Wireless Networking

Small-scale fading and the Rician model

In a mobile channel the received signal is the sum of many multipath components. When there is no dominant component (no line of sight, LOS), the envelope follows the Rayleigh distribution. When a strong dominant component such as a LOS path is present along with scattered multipath, the envelope follows the Rician distribution.

Rician pdf

$p(r)=\frac{r}{\sigma^{2}}\exp\!\left(-\frac{r^{2}+A^{2}}{2\sigma^{2}}\right) I_0\!\left(\frac{A r}{\sigma^{2}}\right),\quad r\ge 0$

where $A$ is the peak amplitude of the dominant (specular) component, $\sigma^2$ is the variance of the scattered components, and $I_0(\cdot)$ is the modified Bessel function of the first kind and zero order.

Rician K-factor

$K=\frac{A^{2}}{2\sigma^{2}},\qquad K_{dB}=10\log_{10}\frac{A^{2}}{2\sigma^{2}}$

K is the ratio of the power of the dominant component to the power of the scattered components.

Relationship to Rayleigh fading

• As $A\to 0$ (so $K\to 0$), $I_0(0)=1$ and the pdf becomes $p(r)=\frac{r}{\sigma^{2}}e^{-r^{2}/2\sigma^{2}}$ which is exactly the Rayleigh pdf. Rayleigh fading is therefore the special case of Rician fading with no dominant path.
• As $K\to\infty$ the fading disappears and the channel approaches a non-fading (AWGN) channel.
• Larger K means shallower fades and better link performance.

Use: Rician fading models LOS microcells, satellite links and indoor environments, while Rayleigh models dense urban NLOS conditions.

Power spectrum and power spectral density

The power spectrum of a signal shows how its power is distributed over frequency (power in each frequency component). For a random process $x(t)$ with autocorrelation $R_x(\tau)$, the power spectral density (PSD) is the Fourier transform of the autocorrelation (Wiener-Khinchin theorem):

$S_x(f)=\int_{-\infty}^{\infty}R_x(\tau)e^{-j2\pi f\tau}\,d\tau$

and the total power is $P=\int_{-\infty}^{\infty}S_x(f)\,df$

PSD is measured in W/Hz and is used to find bandwidth, noise power and interference. For example, white noise has a flat PSD $N_0/2$.

QAM (Quadrature Amplitude Modulation)

QAM carries data by varying both the amplitude and the phase of a carrier, using two orthogonal carriers (cosine and sine) that are each amplitude modulated:

$s(t)=I(t)\cos(2\pi f_c t)-Q(t)\sin(2\pi f_c t)$

Bit groups are mapped to points of a constellation. 16-QAM sends 4 bits per symbol ($I,Q\in\{\pm1,\pm3\}$), and 64-QAM sends 6 bits per symbol. Higher order gives higher data rate but needs a higher SNR. It is used in cable modems, Wi-Fi and LTE.

OFDM (Orthogonal Frequency Division Multiplexing)

OFDM splits a high-rate stream into many low-rate parallel subcarriers whose spacing $\Delta f=1/T_s$ makes them orthogonal, so they can overlap in spectrum without interference. It is implemented with an IFFT at the transmitter and an FFT at the receiver, and a cyclic prefix is added to remove inter-symbol interference from multipath.

Example: with 64 subcarriers in a 20 MHz Wi-Fi channel (802.11a/g), each subcarrier carries QPSK, 16-QAM or 64-QAM. For example, 48 data subcarriers of 64-QAM with 6 bits each, at a symbol time of 4 µs, give $48\times 6/4\ \mu s=72$ Mbit/s before coding. OFDM is used in Wi-Fi, LTE, WiMAX and DVB.

Spread spectrum multiple access (SSMA) vs CDMA

SSMA is the general class of multiple access schemes in which each user's signal is spread over a bandwidth much wider than needed, using a spreading code. CDMA is the main form of SSMA, where users share the same band and time and are separated by unique codes.

In DS-CDMA, data is multiplied by a high-rate code, with processing gain $G_p=W/R$ (chip rate over data rate). The receiver despreads with the same code. Advantages are soft capacity, anti-jamming, multipath resistance and soft handoff. A drawback is the near-far problem.

WLAN standards (IEEE 802.11)

Medium access uses CSMA/CA. Other WLAN standards include HiperLAN.

