbps depending on the complexity of the radio and on the environment. Although today’s 802.11 networks reach data rates of 54Mbps, the actual throughput is typically less than 30 Mbps due to protocol inefficiencies. 802.11n is expected to deliver true 100Mbps throughput as measured at the MAC SAP (service access point) interface and will, therefore, more than triple the transmission speed of WLANs.

MIMO is an innovative advancement in wireless data transmission. It turns the long-time nemesis of wireless – multipath – into a friend. Multipath is a common occurrence indoors where the wireless signal reflects from surfaces thus creating multiple signals that add together in the air. While today’s 802.11a,b,g radios struggle with multipath, MIMO radios actually take advantage of multiple paths to send multiple data streams and thereby increase the rate of transmission.

Figure 1: Multipath creates multiple versions of the signal by virtue of reflections from walls, floors, ceilings, furniture and people. The reflections add together in the air presenting a challenge to the receiver of separating out the original signal. Until now multipath was a problem that limited operating range. Now MIMO radios actually use multipath to achieve gains in operating range.

Pre-standard 802.11 chipsets from Atheros, Broadcom, Marvell, Intel and Airgo (now Qualcomm) feature up to 3 transmitters and 3 receivers, but the standard specifies up to a 4x4 configuration. An NxM MIMO system has N transmitters and M receivers.

Figure 2: An example of a 2x3 MIMO system with 2 transmitters and 3 receivers.

Key elements of 802.11n specification

The IEEE 802.11n standard incorporates two MIMO techniques – Spatial Multiplexing and Beamforming. Spatial Multiplexing splits up a data streams into multiple streams of lower data rate and sends these streams simultaneously via multiple paths in a multi-path channel. These multiple unique streams are re-combined in the receiver to form the original stream of higher data rate. Beamforming optimizes the spatial properties of the beam to improve reception.

802.11n devices can operate in 3 modes: Legacy (802.11a,b,g), Mixed mode (802.11n and 802.11a,b,g) or Green Field (802.11n only). The highest throughput is achieved in Green Field mode when only 802.11n devices are present on the network. The mode of operation impacts network throughput. A single legacy station can slow down the entire 802.11n network.

Figure 3: The PLCP (PHY Layer Convergence Protocol) Frame Formats include L-STF: Legacy Short Training Field; L-LTF: Legacy Long Training Field; L-SIG: Legacy Signal Field; HT-SIG: High Throughput Signal Field; HT-STF: High Throughput Short Training Field; HT-LTF: High Throughput Long Training Field; HT-LTF's: Additional High Throughput Long Training Fields; Data – The data field includes the PSDU (PHY Sub-layer Data Unit)

Green Field 802.11n networks are composed entirely of MIMO devices. 802.11n can support bandwidth-hungry multimedia applications and may be capable of transmitting several HDTV streams simultaneously to display on multiple TV sets in the home.

For handhelds, 802.11n has the potential of improving battery life by minimizing the time required to send and receive data packets and through the use of improved power saving techniques.

802.11n networks use existing unlicensed bands at 2.4GHz and 5GHz matching the frequency plan of legacy networks. While the legacy 802.11 networks use 20 and 25MHz channels, 802.11n networks can use 20 or 40MHz channels.

Modulation Coding Schemes (MSC)

While legacy 802.11a,b,g networks use single-stream DSSS or OFDM modulation across the data rates, 802.11n MIMO networks introduce a concept of Modulation Coding Scheme (MSC) that incorporate 8 variables to implement rate adaptation.

For 802.11n MIMO, each data rate may employ a different modulation scheme defined by the MSC. Each MCS is determined by the following parameters:

To make matters even more interesting, multiple MCSs may have the same PHY rate.

Radios establishing a link must automatically negotiate the optimum MSC based on channel conditions and will dynamically adjust the selection of MSC based on motion of devices, fading of other changes in the channel.

There are 77 MCSs specified in the current draft (v2.0, January 2007) with 8 of them being mandatory.

