Digital Schemes and Multiple Access

Tip

Exam Focus

Compare BPSK, OQPSK, GMSK (constellation, envelope, use); tabulate FDMA/TDMA/CDMA/OFDMA with their separation domain and example systems; and explain handover types with the hysteresis rule $P_{neighbour}>P_{serving}+H$. “Compare the multiple-access techniques” and “explain handover” are both frequent.

Digital Modulation for Wireless

• BPSK: one bit per symbol; carrier phase $0/\pi$; two constellation points. Very robust, low spectral efficiency.

• QPSK: two bits per symbol; four phases. OQPSK (offset QPSK) delays the Q channel by half a symbol so only one bit changes at a time: phase steps are limited to $\pm90^\circ$, avoiding $180^\circ$ transitions and large envelope dips (better for nonlinear amplifiers).

• GMSK: Gaussian-filtered MSK: a constant-envelope, continuous-phase FSK-type scheme with a compact spectrum; used in GSM because it tolerates efficient nonlinear power amplifiers.

Signal-space views: BPSK (two antipodal points), QPSK/OQPSK (four points; OQPSK offsets Q to avoid $180^\circ$ jumps), and constant-envelope GMSK whose trajectory stays on a circle.

Multiple Access Techniques

Technique	Separation	Used in
FDMA	Frequency bands	1G analog
TDMA	Time slots on a carrier	GSM
CDMA	Spreading codes (shared band)	IS-95, WCDMA
OFDMA	Orthogonal subcarrier–time blocks	LTE, 5G NR downlink
SC-FDMA	DFT-spread OFDM (low PAPR)	LTE uplink
SDMA	Spatial beams/antennas	MIMO, 5G

Multiple-access techniques.

Resource sharing: FDMA (frequency bands), TDMA (time slots), CDMA (whole band shared via codes) and OFDMA (subcarrier–time resource blocks).

Handover

Handover (handoff) transfers an ongoing call/session from one cell, sector, channel, beam or RAT to another without interrupting service.

It is triggered by falling RSS/RSRP or SINR, rising interference/BER, cell overload or a better available cell. To avoid the ping-pong effect, a neighbour must lead by a hysteresis margin for a time-to-trigger:

$$P_{neighbour}>P_{serving}+H.$$

Architecture (MS$\to$BTS$\to$BSC$\to$MSC) and the serving/neighbour RSS crossover: handover fires only after the neighbour leads by the hysteresis margin $H$.

Type	Operation / example
Hard	Break-before-make; old link released first (GSM, LTE)
Soft	Make-before-break; two links combined briefly (CDMA)
Softer	Between sectors of the same base station
Horizontal	Between cells of the same RAT (LTE$\to$LTE)
Vertical / inter-system	Between different RATs (Wi-Fi$\to$LTE, NR$\to$LTE)
Intra-/inter-cell	Channel change within a cell / transfer between cells

Handover types.

GSM uses mobile-assisted handover (MAHO): the mobile measures neighbour levels and reports them; the network decides. LTE/5G use hard handover controlled by the eNodeB/gNodeB, with 5G adding beam-level mobility and LTE–NR interworking.

Note

Key Point — Exam-ready model answer — handover trigger

If serving power is $-92\,dBm$, neighbour $-86\,dBm$ and $H=3\,dB$, the neighbour leads by $6\,dB>H$, so after the time-to-trigger the handover executes; brief transient crossings do not trigger, preventing ping-pong.

[!NOTE]

Key Point — Revision — Wireless Communication

• 1G analog/FDMA $\to$ 2G digital voice+SMS (GSM/CDMA) $\to$ 3G WCDMA multimedia $\to$ 4G LTE all-IP (OFDMA+MIMO) $\to$ 5G NR (massive MIMO, mmWave, slicing) with eMBB/URLLC/mMTC.

• Cellular gain is frequency reuse: $N=i^2+ij+j^2$, $D=R\sqrt{3N}$, $Q=D/R=\sqrt{3N}$; smaller $N$ = more capacity, more CCI.

• $L_{fs}=32.44+20\log_{10}d_{km}+20\log_{10}f_{MHz}$; two-ray gives $d^{-4}$; Hata is the closed-form Okumura fit ($150$–$1500$ MHz).

• Doppler $f_d=v/\lambda$; fading is fast/slow and flat/frequency-selective.

• Multiple access: FDMA/TDMA/CDMA/OFDMA; handover hard (break-before-make) vs soft (make-before-break), fired by $P_{nbr}>P_{srv}+H$.