CDMA Solutions: Smart Antenna Technology

Wei-Chiang Wu

Multiple Access Techniques for Wireless Communication

• Frequency Division Multiple Access (FDMA)

• Time Division Multiple Access (TDMA)

• Code Division Multiple Access (CDMA)

• Space Division Multiple Access (SDMA)

• Orthogonal Frequency Division Multiple Access (OFDMA)

• Purpose:Purpose: Allow different users to communicate over the same channel

FDMA - Frequency

divide up the spectrum into non-overlapping sub-bands called channels but can transmit all the timeOrthogonal in frequency domain

TDMA - Time

divide up the time domain into non-overlapping time slotsOrthogonal in time domain

GSM uses 200 kHz channels and 8 slots

1

2

3

Multiple transmitters are divided in time for multiple access to the same carrier

TDMA - Time

CDMA - Code

CDMA uses a code division scheme, each user utilizesthe entire bandwidth all the time with a user-specificsignature sequence

Frequency reuse: Same frequency is used in different geographic areas or "cells"

SDMA - Spatial

Cell Sectoring

Cell Sectoring

• 120° cell sectoring reduces the number of co-channel base stations from 6 to 2.

Characteristics of CDMA (1)

• A multiple access technique that each user is assigned a unique signature waveform (PN code) upon which data bits are modulated– In FDMA, all users transmit simultaneously, but use

disjoint frequency bands– In TDMA, all users occupy the same BW, but transmit

sequentially in time– In CDMA, users are allowed to transmit

simultaneously in time and occupy the same BW

Characteristics of CDMA (2)

• Multiple Access Interference (MAI) limited

• The signature waveforms should perform– Delta-like autocorrelation for identification and sy

nchronization– Low crosscorrelation (approximate orthogonal) to

suppress MAI.– The signature waveforms among each user should

be linear independent in order not to cancel each other’s transmission

Characteristics of CDMA (3)

• Anti-multipath fading capability• Relatively easy to add additional users to the system• CDMA Capacity: How many users can coexist in a

CDMA cell? – mainly determined by processing gain and the required SI

R– The capacity is soft

• Same frequency can be used in all cells, BW efficient for multiple users

• CDMA air-interface is well-standardized

Multipath Fading

Tapped Delay Line Channel Model

CDMA Path Diversity_ RAKE Receiver

RAKE Receiver for CDMA

• CDMA takes advantage of the multipath signals to improve signal quality

• Using multiple correlating receivers that matched to each path

• Maximum ratio combining– coherently recombine the output signals from each fin

ger

Characteristics of CDMA (4)

• Anti-jamming capability• Security, ECCM• FH/CDMA systems achieve their processing

gain through Interference avoidance• DS/CDMA systems achieve their processing

gain through Interference attenuation• Soft-handover• Near-Far Problem: rx powers from different

users are unequal such that stronger user may degrade weak user’s transmission

Characteristics of CDMA (5)

• Power control: counter the near-far problem by dynamically adjusting transmitted power, based on feedback information from the receiver

• Multiuser detection: robust signal processing technique to design near-far resistant detectors, timing estimators,…

information source for user # 1

spreading code generatorfor user # 1

data modulator

information source for user # K

spreading code generatorfor user # K

data modulator

noise

antenna array(optional)

Multiuserdetector

Multiusertiming

estimator

Identification of active

users

Information sink for user # 1

Information sink for user # K

Fig. 1.1: The block diagram of a DS/CDMA communication system

Power Control

• A way to solve the near-far problems such that all the mobiles’ transmitting powers are equally-received by the Base station– For the near-end mobile, it can be asked to

transmit lower power– For the far-end mobile, it can be asked to increase

transmit power

• All mobiles are power controlled to the minimum power so as to maintain the link

Advantages of Cellular CDMA

• Multipath diversity can be gained with a RAKE receiver

• High frequency reuse efficiency

• Soft handoff capability

• Rejection of narrowband interference

• Ability to adapt to nonstationary traffic

• No need for guard intervals as in TDMA

Soft Handoff and Hard handoff

• Hard handoff– a physical change in the assigned channel– Break before make

• Soft Handoff – Soft Handoff is performed by exploiting two or more

base stations as a giant diversity system such that multiple BSs simultaneously talk to the mobile during a handoff

– a different BS handles the communication

Soft Handoff

Soft Handoff (1)

• Power control during soft handoff– If ANY BS orders the MS to reduce its power, it m

ust do so, but it may increase power only when directed by ALL BSs

• Macro-diversity– On the reverse link, each BS receives the signal fro

m MS and the demodulated signal are combined (fade resistant)

– On the forward link, MS receives the signal from BSs by RAKE and then combined

Soft Handoff (2)

• Advantage– improve link performance at the edge of the

cell

• Disadvantage– Reduce forward link capacity since each

takes up a traffic channel

Overview of Smart Antenna System (SAS) for Mobile Communications

• SAS is equipped in basestation (BS) with a pattern that is not fixed but adapts to the current radio conditions.

• Traditional omnidirectional or sectored antennas not only “Waste” power but also interfere to other users

• Smart antenna is a “Spatial Filter” that directs a beam toward desired user only

• It consists of a number of radiating elements, a combining/dividing network, a control unit (DSP).