With so many MSCs to choose from, the complexity of rate adaptation decisions becomes considerably higher than in legacy networks. This level of complexity is likely to make interoperability between devices from multiple vendors challenging.

MIMO channel models

Since multipath environment is inherent to making MIMO work, the throughput performance of MIMO networks and selection of MCSs are highly dependent on the physical space.

Figure 6: Radio signals reflect from walls, furniture and other conductive surfaces, which causes the receiver to ‘see’ multiple clusters of the same signal arriving at different times and with different amplitudes. In this figure we see 3 or 4 major clusters.

Multipath reflections come in “clusters”. Each cluster is caused by a specific group of reflectors. Reflections in a cluster arrive at a receiver from the same general direction.

Following an extensive analysis of cluster statistics, the IEEE 802.11n group defined 6 channel models, A through F (“TGn Channel Models,” V. Erceg et al, IEEE 802.11 document 11-03/0940r4). Model A is a test mode. Model B represents a typical small office environment. Model F represents large metropolitan spaces.

Figure 7: Key parameters in the IEEE 802.11n models A-F. Delay spread and number of clusters increase as the modeled physical space gets bigger. The number of taps also increases as a function of physical size to provide sufficient resolution of the emulation.

Delay spread and the number of clusters vary based on the size of the modeled environment. The number of clusters represents number of independent propagation paths modeled.

IEEE models A-F are defined in the form of tapped delay lines or FIR (Finite Impulse Response) filters. These models assume linear antenna arrays for transmitters and receivers with ½, 1 and 4 wavelength element spacing.

Figure 8: Time-varying FIR filter weights are spatially correlated: H11 correlated with H12, etc., according to antenna spacing and cluster statistics. The coefficients are also time correlated according to the Doppler model.

Since each model defines some particular representative environment (e.g. a typical floor of an office building) the multiple signal paths are correlated based on the model of the physical space.

The models include Doppler shifts, which are amplitude fluctuation of signals at the receiver. The fluctuations are caused by moving objects that reflect RF propagation – people, cars, etc. The Doppler shifts are modeled assuming reflectors are moving at 1.2 km/h, which corresponds to about 6 Hz in the 5 GHz band and 3 Hz in 2.4 GHz band.

Measuring MIMO range performance

When measuring MIMO range performance and comparing performance of different products, the preferred method is through a channel emulator.

Channel emulators used in 802.11n testing should be capable of modeling the IEEE 802.11n models of representative physical settings. The IEEE channel models are an objective means of comparing the range performance of competing MIMO products.

Figure 9: MIMO channel emulator block diagram. In a 4x4 emulator, 16 paths (n^2) are modeled with the coupling from each transmitter to each receiver. The 16 paths must be modeled bidirectionally so that the channel effects are applied to signals injected from either port.

Channel emulation must be bidirectional since the Beamforming method requires channel sounding. The signal traveling from one port of the emulator to the other must go through the same channel as the signal going in the return direction. The beamforming transmitter may derive channel information from the ACK frames sent by the receiver and use this information to select the optimum MCS. Bidirectionality is required for any of the channel sounding methods being considered by the IEEE.

The defined MSCs support up to 4 MIMO streams. Therefore, the channel emulator should offer a 4x4 configuration. A typical channel emulator down-converts the inbound RF signal to a lower IF frequency. It then digitizes the signal and implements the IEEE models using Digital Signal Processing (DSP). The IF signal processed by the DSP is up-converted and presented to the station at the opposite port.

Each MIMO receiver in the radio has to train on one of the transmit signals. In spatial multiplexing each transmit stream must be received since the transmit streams carry different data to be combined into a single stream at the receiver. In the case of Beamforming, the streams carry the same data, so Maximum Ratio Combining (MRC) of multiple streams can be used by the MIMO receivers.

See our recent Wi-Fi Alliance white paper, EE Times article and Test & Measurement World article on 802.11n.

