DSP TECHNIQUES AND APPLIACTIONS

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wimax vs 3g vs wifi

How does WiMAX compare with the existing and emerging capabilities of 3G and Wi-Fi? The

throughput capabilities of WiMAX depend on the channel bandwidth used. Unlike 3G systems,

which have a fixed channel bandwidth, WiMAX defines a selectable channel bandwidth from

1.25MHz to 20MHz, which allows for a very flexible deployment. When deployed using the

more likely 10MHz TDD (time division duplexing) channel, assuming a 3:1 downlink-to-uplink

split and 2 × 2 MIMO, WiMAX offers 46Mbps peak downlink throughput and 7Mbps uplink.

The reliance of Wi-Fi and WiMAX on OFDM modulation, as opposed to CDMA as in 3G,

allows them to support very high peak rates. The need for spreading makes very high data rates

more difficult in CDMA systems.

More important than peak data rate offered over an individual link is the average throughput

and overall system capacity when deployed in a multicellular environment. From a capacity

standpoint, the more pertinent measure of system performance is spectral efficiency. In Chapter

12, we provide a detailed analysis of WiMAX system capacity and show that WiMAX can

achieve spectral efficiencies higher than what is typically achieved in 3G systems. The fact that

WiMAX specifications accommodated multiple antennas right from the start gives it a boost in

spectral efficiency. In 3G systems, on the other hand, multiple-antenna support is being added in

the form of revisions. Further, the OFDM physical layer used by WiMAX is more amenable to

MIMO implementations than are CDMA systems from the standpoint of the required complexity

for comparable gain. OFDM also makes it easier to exploit frequency diversity and multiuser

diversity to improve capacity. Therefore, when compared to 3G, WiMAX offers higher peak

data rates, greater flexibility, and higher average throughput and system capacity.

Another advantage of WiMAX is its ability to efficiently support more symmetric links—

useful for fixed applications, such as T1 replacement—and support for flexible and dynamic

adjustment of the downlink-to-uplink data rate ratios. Typically, 3G systems have a fixed asymmetric data rate ratio between downlink and uplink.

What about in terms of supporting advanced IP applications, such as voice, video, and multimedia?

How do the technologies compare in terms of prioritizing traffic and controlling quality?

The WiMAX media access control layer is built from the ground up to support a variety of

traffic mixes, including real-time and non-real-time constant bit rate and variable bit rate traffic,prioritized data, and best-effort data. Such 3G solutions as HSDPA and 1x EV-DO were also designed for a variety of QoS levels.

Perhaps the most important advantage for WiMAX may be the potential for lower cost

owing to its lightweight IP architecture. Using an IP architecture simplifies the core network—

3G has a complex and separate core network for voice and data—and reduces the capital and

operating expenses. IP also puts WiMAX on a performance/price curve that is more in line with

general-purpose processors (Moore’s Law), thereby providing greater capital and operational

efficiencies. IP also allows for easier integration with third-party application developers and

makes convergence with other networks and applications easier.

In terms of supporting roaming and high-speed vehicular mobility, WiMAX capabilities are

somewhat unproven when compared to those of 3G. In 3G, mobility was an integral part of the

design; WiMAX was designed as a fixed system, with mobility capabilities developed as an addon

feature.

In summary, WiMAX occupies a somewhat middle ground between Wi-Fi and 3G technologies

when compared in the key dimensions of data rate, coverage, QoS, mobility, and price.

Communication System Building Blocks

All wireless digital communication systems must possess a few key building blocks. Even in a reasonably complicated wireless network, the entire system can be broken

down into a collection of links, each consisting of a transmitter, a channel, and a receiver.

The transmitter receives packets of bits from a higher protocol layer and sends those bits as

electromagnetic waves toward the receiver. The key steps in the digital domain are encoding and

modulation. The encoder generally adds redundancy that will allow error correction at the

receiver. The modulator prepares the digital signal for the wireless channel and may comprise a

number of operations. The modulated digital signal is converted into a representative analog

waveform by a digital-to-analog convertor (DAC) and then upconverted to one of the desired

WiMAX radio frequency (RF) bands. This RF signal is then radiated as electromagnetic waves

by a suitable antenna.

The receiver performs essentially the reverse of these operations. After downconverting the

received RF signal and filtering out signals at other frequencies, the resulting baseband signal is

converted to a digital signal by an analog-to-digital convertor (ADC). This digital signal can

then be demodulated and decoded with energy and space-efficient integrated circuits to, ideally,

reproduce the original bit stream.

Naturally, the devil is in the details. As we will see, the designer of a digital communication

system has an endless number of choices. It is important to note that the IEEE 802.16 standard

and WiMAX focus almost exclusively on the digital aspects of wireless communication, in particular

at the transmitter side. The receiver implementation is unspecified; each equipment manufacturer

is welcome to develop efficient proprietary receiver algorithms. Aside from agreeing

on a carrier frequency and transmit spectrum mask, few requirements are placed on the RF units.

The standard is interested primarily in the digital transmitter because the receiver must understand what the transmitter did in order to make sense of the received signal—but not vice versa.

Simulating OFDM systems

The popular LabVIEW simulation package from National Instruments can be used to develop

virtual instruments (VI’s) that implement OFDM in a graphical user interface. See also [1].