Difference of BS radiation pattern between a traditional antenna and SAS

Principle of a SAS

Sharing the radio spectrum: SDMA (Space Division Multiple Access)

• Users in the same cell can use the same physical communication channel (carrier frequency, time slot, spreading code) as long as their angles are different

• The next step in an evolutionary path toward increasing the capacity of cellular systems– Dr. Andrew Viterbi

Advantages of Smart Antenna Technology

• Enhance coverage through Range extension• Improve building penetration and Hole filling• Reduce delay spread (time dispersion) because fewer

scatters are illuminated• Reduce co-channel interference (CCI) and multiple

access interference (MAI)• Link quality can be improved through multipath man

agement• Improve system capacity• Helps to isolate the uplink signals from different user

s, reduce the power control burden

Level of intelligence

• Switched Beam system– A switch is used to select the best beam to receive a particu

lar signal– Rx power level fluctuates as a subscriber travels in an arc– Unable to take advantage of path diversity of multipath sig

nal

• Dynamically phased array• Adaptive antenna system

– Interference nulling– Multipath diversity– Main beam steering

Different levels of SAS

Challenges of optimizing CDMA capacity

• Traffic load balancing

• Handoff overhead management

• Interference control

Traffic loading Challenge

• The time-varying traffic load is usually distributed unevenly among the cell sites or even within the sectors of an individual cell. Consequently, the capacity limit may be attained in the heavily loaded sectors (cells) even though quite a few channels (codes) remain available in the lightly loaded sectors (cells). The problem arises that as the capacity limit is reached in a specific heavily loaded sector, the unused capacity in some more lightly loaded sectors is inaccessible to subscribers.

Load Balancing: SAS rotates sectors and changes beamwidth

Traffic load balancing

• Consider a conventional 3-sector cell, one has to climb the antenna tower, physically rotate the orientation of the antennas such that the traffic is redistributed, which seem to be time-consuming and inefficient

• Smart antennas not only redirect the sector orientation, the beamwidth or coverage area of each sector is also changed. Furthermore, to reflect the fact that the load distribution pattern is time-varying, a software controlled sector orientation and beamwidth should be applied.

Dynamic sector synthesis

Handoff Overhead Challenge

• In the existed or third generation CDMA network, e.g. IS-95, cdma2000, and W-CDMA, Soft/softer handoff is a well-published mean to enhance the link quality in the cell edge. It is a giant diversity system that several base stations (BSs) simultaneously communicate to a single mobile station. Reliable link performance and fade resistance can be attained in the handoff process. However, since a MS contacts to several BSs via different channels (codes), Soft/softer handoff does exact a significant cost in capacity.

Handoff Overhead Management

• Goals– Reduce the size of handoff zones– Shift the handoff zone from heavy to light traffic area

• Cannot be overcome by conventional antennas, since the off-the-shelf antennas typically display very gradual roll off characteristics

• Best solution: apply array of antennas to produce narrower beams

• Smart antennas can create beams with sharper roll off especially in the high traffic area, the unnecessary handoff zones can be reduced and handoff overhead is lessened. Thereby, system capacity is increased.

Handoff Overhead plot

Interference Challenge

• Pilots interference: The MS usually receives various pilot signals from the surrounding cell sites and always chooses the strongest one to register. In some environment, the strength of the received pilot signals are approximately equal. This leads to unreliable handoff, synchronization problem, and moreover, reduced system capacity. It is not uncommon since many high elevation sites with RF coverage footprints much larger than normal sites due to line-of-sight. To reduce the transmitting power, downtilt or reduce the elevation height to the offending antenna seem to be the solution to prevent interference from too many pilots. However, the overall coverage is reduced.

• Multiuser interference and near-far problem

Pilot pollution: results from too many dominant servers

Interference Control

• It is required to propose a per-beam gain control scheme – the beam that is dedicated to provide service to the

in-building or tunnel should have a larger gain– the beam that is directed to an open area should

have lower gain in order not to induce interference to neighboring sites

Per-beam gain control

The CDMA Spatial Processing RAKE receiver

• Each RAKE finger uses the adaptive antenna to reject multipath component to which the finger is locked

• Diversity combining (MRC) is then used to combine the output from each RAKE finger to maximize the SINR for that finger

Downlink Beamforming for CDMA (1)

• Downlink Beamforming is used to significantly reduce the overall transmitted power of the BS, since it focuses only sufficient power needed to meet FER requirements for a subscriber receiver

• CDMA BS transmitter uses a broad beam to provide universal pilot, sync, paging channels throughout coverage (otherwise, MSs in those areas may not detect that the cell is available to provide service)

Downlink Beamforming for CDMA (2)

• Individual traffic channel is focused where they are required using Downlink Beamforming technique

• Coherent demodulation is degraded– Traffic signals (narrow beam) are phase-modulated r

elative to pilot (broad beam)– They may encounter phase differences, since broad

beam encounter more scatters– Add auxiliary pilot in cdma2000

Conclusion (1/2)

• CDMA network capacity strain is a paradox, because most networks use far less capacity than they have available

• Key constraint on capacity is peak loading in hot spots. Load balancing can “unlock” capacity by reducing peak loading

• SAS can generate a set of “flexible” beams. In what follows, the pattern, beamwidth, and gain of each beam can be adjusted even adapted individually to optimize the capacity of a CDMA network.

Conclusion (2/2)

• Specifically, unlike the conventional scheme, no tower climbing and no physical changing of antenna orientation are needed. All the required sector orientation and beamwidths are software-controllable, remotely configurable.