Small-scale vs large-scale fading

Fading due to multipath delay spread

Multipath causes copies of the signal to arrive at different times. The delay spread $\sigma_\tau$ (the rms spread of arrival times) determines the coherence bandwidth $B_c\approx 1/(5\sigma_\tau)$.

• Flat fading: if signal bandwidth $B_s \ll B_c$ (symbol period $T_s \gg \sigma_\tau$), all frequency components fade together. The spectrum is preserved but the amplitude varies, as in Rayleigh and Rician fading.
• Frequency-selective fading: if $B_s > B_c$ ($T_s < \sigma_\tau$), different frequency components fade differently, which causes inter-symbol interference and signal distortion. Remedies are equalisation, OFDM and RAKE receivers.

4G (LTE/LTE-Advanced)

Peak rates are about 100 Mbps (mobile) to 1 Gbps (stationary), with an all-IP, packet-switched core (EPC).

• OFDMA in the downlink and SC-FDMA in the uplink.
• MIMO (spatial multiplexing and diversity) for higher rate and reliability.
• Adaptive modulation and coding (QPSK to 64-QAM) and HARQ.
• Carrier aggregation and relays/ CoMP in LTE-Advanced.
• Small cells and femtocells for coverage and capacity.

5G (IMT-2020)

Targets about 10-20 Gbps peak, under 1 ms latency and 10^6 devices per km².

• Massive MIMO and beamforming with many antennas.
• Millimetre-wave bands (24-100 GHz) plus sub-6 GHz bands for wide bandwidth.
• Network slicing, SDN/NFV and cloud-native core.
• Edge computing for low latency.
• Flexible numerology in OFDM, and ultra-dense small cells.
• Service classes: eMBB, URLLC, mMTC.

Modulation is the process of varying a property (amplitude, frequency or phase) of a high-frequency carrier $c(t)=A_c\cos(2\pi f_c t)$ in accordance with the message (baseband) signal $m(t)$, so that the information can be sent efficiently over a channel, with practical antenna sizes and with multiplexing.

AM (amplitude modulation): the carrier amplitude varies with the message. $s(t)=A_c[1+m\,\cos(2\pi f_m t)]\cos(2\pi f_c t)$ Bandwidth is $2f_m$. It is simple but noise sensitive. Example: medium-wave radio broadcast.

FM (frequency modulation): the carrier frequency varies with the message amplitude, and its amplitude is constant. Bandwidth by Carson's rule is $2(\Delta f+f_m)$. It is more noise resistant but needs more bandwidth. Example: FM radio (88-108 MHz).

PM (phase modulation): the carrier phase varies with the message, $s(t)=A_c\cos(2\pi f_c t+k_p m(t))$. It is closely related to FM, since frequency is the derivative of phase. Example: digital forms such as PSK in modems and satellite links.

Handoff (handover) management is the process of transferring an ongoing call or data session from one base station (cell/channel) to another when the mobile moves, so that the connection continues without interruption. It is triggered when signal strength falls below a threshold or when the quality or interference level becomes unacceptable.

Types of handoff

1. Hard handoff (break-before-make): the old link is released before the new one is made, as in GSM and FDMA/TDMA. There may be a brief interruption.
2. Soft handoff (make-before-break): the mobile is connected to two or more base stations at once, as in CDMA, and then drops the old one. It is smoother, with fewer dropped calls.
3. Softer handoff: soft handoff between sectors of the same base station.
4. Intra-cell handoff: a change of channel within the same cell, to avoid interference.
5. Inter-cell handoff: movement between cells.
6. Horizontal handoff: between the same network technology (e.g. LTE to LTE).
7. Vertical handoff: between different technologies (e.g. Wi-Fi to LTE).
8. Mobile-controlled, network-controlled and mobile-assisted (MAHO) are classes by who makes the decision.

Internetworking is the interconnection of two or more separate networks (possibly of different technologies and protocols) so that they work as one large network and devices in them can communicate, as in the Internet.

Internetworking devices in wireless networks

• Repeater: regenerates and amplifies signals to extend range (physical layer).
• Hub: connects wired devices and broadcasts frames.
• Access Point (AP): connects wireless clients to a wired LAN and acts as a bridge between 802.11 and 802.3. It operates at the data link layer.
• Bridge / switch: connects LAN segments and filters frames by MAC address.
• Router: connects different networks and forwards packets by IP address (network layer). A wireless router combines an AP, switch and router.
• Gateway: connects networks with different protocols and translates between them (e.g. WAP gateway, VoIP gateway).
• Base station / base station controller, MSC: connect cellular radio networks to the core network and PSTN.
• Mobility agents (home agent, foreign agent): support Mobile IP routing.