Wireless Mesh Networks

Speed of deployment and flexibility favorably position wireless mesh technology for important applications including mobile networks contending with 3G/4G, military airborne networks and home networks. Requiring only power, wireless mesh nodes can be mounted on street lights to form city- or campus-wide networks expediently and cost-effectively without running a cable to every node. In the home, small wireless mesh nodes can plug into readily available electrical outlets to interconnect data, voice and video devices. Mesh nodes automatically configure themselves into coherent wireless networks using sophisticated routing algorithms to deliver data, voice and video traffic.

According to a recent Wall Street Journal article (“Companies That Fought Cities On Wi-Fi, Now Rush to Join In” by Amol Sharma, Wall Street Journal March 20, 2006), more than 50 municipalities around the country have already installed Wi-Fi metropolitan networks and many more are in the deployment process, including Philadelphia, Chicago, San Francisco and Houston. The Wireless Silicon Valley Wi-Fi mesh network announced in September of 2006 will spread across 1,500 square miles and cover 42 municipalities including Fremont, Newark, Santa Cruz, San Mateo and Santa Clara. By 2010, ABI Research forecasts a $1.2 billion market for municipal Wi-Fi networks.

Figure 1: Clusters of mesh nodes configure themselves into powerful networking infrastructure without requiring a cabled connection for every node.

While today most mesh networks are based on Wi-Fi, WiMAX is expected to play a role in the near future. Although traditional WiMAX architecture calls for a cabled backhaul connection to each base station, innovative companies are looking to adapt WiMAX base stations for operation as wireless mesh nodes.

Wireless mesh standards

The IEEE 802 committee is developing wireless mesh standards for Wi-Fi based Wireless Local Area Networks (WLANs), WiMAX based Wireless Metropolitan Area Networks (WMANs) and Wireless Personal Area Networks (WPANs). The Wi-Fi mesh protocol is governed by the emerging IEEE 802.11s specification that got underway in May 2004 and is expected to be ratified in June 2008. The WiMAX mesh related draft, 802.16j, does not define a true ad hoc mesh as does 802.11s, but rather a simpler wireless relay scheme for extending the range of a base station. The IEEE 802.15 group working on WPAN specifications is also looking at standardizing wireless mesh architecture optimized for consumer devices in the home. All three IEEE groups - 802.11s, 802.16j and 802.15 - have presented tutorials on their technologies and are in discussions to harmonize on the emerging mesh architectures.

Wireless mesh architecture

Network designers strive to maximize the number of mesh nodes per wired backhaul connection since backhaul links add cost to mesh installations. Minimizing the number of backhaul links can increase the number of hops in the mesh network. Each mesh node represents a hop as it forwards the client traffic through to its neighboring nodes.

QoS for services such as voice and video is highly dependent on throughput, packet loss, delay and jitter, all of which degrade per hop in a mesh network.

Performance tends to be better for mesh networks employing multi-radio nodes that use different channels to communicate with neighboring nodes and with local clients. Early Wi-Fi mesh implementations had to share one radio for client and backhaul traffic. Multiple radios enable the mesh node to transport local client traffic and backhaul traffic simultaneously on different channels.

Figure 2: Mesh nodes can have one or more radios and in some products the number of radios is configurable. One of the radios is typically configured to communicate with local clients and the other radios are dedicated to routing traffic on the wireless backhaul.

In a multi-radio mesh architecture, mesh nodes discover each other and determine the optimum frequency scheme for communicating with their neighboring nodes and with local clients. Mesh nodes must be constantly aware of the network conditions and promptly respond to any changes in the environment and to fault conditions by rerouting traffic or by reconfiguring the channel frequency scheme.

These self-configuring and self-healing capabilities are key to network robustness and must be tested in a variety of topologies and physical layer conditions prior to deployment.

Today's wireless mesh implementations are proprietary requiring the use of mesh nodes from the same vendor throughout the network. As the emerging IEEE mesh standards mature mesh nodes from multiple vendors are expected to interoperate in the same network.

See our latest mesh seminar and RF Design cover story on mesh network performance requirements and test.

Fixed Mobile Convergence (FMC)