Wireless Notes

OFDMA Orthogonal Frequency Division Multiple Access

Learn OFDMA with OFDM basics, subcarrier allocation, resource blocks, cyclic prefix, SC-FDMA uplink, 4G LTE 5G NR implementation, scheduling, and advantages over CDMA for engineering students.

OFDMA is the foundation of modern wireless (4G LTE, 5G NR, WiFi 6). It converts the OFDM multiplexing technique into multiple access – dividing bandwidth into many narrow subcarriers and assigning them to different users.

📡 OFDM Basics

OFDM (Orthogonal Frequency Division Multiplexing) divides a wideband channel into many narrowband subcarriers. Each subcarrier is independently modulated.

│ Traditional single-carrier	│
│ OFDM (many narrow subcarriers)	│
│ Each: slow, narrow, long symbol	NO ISI! ✅ │
│ Together	high total data rate │
│ Key	Subcarriers are ORTHOGONAL (overlap but don't interfere) │

Why OFDM Solves Multipath:

Each subcarrier bandwidth << Coherence bandwidth
Each subcarrier sees flat fading (easy equalization!)
Long symbol duration >> delay spread → No ISI
Cyclic prefix absorbs remaining multipath

🎯 What is OFDMA?

OFDMA = OFDM + Multiple Access. OFDM is for a single user, OFDMA is for multiple users – different subcarriers are allocated to different users.

┌─────────────────────────────────────────────────────────────────────┐ │ OFDMA – RESOURCE ALLOCATION │ │ │ │ Frequency ↑ │ │ (subcarriers) │ │ │ ┌──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┬──┐ │ │ SC 12 │ │U1│U1│U3│U3│U2│U2│U1│U4│U4│U3│U2│U1│ │ │ SC 11 │ │U1│U1│U3│U3│U2│U2│U1│U4│U4│U3│U2│U1│ │ │ SC 10 │ │U2│U2│U2│U4│U4│U1│U1│U3│U3│U2│U1│U4│ │ │ SC 9 │ │U2│U2│U2│U4│U4│U1│U1│U3│U3│U2│U1│U4│ │ │ SC 8 │ │U3│U3│U1│U1│U1│U4│U4│U2│U2│U4│U3│U3│ │ │ SC 7 │ │U3│U3│U1│U1│U1│U4│U4│U2│U2│U4│U3│U3│ │ │ SC 6 │ │U4│U4│U4│U2│U3│U3│U2│U1│U1│U1│U4│U2│ │ │ ... │ └──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┴──┘ │ │ └──────────────────────────────────────────▶ Time │ │ (OFDM symbols / time slots) │ │ │ │ U1 = User 1 (blue), U2 = User 2 (green) │ │ U3 = User 3 (red), U4 = User 4 (yellow) │ │ │ │ DYNAMIC allocation: scheduler assigns based on channel quality │ │ User gets subcarriers where THEIR channel is good! │ └─────────────────────────────────────────────────────────────────────┘

📊 Resource Allocation

Resource Block (RB) in LTE:

	12 subcarriers	← 180 kHz wide
	×
	7 OFDM symbols	← 0.5 ms (1 slot)
	= 84 Resource Elements

Scheduling Types:

Strategy	How	Benefit
Frequency-selective	Give user RBs where channel is best	Maximum throughput
Round-robin	Equal RBs to all users	Fairness
Proportional fair	Balance throughput + fairness	Practical balance
Max throughput	Best channel gets priority	System capacity

🛡️ Cyclic Prefix

Cyclic prefix (CP) copies the end of an OFDM symbol and places it at the beginning. This absorbs multipath delay and prevents ISI.

Original OFDM symbol	[───────── Symbol data ─────────]
With CP	[CP][───────── Symbol data ─────────]
CP discarded	data region is ISI-free ✅
LTE	Normal CP = 4.7 μs, Extended CP = 16.7 μs
5G NR	Varies with subcarrier spacing (SCS)

📱 SC-FDMA (Uplink) {#sc-fdma-uplink}

The problem with OFDMA: high PAPR (Peak-to-Average Power Ratio). For mobile phones, this causes battery drain. Solution: SC-FDMA (Single Carrier FDMA) for uplink.