Other communication system building blocks and multicarrier modulation tools are available in

the National Instruments Modulation Toolkit.

OFDM functions can also be developed in Matlab.6 Here, we provide Matlab code for a

baseband OFDM transmitter and receiver for QPSK symbols. These functions can be modified

to transmit and receive M-QAM symbols or passband—complex baseband—signals.

function x=OFDMTx(N, bits, num, nu)

%==========================================================

% x=OFDMTx( N, bits, num, nu,zero_tones)

%

% APSK transmitter for OFDM

%

% N is the FFT size

% bits is a {1,-1} stream of length (N/2-1)*2*num (baseband, zero DC)

% num is the number of OFDM symbols to produce

% nu is the cyclic prefix length

%==========================================================

if length(bits) ~= (N/2-1)*2*num

error('bits vector of improper length -- Aborting');

end x=[];

real_index = -1; %initial values

imag_index = 0;

for a=1:num

real_index = max(real_index)+2:2:max(real_index)+N-1;

imag_index = max(imag_index)+2:2:max(imag_index)+N-1;

X = (bits(real_index) + j*bits(imag_index))/2;

X=[0 X 0]; % zero nyquist and DC

x_hold=sqrt(N)*ifft([X,conj(fliplr(X(2:length(X)-1)))]); %Baseband so symmeteic

x_hold=[x_hold(length(x_hold)-nu+1:length(x_hold)),x_hold]; %Add CP

x=[x,x_hold];

end

x=real(x);

function bits_out = OFDMRx(N,y,h,num_symbols,nu)

%=========================================================

% bits_out = OFDMRx(M,N,y,num_symbols,nu,zero_tones)

%

% N is the FFT size

% y is received channel output for all symbols (excess delay spread removed)

% h is the (estimated) channel impulse response

% num_symbols is the # of symbols (each of N*sqrt(M) bits and N+nu samples)

% nu is the cyclic prefix length

%=========================================================

if length(y) ~= (N + nu)*num_symbols

error('received vector of improper length -- Aborting');

end bits_out=[]; bits_out_cur = zeros(1,N-2);

for a=1:num_symbols

y_cur = y((a-1)*(N+nu)+1+nu:a*(N+nu)); % Get current OFDM symbol, strip CP

X_hat = 1/sqrt(N)*fft(y_cur)./fft(h,N); %FEQ

X_hat = X_hat(1:N/2);

real_index = 1:2:N-2-1; % Don’t inc. X_hat (1) because is DC, zeroed

imag_index = 2:2:N-2;

bits_out_cur(real_index) = sign(real(X_hat(2:length(X_hat))));

bits_out_cur(imag_index) = sign(imag(X_hat(2:length(X_hat))));

bits_out=[bits_out,bits_out_cur];

end

Network topology

The basic issue in communication networks is the transmission of messages to achieve a prescribed message throughput (Quantity of Service) and Quality of Service (QoS). QoS can be specified in terms of message delay, message due dates, bit error rates, packet loss, economic cost of transmission, transmission power, etc. Depending on QoS, the installation environment, economic considerations, and the application, one of several basic network topologies may be used.

A communication network is composed of nodes, each of which has computing power and can transmit and receive messages over communication links, wireless or cabled. The basic network topologies are shown in the figure and include fully connected, mesh, star, ring, tree, bus. A single network may consist of several interconnected subnets of different topologies. Networks are further classified as Local Area Networks (LAN), e.g. inside one building, or Wide Area Networks (WAN), e.g. between buildings.

Fully connected networks suffer from problems of NP-complexity; as additional nodes are added, the number of links increases exponentially. Therefore, for large networks, the routing problem is computationally intractable even with the availability of large amounts of computing power.

Mesh networks are regularly distributed networks that generally allow transmission only to a node’s nearest neighbors. The nodes in these networks are generally identical, so that mesh nets are also referred to as peer-to-peer (see below) nets. Mesh nets can be good models for large-scale networks of wireless sensors that are distributed over a geographic region, e.g. personnel or vehicle security surveillance systems. Note that the regular structure reflects the communications topology; the actual geographic distribution of the nodes need not be a regular mesh. Since there are generally multiple routing paths between nodes, these nets are robust to failure of individual nodes or links. An advantage of mesh nets is that, although all nodes may be identical and have the same computing and transmission capabilities, certain nodes can be designated as ‘group leaders’ that take on additional functions. If a group leader is disabled, another node can then take over these duties.

All nodes of the star topology are connected to a single hub node. The hub requires greater message handling, routing, and decision-making capabilities than the other nodes. If a communication link is cut, it only affects one node. However, if the hub is incapacitated the network is destroyed. In the ring topology all nodes perform the same function and there is no leader node. Messages generally travel around the ring in a single direction.

However, if the ring is cut, all communication is lost. The self-healing ring network (SHR) shown has two rings and is more fault tolerant. Self-Healing RingPrimaryringBackupringSelf-RingPrimaryringBackupring

In the bus topology, messages are broadcast on the bus to all nodes. Each node checks the destination address in the message header, and processes the messages addressed to it. The bus topology is passive in that each node simply listens for messages and is not responsible for retransmitting any messages.