Why diversity is used

In a fading channel the received signal can drop deeply. Diversity transmits or receives the same information over several independent fading paths (in space, time, frequency, polarization or angle). Because it is unlikely that all paths fade together, combining them reduces the probability of deep fades, improves SNR and reliability, and increases link quality or range without raising transmit power.

Maximum ratio combining (MRC)

Each branch signal is weighted by its own amplitude and co-phased before summing. With branch gains $a_i$ and noise power $N$, weights are $w_i = a_i/N$ (proportional to signal amplitude over noise power). The output SNR is the sum of branch SNRs:

$\gamma_{MRC}=\sum_{i=1}^{M}\gamma_i$

It is the optimum linear combiner, but it needs channel-amplitude and phase estimates for every branch, so it is more complex.

Equal gain combining (EGC)

All branches are co-phased and added with equal weight ($w_i=1$), without amplitude weighting:

$\gamma_{EGC}=\frac{\left(\sum_{i}\sqrt{\gamma_i}\right)^{2}}{M}$

It is simpler than MRC and only slightly worse in performance. Both are better than selection combining, which uses only the best branch.

PCS (Personal Communication Services) network

A PCS network is a digital wireless system offering voice, data, messaging and other services to users anywhere and anytime, using small cells, low-power portable handsets and a personal number. Its parts are mobile stations, base stations, a base station controller, mobile switching centres (MSC), location databases (HLR and VLR) and the PSTN/signalling network (SS7).

Location management

Location management tracks where a mobile is, so incoming calls can be delivered. It has two parts:

1. Location registration (update): the mobile reports its location area to the network when it moves into a new location area (LA) or periodically. The HLR stores the current VLR/MSC of the user, and the VLR holds the visiting user's record.
2. Call delivery (paging): on an incoming call, the HLR is queried to find the serving MSC and VLR, then the cells of the location area are paged to find the mobile.

There is a trade-off. Frequent updates cost signalling but reduce paging cost, and the reverse is also true. Strategies include location areas, time-based, movement-based and distance-based updates, and two-tier schemes with pointers and caching.

MANET (Mobile Ad hoc Network)

A MANET is a self-organising, infrastructure-less wireless network of mobile nodes that communicate directly or through multi-hop forwarding by intermediate nodes, each node acting as both host and router. Topology changes dynamically. Features are no fixed infrastructure, limited bandwidth and battery, and frequent link breaks. Routing protocols are AODV, DSR (reactive), DSDV and OLSR (proactive). Uses are the battlefield, disaster relief and sensor networks.

Mobility handling in IPv6 (Mobile IPv6)

• Each mobile node has a permanent home address on its home network and gets a temporary care-of address (CoA) on a visited network by stateless autoconfiguration or DHCPv6.
• The node registers its CoA with its home agent by a binding update, and the home agent keeps the binding.
• Packets sent to the home address are intercepted by the home agent and tunnelled to the CoA.
• With route optimization, the mobile informs the correspondent node of the CoA by binding updates, and the correspondent sends packets directly (using a routing header), avoiding triangular routing.
• Unlike Mobile IPv4, no foreign agent is needed, since IPv6 has built-in autoconfiguration and neighbour discovery.

(a) WAP (Wireless Application Protocol)

WAP is an open standard that lets mobile devices with small screens and limited bandwidth and memory access Internet content. A WAP gateway translates between the WAP protocol and Web (HTTP/TCP) protocols. Pages are written in WML (Wireless Markup Language), with WMLScript. The protocol stack has WAE (application environment), WSP (session), WTP (transaction), WTLS (security) and WDP (datagram) over bearers such as SMS and GPRS. It is the basis of early mobile browsing, and was later replaced by smartphone web browsers.

(b) DPSK (Differential Phase Shift Keying)

DPSK is a form of PSK in which information is carried by the change of phase between successive symbols, rather than the absolute phase. For binary DPSK, a bit 1 leaves the phase unchanged and a bit 0 reverses it by 180° (or the reverse convention). The receiver compares the phase of the current symbol with the previous one, so no coherent carrier recovery is required and phase ambiguity is avoided. It is simpler, but has a slightly higher bit error rate than coherent PSK. It is used in modems, and $\pi/4$-DQPSK is used in digital cellular systems.