OFDMA: Multiple subcarriers	high peaks → needs linear PA → expensive
SC-FDMA: Single carrier-like	lower PAPR → efficient PA → cheap phone
Used in	LTE uplink, 5G NR uplink (optional)

	OFDMA (DL)	SC-FDMA (UL)
PAPR	High (7-12 dB)	Low (3-5 dB)
PA efficiency	Needs linear	Can use non-linear
Used by	Base station (has power)	Phone (battery limited)
Performance	Slightly better	Slightly worse
5G NR	DL + optional UL	Default UL (CP-OFDM also option)

📶 OFDMA in LTE & 5G

LTE (4G):

Parameter	Value
DL multiple access	OFDMA
UL multiple access	SC-FDMA
Subcarrier spacing	15 kHz
FFT sizes	128 to 2048
Bandwidths	1.4, 3, 5, 10, 15, 20 MHz
Symbol duration	66.7 μs
CP (normal)	4.7 μs
RB	12 subcarriers × 7 symbols
Scheduling	Every 1 ms (TTI)

5G NR:

Parameter	Value
DL & UL	CP-OFDM (both!)
Subcarrier spacings	15, 30, 60, 120, 240 kHz
Bandwidths	Up to 400 MHz (FR2)
Slot duration	0.5 ms (30 kHz SCS)
Max RBs	275 per carrier
Carrier aggregation	Up to 16 carriers
Scheduling	Mini-slot (2-7 symbols) possible

5G Numerology (Flexible SCS):

SCS	Symbol	CP	Use Case
15 kHz	66.7 μs	4.7 μs	Low-band, large cells
30 kHz	33.3 μs	2.3 μs	Mid-band (3.5 GHz)
60 kHz	16.7 μs	1.2 μs	Mid-band, URLLC
120 kHz	8.3 μs	0.59 μs	mmWave
240 kHz	4.2 μs	0.29 μs	mmWave sync

✅ Advantages & Disadvantages

Advantages:

Advantage	Hindi
Excellent multipath resistance	Multipath se bacha hua
Flexible resource allocation	Dynamic scheduling
High spectral efficiency	Spectrum achhe se use
Scalable bandwidth	Supports small and large BW
Simple equalization (1-tap per SC)	Simple receiver per subcarrier
Supports MIMO easily	Works well with MIMO
Frequency-selective scheduling	Best subcarriers give to best user
Adaptive modulation per subcarrier	Har SC pe alag MCS possible

Disadvantages:

Disadvantage	Hindi
High PAPR	Peak power is high (amplifier issue)
Sensitive to frequency offset	Frequency error se performance drop
Sensitive to Doppler (high speed)	High speed pe subcarriers interfere
CP overhead (wastes some capacity)	Some capacity wasted in CP
Requires synchronization	Time/freq sync is essential
Complex DSP (FFT/IFFT)	Heavy signal processing

⚔️ OFDMA vs CDMA

Parameter	CDMA (3G)	OFDMA (4G/5G)
Interference	Self-interference (all users)	Orthogonal (zero intra-cell)
Scheduling	Power-based	Frequency+time flexible
Multipath	RAKE receiver	CP handles it
Peak rate	~42 Mbps (HSPA+)	Gbps (LTE-A, 5G)
Spectral eff.	~1-2 bps/Hz	5-15 bps/Hz
MIMO support	Limited	Excellent (per-subcarrier)
Frequency planning	Not needed (reuse 1)	Not needed (reuse 1)
Complexity	Moderate	Higher DSP but cheaper overall

📝 Summary

Concept	Key Point
OFDM	Wideband → many narrow orthogonal subcarriers
OFDMA	OFDM + assign subcarriers to different users
ISI solution	Long symbols + cyclic prefix
Equalization	Simple 1-tap per subcarrier
Resource Block	Smallest unit: 12 SCs × 7 symbols (LTE)
Scheduling	Dynamic, frequency-selective
SC-FDMA	Low-PAPR variant for uplink (phone)
Used in	4G LTE, 5G NR, WiFi 6/7, WiMAX
Key win over CDMA	Orthogonal = no self-interference

❓ FAQ

Q: If users are orthogonal in OFDMA, is interference zero? A: Within the same cell (intra-cell), yes – orthogonal subcarriers = zero interference. But interference comes from neighboring cells (inter-cell). This is handled through coordination (ICIC, CoMP).

Q: WiFi 6 mein OFDMA kya naya kiya? A: Until WiFi 5, a single user used the entire bandwidth (OFDM, not OFDMA). WiFi 6 introduced multi-user OFDMA – multiple users simultaneously on different subcarrier groups. This significantly improved latency and efficiency.

Q: 5G mein subcarrier spacing kyun variable hai? A: Different use cases: Low SCS (15 kHz) = long CP = large cells (rural). High SCS (120 kHz) = short symbols = low latency (mmWave, URLLC). Flexibility!

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for OFDMA Orthogonal Frequency Division Multiple Access.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Wireless Communications topic.

Search Terms

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